TypeScript/src/compiler/program.ts

/// <reference path="sys.ts" />
/// <reference path="emitter.ts" />
/// <reference path="core.ts" />

namespace ts {
    /** The version of the TypeScript compiler release */

    export const version = "2.1.0";

    const emptyArray: any[] = [];

    export function findConfigFile(searchPath: string, fileExists: (fileName: string) => boolean, configName = "tsconfig.json"): string {
        while (true) {
            const fileName = combinePaths(searchPath, configName);
            if (fileExists(fileName)) {
                return fileName;
            }
            const parentPath = getDirectoryPath(searchPath);
            if (parentPath === searchPath) {
                break;
            }
            searchPath = parentPath;
        }
        return undefined;
    }

    export function resolveTripleslashReference(moduleName: string, containingFile: string): string {
        const basePath = getDirectoryPath(containingFile);
        const referencedFileName = isRootedDiskPath(moduleName) ? moduleName : combinePaths(basePath, moduleName);
        return normalizePath(referencedFileName);
    }

    /* @internal */
    export function computeCommonSourceDirectoryOfFilenames(fileNames: string[], currentDirectory: string, getCanonicalFileName: (fileName: string) => string): string {
        let commonPathComponents: string[];
        const failed = forEach(fileNames, sourceFile => {
            // Each file contributes into common source file path
            const sourcePathComponents = getNormalizedPathComponents(sourceFile, currentDirectory);
            sourcePathComponents.pop(); // The base file name is not part of the common directory path

            if (!commonPathComponents) {
                // first file
                commonPathComponents = sourcePathComponents;
                return;
            }

            for (let i = 0, n = Math.min(commonPathComponents.length, sourcePathComponents.length); i < n; i++) {
                if (getCanonicalFileName(commonPathComponents[i]) !== getCanonicalFileName(sourcePathComponents[i])) {
                    if (i === 0) {
                        // Failed to find any common path component
                        return true;
                    }

                    // New common path found that is 0 -> i-1
                    commonPathComponents.length = i;
                    break;
                }
            }

            // If the sourcePathComponents was shorter than the commonPathComponents, truncate to the sourcePathComponents
            if (sourcePathComponents.length < commonPathComponents.length) {
                commonPathComponents.length = sourcePathComponents.length;
            }
        });

        // A common path can not be found when paths span multiple drives on windows, for example
        if (failed) {
            return "";
        }

        if (!commonPathComponents) { // Can happen when all input files are .d.ts files
            return currentDirectory;
        }

        return getNormalizedPathFromPathComponents(commonPathComponents);
    }

    interface OutputFingerprint {
        hash: string;
        byteOrderMark: boolean;
        mtime: Date;
    }

    export function createCompilerHost(options: CompilerOptions, setParentNodes?: boolean): CompilerHost {
        const existingDirectories = createMap<boolean>();

        function getCanonicalFileName(fileName: string): string {
            // if underlying system can distinguish between two files whose names differs only in cases then file name already in canonical form.
            // otherwise use toLowerCase as a canonical form.
            return sys.useCaseSensitiveFileNames ? fileName : fileName.toLowerCase();
        }

        // returned by CScript sys environment
        const unsupportedFileEncodingErrorCode = -2147024809;

        function getSourceFile(fileName: string, languageVersion: ScriptTarget, onError?: (message: string) => void): SourceFile {
            let text: string;
            try {
                performance.mark("beforeIORead");
                text = sys.readFile(fileName, options.charset);
                performance.mark("afterIORead");
                performance.measure("I/O Read", "beforeIORead", "afterIORead");
            }
            catch (e) {
                if (onError) {
                    onError(e.number === unsupportedFileEncodingErrorCode
                        ? createCompilerDiagnostic(Diagnostics.Unsupported_file_encoding).messageText
                        : e.message);
                }
                text = "";
            }

            return text !== undefined ? createSourceFile(fileName, text, languageVersion, setParentNodes) : undefined;
        }

        function directoryExists(directoryPath: string): boolean {
            if (directoryPath in existingDirectories) {
                return true;
            }
            if (sys.directoryExists(directoryPath)) {
                existingDirectories[directoryPath] = true;
                return true;
            }
            return false;
        }

        function ensureDirectoriesExist(directoryPath: string) {
            if (directoryPath.length > getRootLength(directoryPath) && !directoryExists(directoryPath)) {
                const parentDirectory = getDirectoryPath(directoryPath);
                ensureDirectoriesExist(parentDirectory);
                sys.createDirectory(directoryPath);
            }
        }

        let outputFingerprints: Map<OutputFingerprint>;

        function writeFileIfUpdated(fileName: string, data: string, writeByteOrderMark: boolean): void {
            if (!outputFingerprints) {
                outputFingerprints = createMap<OutputFingerprint>();
            }

            const hash = sys.createHash(data);
            const mtimeBefore = sys.getModifiedTime(fileName);

            if (mtimeBefore && fileName in outputFingerprints) {
                const fingerprint = outputFingerprints[fileName];

                // If output has not been changed, and the file has no external modification
                if (fingerprint.byteOrderMark === writeByteOrderMark &&
                    fingerprint.hash === hash &&
                    fingerprint.mtime.getTime() === mtimeBefore.getTime()) {
                    return;
                }
            }

            sys.writeFile(fileName, data, writeByteOrderMark);

            const mtimeAfter = sys.getModifiedTime(fileName);

            outputFingerprints[fileName] = {
                hash,
                byteOrderMark: writeByteOrderMark,
                mtime: mtimeAfter
            };
        }

        function writeFile(fileName: string, data: string, writeByteOrderMark: boolean, onError?: (message: string) => void) {
            try {
                performance.mark("beforeIOWrite");
                ensureDirectoriesExist(getDirectoryPath(normalizePath(fileName)));

                if (isWatchSet(options) && sys.createHash && sys.getModifiedTime) {
                    writeFileIfUpdated(fileName, data, writeByteOrderMark);
                }
                else {
                    sys.writeFile(fileName, data, writeByteOrderMark);
                }

                performance.mark("afterIOWrite");
                performance.measure("I/O Write", "beforeIOWrite", "afterIOWrite");
            }
            catch (e) {
                if (onError) {
                    onError(e.message);
                }
            }
        }

        function getDefaultLibLocation(): string {
            return getDirectoryPath(normalizePath(sys.getExecutingFilePath()));
        }

        const newLine = getNewLineCharacter(options);
        const realpath = sys.realpath && ((path: string) => sys.realpath(path));

        return {
            getSourceFile,
            getDefaultLibLocation,
            getDefaultLibFileName: options => combinePaths(getDefaultLibLocation(), getDefaultLibFileName(options)),
            writeFile,
            getCurrentDirectory: memoize(() => sys.getCurrentDirectory()),
            useCaseSensitiveFileNames: () => sys.useCaseSensitiveFileNames,
            getCanonicalFileName,
            getNewLine: () => newLine,
            fileExists: fileName => sys.fileExists(fileName),
            readFile: fileName => sys.readFile(fileName),
            trace: (s: string) => sys.write(s + newLine),
            directoryExists: directoryName => sys.directoryExists(directoryName),
            getEnvironmentVariable: name => sys.getEnvironmentVariable ? sys.getEnvironmentVariable(name) : "",
            getDirectories: (path: string) => sys.getDirectories(path),
            realpath
        };
    }

    export function getPreEmitDiagnostics(program: Program, sourceFile?: SourceFile, cancellationToken?: CancellationToken): Diagnostic[] {
        let diagnostics = program.getOptionsDiagnostics(cancellationToken).concat(
            program.getSyntacticDiagnostics(sourceFile, cancellationToken),
            program.getGlobalDiagnostics(cancellationToken),
            program.getSemanticDiagnostics(sourceFile, cancellationToken));

        if (program.getCompilerOptions().declaration) {
            diagnostics = diagnostics.concat(program.getDeclarationDiagnostics(sourceFile, cancellationToken));
        }

        return sortAndDeduplicateDiagnostics(diagnostics);
    }

    export interface FormatDiagnosticsHost {
        getCurrentDirectory(): string;
        getCanonicalFileName(fileName: string): string;
        getNewLine(): string;
    }

    export function formatDiagnostics(diagnostics: Diagnostic[], host: FormatDiagnosticsHost): string {
        let output = "";

        for (const diagnostic of diagnostics) {
            if (diagnostic.file) {
                const { line, character } = getLineAndCharacterOfPosition(diagnostic.file, diagnostic.start);
                const fileName = diagnostic.file.fileName;
                const relativeFileName = convertToRelativePath(fileName, host.getCurrentDirectory(), fileName => host.getCanonicalFileName(fileName));
                output += `${relativeFileName}(${line + 1},${character + 1}): `;
            }

            const category = DiagnosticCategory[diagnostic.category].toLowerCase();
            output += `${category} TS${diagnostic.code}: ${flattenDiagnosticMessageText(diagnostic.messageText, host.getNewLine())}${host.getNewLine()}`;
        }
        return output;
    }

    export function flattenDiagnosticMessageText(messageText: string | DiagnosticMessageChain, newLine: string): string {
        if (typeof messageText === "string") {
            return messageText;
        }
        else {
            let diagnosticChain = messageText;
            let result = "";

            let indent = 0;
            while (diagnosticChain) {
                if (indent) {
                    result += newLine;

                    for (let i = 0; i < indent; i++) {
                        result += "  ";
                    }
                }
                result += diagnosticChain.messageText;
                indent++;
                diagnosticChain = diagnosticChain.next;
            }

            return result;
        }
    }

    function loadWithLocalCache<T>(names: string[], containingFile: string, loader: (name: string, containingFile: string) => T): T[] {
        if (names.length === 0) {
            return [];
        }
        const resolutions: T[] = [];
        const cache = createMap<T>();
        for (const name of names) {
            const result = name in cache
                ? cache[name]
                : cache[name] = loader(name, containingFile);
            resolutions.push(result);
        }
        return resolutions;
    }

    export function createProgram(rootNames: string[], options: CompilerOptions, host?: CompilerHost, oldProgram?: Program): Program {
        let program: Program;
        let files: SourceFile[] = [];
        let commonSourceDirectory: string;
        let diagnosticsProducingTypeChecker: TypeChecker;
        let noDiagnosticsTypeChecker: TypeChecker;
        let classifiableNames: Map<string>;

        let resolvedTypeReferenceDirectives = createMap<ResolvedTypeReferenceDirective>();
        let fileProcessingDiagnostics = createDiagnosticCollection();

        // The below settings are to track if a .js file should be add to the program if loaded via searching under node_modules.
        // This works as imported modules are discovered recursively in a depth first manner, specifically:
        // - For each root file, findSourceFile is called.
        // - This calls processImportedModules for each module imported in the source file.
        // - This calls resolveModuleNames, and then calls findSourceFile for each resolved module.
        // As all these operations happen - and are nested - within the createProgram call, they close over the below variables.
        // The current resolution depth is tracked by incrementing/decrementing as the depth first search progresses.
        const maxNodeModuleJsDepth = typeof options.maxNodeModuleJsDepth === "number" ? options.maxNodeModuleJsDepth : 0;
        let currentNodeModulesDepth = 0;

        // If a module has some of its imports skipped due to being at the depth limit under node_modules, then track
        // this, as it may be imported at a shallower depth later, and then it will need its skipped imports processed.
        const modulesWithElidedImports = createMap<boolean>();

        // Track source files that are source files found by searching under node_modules, as these shouldn't be compiled.
        const sourceFilesFoundSearchingNodeModules = createMap<boolean>();

        performance.mark("beforeProgram");

        host = host || createCompilerHost(options);

        let skipDefaultLib = options.noLib;
        const programDiagnostics = createDiagnosticCollection();
        const currentDirectory = host.getCurrentDirectory();
        const supportedExtensions = getSupportedExtensions(options);

        // Map storing if there is emit blocking diagnostics for given input
        const hasEmitBlockingDiagnostics = createFileMap<boolean>(getCanonicalFileName);

        let resolveModuleNamesWorker: (moduleNames: string[], containingFile: string) => ResolvedModuleFull[];
        if (host.resolveModuleNames) {
            resolveModuleNamesWorker = (moduleNames, containingFile) => host.resolveModuleNames(moduleNames, containingFile).map(resolved => {
                // An older host may have omitted extension, in which case we should infer it from the file extension of resolvedFileName.
                if (!resolved || (resolved as ResolvedModuleFull).extension !== undefined) {
                    return resolved as ResolvedModuleFull;
                }
                const withExtension = clone(resolved) as ResolvedModuleFull;
                withExtension.extension = extensionFromPath(resolved.resolvedFileName);
                return withExtension;
            });
        }
        else {
            const loader = (moduleName: string, containingFile: string) => resolveModuleName(moduleName, containingFile, options, host).resolvedModule;
            resolveModuleNamesWorker = (moduleNames, containingFile) => loadWithLocalCache(moduleNames, containingFile, loader);
        }

        let resolveTypeReferenceDirectiveNamesWorker: (typeDirectiveNames: string[], containingFile: string) => ResolvedTypeReferenceDirective[];
        if (host.resolveTypeReferenceDirectives) {
            resolveTypeReferenceDirectiveNamesWorker = (typeDirectiveNames, containingFile) => host.resolveTypeReferenceDirectives(typeDirectiveNames, containingFile);
        }
        else {
            const loader = (typesRef: string, containingFile: string) => resolveTypeReferenceDirective(typesRef, containingFile, options, host).resolvedTypeReferenceDirective;
            resolveTypeReferenceDirectiveNamesWorker = (typeReferenceDirectiveNames, containingFile) => loadWithLocalCache(typeReferenceDirectiveNames, containingFile, loader);
        }

        const filesByName = createFileMap<SourceFile>();
        // stores 'filename -> file association' ignoring case
        // used to track cases when two file names differ only in casing
        const filesByNameIgnoreCase = host.useCaseSensitiveFileNames() ? createFileMap<SourceFile>(fileName => fileName.toLowerCase()) : undefined;

        if (!tryReuseStructureFromOldProgram()) {
            forEach(rootNames, name => processRootFile(name, /*isDefaultLib*/ false));

            // load type declarations specified via 'types' argument or implicitly from types/ and node_modules/@types folders
            const typeReferences: string[] = getAutomaticTypeDirectiveNames(options, host);

            if (typeReferences.length) {
                // This containingFilename needs to match with the one used in managed-side
                const containingFilename = combinePaths(host.getCurrentDirectory(), "__inferred type names__.ts");
                const resolutions = resolveTypeReferenceDirectiveNamesWorker(typeReferences, containingFilename);
                for (let i = 0; i < typeReferences.length; i++) {
                    processTypeReferenceDirective(typeReferences[i], resolutions[i]);
                }
            }

            // Do not process the default library if:
            //  - The '--noLib' flag is used.
            //  - A 'no-default-lib' reference comment is encountered in
            //      processing the root files.
            if (!skipDefaultLib) {
                // If '--lib' is not specified, include default library file according to '--target'
                // otherwise, using options specified in '--lib' instead of '--target' default library file
                if (!options.lib) {
                    processRootFile(host.getDefaultLibFileName(options), /*isDefaultLib*/ true);
                }
                else {
                    const libDirectory = host.getDefaultLibLocation ? host.getDefaultLibLocation() : getDirectoryPath(host.getDefaultLibFileName(options));
                    forEach(options.lib, libFileName => {
                        processRootFile(combinePaths(libDirectory, libFileName), /*isDefaultLib*/ true);
                    });
                }
            }
        }

        // unconditionally set oldProgram to undefined to prevent it from being captured in closure
        oldProgram = undefined;

        program = {
            getRootFileNames: () => rootNames,
            getSourceFile,
            getSourceFileByPath,
            getSourceFiles: () => files,
            getCompilerOptions: () => options,
            getSyntacticDiagnostics,
            getOptionsDiagnostics,
            getGlobalDiagnostics,
            getSemanticDiagnostics,
            getDeclarationDiagnostics,
            getTypeChecker,
            getClassifiableNames,
            getDiagnosticsProducingTypeChecker,
            getCommonSourceDirectory,
            emit,
            getCurrentDirectory: () => currentDirectory,
            getNodeCount: () => getDiagnosticsProducingTypeChecker().getNodeCount(),
            getIdentifierCount: () => getDiagnosticsProducingTypeChecker().getIdentifierCount(),
            getSymbolCount: () => getDiagnosticsProducingTypeChecker().getSymbolCount(),
            getTypeCount: () => getDiagnosticsProducingTypeChecker().getTypeCount(),
            getFileProcessingDiagnostics: () => fileProcessingDiagnostics,
            getResolvedTypeReferenceDirectives: () => resolvedTypeReferenceDirectives,
            isSourceFileFromExternalLibrary,
            dropDiagnosticsProducingTypeChecker
        };

        verifyCompilerOptions();
        performance.mark("afterProgram");
        performance.measure("Program", "beforeProgram", "afterProgram");

        return program;

        function getCommonSourceDirectory() {
            if (typeof commonSourceDirectory === "undefined") {
                if (options.rootDir && checkSourceFilesBelongToPath(files, options.rootDir)) {
                    // If a rootDir is specified and is valid use it as the commonSourceDirectory
                    commonSourceDirectory = getNormalizedAbsolutePath(options.rootDir, currentDirectory);
                }
                else {
                    commonSourceDirectory = computeCommonSourceDirectory(files);
                }
                if (commonSourceDirectory && commonSourceDirectory[commonSourceDirectory.length - 1] !== directorySeparator) {
                    // Make sure directory path ends with directory separator so this string can directly
                    // used to replace with "" to get the relative path of the source file and the relative path doesn't
                    // start with / making it rooted path
                    commonSourceDirectory += directorySeparator;
                }
            }
            return commonSourceDirectory;
        }

        function getClassifiableNames() {
            if (!classifiableNames) {
                // Initialize a checker so that all our files are bound.
                getTypeChecker();
                classifiableNames = createMap<string>();

                for (const sourceFile of files) {
                    copyProperties(sourceFile.classifiableNames, classifiableNames);
                }
            }

            return classifiableNames;
        }

        interface OldProgramState {
            program: Program;
            file: SourceFile;
            modifiedFilePaths: Path[];
        }

        function resolveModuleNamesReusingOldState(moduleNames: string[], containingFile: string, file: SourceFile, oldProgramState?: OldProgramState) {
            if (!oldProgramState && !file.ambientModuleNames.length) {
                // if old program state is not supplied and file does not contain locally defined ambient modules
                // then the best we can do is fallback to the default logic
                return resolveModuleNamesWorker(moduleNames, containingFile);
            }

            // at this point we know that either 
            // - file has local declarations for ambient modules
            // OR
            // - old program state is available
            // OR
            // - both of items above
            // With this it is possible that we can tell how some module names from the initial list will be resolved
            // without doing actual resolution (in particular if some name was resolved to ambient module).
            // Such names should be excluded from the list of module names that will be provided to `resolveModuleNamesWorker`
            // since we don't want to resolve them again.

            // this is a list of modules for which we cannot predict resolution so they should be actually resolved
            let unknownModuleNames: string[];
            // this is a list of combined results assembles from predicted and resolved results.
            // Order in this list matches the order in the original list of module names `moduleNames` which is important
            // so later we can split results to resolutions of modules and resolutions of module augmentations.
            let result: ResolvedModuleFull[];
            // a transient placeholder that is used to mark predicted resolution in the result list
            const predictedToResolveToAmbientModuleMarker: ResolvedModuleFull = <any>{};

            for (let i = 0; i < moduleNames.length; i++) {
                const moduleName = moduleNames[i];
                // module name is known to be resolved to ambient module if
                // - module name is contained in the list of ambient modules that are locally declared in the file
                // - in the old program module name was resolved to ambient module whose declaration is in non-modified file
                //   (so the same module declaration will land in the new program)
                let isKnownToResolveToAmbientModule = false;
                if (contains(file.ambientModuleNames, moduleName)) {
                    isKnownToResolveToAmbientModule = true;
                    if (isTraceEnabled(options, host)) {
                        trace(host, Diagnostics.Module_0_was_resolved_as_locally_declared_ambient_module_in_file_1, moduleName, containingFile);
                    }
                }
                else {
                    isKnownToResolveToAmbientModule = checkModuleNameResolvedToAmbientModuleInNonModifiedFile(moduleName, oldProgramState);
                }

                if (isKnownToResolveToAmbientModule) {
                    if (!unknownModuleNames) {
                        // found a first module name for which result can be prediced
                        // this means that this module name should not be passed to `resolveModuleNamesWorker`.
                        // We'll use a separate list for module names that are definitely unknown.
                        result = new Array(moduleNames.length);
                        // copy all module names that appear before the current one in the list
                        // since they are known to be unknown
                        unknownModuleNames = moduleNames.slice(0, i);
                    }
                    // mark prediced resolution in the result list
                    result[i] = predictedToResolveToAmbientModuleMarker;
                }
                else if (unknownModuleNames) {
                    // found unknown module name and we are already using separate list for those - add it to the list
                    unknownModuleNames.push(moduleName);
                }
            }

            if (!unknownModuleNames) {
                // we've looked throught the list but have not seen any predicted resolution
                // use default logic
                return resolveModuleNamesWorker(moduleNames, containingFile);
            }

            const resolutions = unknownModuleNames.length
                ? resolveModuleNamesWorker(unknownModuleNames, containingFile)
                : emptyArray;

            // combine results of resolutions and predicted results
            let j = 0;
            for (let i = 0; i < result.length; i++) {
                if (result[i] == predictedToResolveToAmbientModuleMarker) {
                    result[i] = undefined;
                }
                else {
                    result[i] = resolutions[j];
                    j++;
                }
            }
            Debug.assert(j === resolutions.length);
            return result;

            function checkModuleNameResolvedToAmbientModuleInNonModifiedFile(moduleName: string, oldProgramState?: OldProgramState): boolean {
                if (!oldProgramState) {
                    return false;
                }
                const resolutionToFile = getResolvedModule(oldProgramState.file, moduleName);
                if (resolutionToFile) {
                    // module used to be resolved to file - ignore it
                    return false;
                }
                const ambientModule = oldProgram.getTypeChecker().tryFindAmbientModuleWithoutAugmentations(moduleName);
                if (!(ambientModule && ambientModule.declarations)) {
                    return false;
                }

                // at least one of declarations should come from non-modified source file
                const firstUnmodifiedFile = forEach(ambientModule.declarations, d => {
                    const f = getSourceFileOfNode(d);
                    return !contains(oldProgramState.modifiedFilePaths, f.path) && f;
                });

                if (!firstUnmodifiedFile) {
                    return false;
                }

                if (isTraceEnabled(options, host)) {
                    trace(host, Diagnostics.Module_0_was_resolved_as_ambient_module_declared_in_1_since_this_file_was_not_modified, moduleName, firstUnmodifiedFile.fileName);
                }
                return true;
            }
        }

        function tryReuseStructureFromOldProgram(): boolean {
            if (!oldProgram) {
                return false;
            }

            // check properties that can affect structure of the program or module resolution strategy
            // if any of these properties has changed - structure cannot be reused
            const oldOptions = oldProgram.getCompilerOptions();
            if (changesAffectModuleResolution(oldOptions, options)) {
                return false;
            }

            Debug.assert(!oldProgram.structureIsReused);

            // there is an old program, check if we can reuse its structure
            const oldRootNames = oldProgram.getRootFileNames();
            if (!arrayIsEqualTo(oldRootNames, rootNames)) {
                return false;
            }

            if (!arrayIsEqualTo(options.types, oldOptions.types)) {
                return false;
            }

            // check if program source files has changed in the way that can affect structure of the program
            const newSourceFiles: SourceFile[] = [];
            const filePaths: Path[] = [];
            const modifiedSourceFiles: { oldFile: SourceFile, newFile: SourceFile }[] = [];

            for (const oldSourceFile of oldProgram.getSourceFiles()) {
                let newSourceFile = host.getSourceFileByPath
                    ? host.getSourceFileByPath(oldSourceFile.fileName, oldSourceFile.path, options.target)
                    : host.getSourceFile(oldSourceFile.fileName, options.target);

                if (!newSourceFile) {
                    return false;
                }

                newSourceFile.path = oldSourceFile.path;
                filePaths.push(newSourceFile.path);

                if (oldSourceFile !== newSourceFile) {
                    if (oldSourceFile.hasNoDefaultLib !== newSourceFile.hasNoDefaultLib) {
                        // value of no-default-lib has changed
                        // this will affect if default library is injected into the list of files
                        return false;
                    }

                    // check tripleslash references
                    if (!arrayIsEqualTo(oldSourceFile.referencedFiles, newSourceFile.referencedFiles, fileReferenceIsEqualTo)) {
                        // tripleslash references has changed
                        return false;
                    }

                    // check imports and module augmentations
                    collectExternalModuleReferences(newSourceFile);
                    if (!arrayIsEqualTo(oldSourceFile.imports, newSourceFile.imports, moduleNameIsEqualTo)) {
                        // imports has changed
                        return false;
                    }
                    if (!arrayIsEqualTo(oldSourceFile.moduleAugmentations, newSourceFile.moduleAugmentations, moduleNameIsEqualTo)) {
                        // moduleAugmentations has changed
                        return false;
                    }

                    if (!arrayIsEqualTo(oldSourceFile.typeReferenceDirectives, newSourceFile.typeReferenceDirectives, fileReferenceIsEqualTo)) {
                        // 'types' references has changed
                        return false;
                    }

                    // tentatively approve the file
                    modifiedSourceFiles.push({ oldFile: oldSourceFile, newFile: newSourceFile });
                }
                else {
                    // file has no changes - use it as is
                    newSourceFile = oldSourceFile;
                }

                // if file has passed all checks it should be safe to reuse it
                newSourceFiles.push(newSourceFile);
            }

            const modifiedFilePaths = modifiedSourceFiles.map(f => f.newFile.path);
            // try to verify results of module resolution 
            for (const { oldFile: oldSourceFile, newFile: newSourceFile } of modifiedSourceFiles) {
                const newSourceFilePath = getNormalizedAbsolutePath(newSourceFile.fileName, currentDirectory);
                if (resolveModuleNamesWorker) {
                    const moduleNames = map(concatenate(newSourceFile.imports, newSourceFile.moduleAugmentations), getTextOfLiteral);
                    const resolutions = resolveModuleNamesReusingOldState(moduleNames, newSourceFilePath, newSourceFile, { file: oldSourceFile, program: oldProgram, modifiedFilePaths });
                    // ensure that module resolution results are still correct
                    const resolutionsChanged = hasChangesInResolutions(moduleNames, resolutions, oldSourceFile.resolvedModules, moduleResolutionIsEqualTo);
                    if (resolutionsChanged) {
                        return false;
                    }
                }
                if (resolveTypeReferenceDirectiveNamesWorker) {
                    const typesReferenceDirectives = map(newSourceFile.typeReferenceDirectives, x => x.fileName);
                    const resolutions = resolveTypeReferenceDirectiveNamesWorker(typesReferenceDirectives, newSourceFilePath);
                    // ensure that types resolutions are still correct
                    const resolutionsChanged = hasChangesInResolutions(typesReferenceDirectives, resolutions, oldSourceFile.resolvedTypeReferenceDirectiveNames, typeDirectiveIsEqualTo);
                    if (resolutionsChanged) {
                        return false;
                    }
                }
                // pass the cache of module/types resolutions from the old source file
                newSourceFile.resolvedModules = oldSourceFile.resolvedModules;
                newSourceFile.resolvedTypeReferenceDirectiveNames = oldSourceFile.resolvedTypeReferenceDirectiveNames;
            }

            // update fileName -> file mapping
            for (let i = 0, len = newSourceFiles.length; i < len; i++) {
                filesByName.set(filePaths[i], newSourceFiles[i]);
            }

            files = newSourceFiles;
            fileProcessingDiagnostics = oldProgram.getFileProcessingDiagnostics();

            for (const modifiedFile of modifiedSourceFiles) {
                fileProcessingDiagnostics.reattachFileDiagnostics(modifiedFile.newFile);
            }
            resolvedTypeReferenceDirectives = oldProgram.getResolvedTypeReferenceDirectives();
            oldProgram.structureIsReused = true;

            return true;
        }

        function getEmitHost(writeFileCallback?: WriteFileCallback): EmitHost {
            return {
                getCanonicalFileName,
                getCommonSourceDirectory: program.getCommonSourceDirectory,
                getCompilerOptions: program.getCompilerOptions,
                getCurrentDirectory: () => currentDirectory,
                getNewLine: () => host.getNewLine(),
                getSourceFile: program.getSourceFile,
                getSourceFileByPath: program.getSourceFileByPath,
                getSourceFiles: program.getSourceFiles,
                isSourceFileFromExternalLibrary,
                writeFile: writeFileCallback || (
                    (fileName, data, writeByteOrderMark, onError, sourceFiles) => host.writeFile(fileName, data, writeByteOrderMark, onError, sourceFiles)),
                isEmitBlocked,
            };
        }

        function isSourceFileFromExternalLibrary(file: SourceFile): boolean {
            return sourceFilesFoundSearchingNodeModules[file.path];
        }

        function getDiagnosticsProducingTypeChecker() {
            return diagnosticsProducingTypeChecker || (diagnosticsProducingTypeChecker = createTypeChecker(program, /*produceDiagnostics:*/ true));
        }

        function dropDiagnosticsProducingTypeChecker() {
            diagnosticsProducingTypeChecker = undefined;
        }

        function getTypeChecker() {
            return noDiagnosticsTypeChecker || (noDiagnosticsTypeChecker = createTypeChecker(program, /*produceDiagnostics:*/ false));
        }

        function emit(sourceFile?: SourceFile, writeFileCallback?: WriteFileCallback, cancellationToken?: CancellationToken, emitOnlyDtsFiles?: boolean): EmitResult {
            return runWithCancellationToken(() => emitWorker(program, sourceFile, writeFileCallback, cancellationToken, emitOnlyDtsFiles));
        }

        function isEmitBlocked(emitFileName: string): boolean {
            return hasEmitBlockingDiagnostics.contains(toPath(emitFileName, currentDirectory, getCanonicalFileName));
        }

        function emitWorker(program: Program, sourceFile: SourceFile, writeFileCallback: WriteFileCallback, cancellationToken: CancellationToken, emitOnlyDtsFiles?: boolean): EmitResult {
            let declarationDiagnostics: Diagnostic[] = [];

            if (options.noEmit) {
                return { diagnostics: declarationDiagnostics, sourceMaps: undefined, emittedFiles: undefined, emitSkipped: true };
            }

            // If the noEmitOnError flag is set, then check if we have any errors so far.  If so,
            // immediately bail out.  Note that we pass 'undefined' for 'sourceFile' so that we
            // get any preEmit diagnostics, not just the ones
            if (options.noEmitOnError) {
                const diagnostics = program.getOptionsDiagnostics(cancellationToken).concat(
                    program.getSyntacticDiagnostics(sourceFile, cancellationToken),
                    program.getGlobalDiagnostics(cancellationToken),
                    program.getSemanticDiagnostics(sourceFile, cancellationToken));

                if (diagnostics.length === 0 && program.getCompilerOptions().declaration) {
                    declarationDiagnostics = program.getDeclarationDiagnostics(/*sourceFile*/ undefined, cancellationToken);
                }

                if (diagnostics.length > 0 || declarationDiagnostics.length > 0) {
                    return {
                        diagnostics: concatenate(diagnostics, declarationDiagnostics),
                        sourceMaps: undefined,
                        emittedFiles: undefined,
                        emitSkipped: true
                    };
                }
            }

            // Create the emit resolver outside of the "emitTime" tracking code below.  That way
            // any cost associated with it (like type checking) are appropriate associated with
            // the type-checking counter.
            //
            // If the -out option is specified, we should not pass the source file to getEmitResolver.
            // This is because in the -out scenario all files need to be emitted, and therefore all
            // files need to be type checked. And the way to specify that all files need to be type
            // checked is to not pass the file to getEmitResolver.
            const emitResolver = getDiagnosticsProducingTypeChecker().getEmitResolver((options.outFile || options.out) ? undefined : sourceFile);

            performance.mark("beforeEmit");

            const emitResult = emitFiles(
                emitResolver,
                getEmitHost(writeFileCallback),
                sourceFile,
                emitOnlyDtsFiles);

            performance.mark("afterEmit");
            performance.measure("Emit", "beforeEmit", "afterEmit");
            return emitResult;
        }

        function getSourceFile(fileName: string): SourceFile {
            return getSourceFileByPath(toPath(fileName, currentDirectory, getCanonicalFileName));
        }

        function getSourceFileByPath(path: Path): SourceFile {
            return filesByName.get(path);
        }

        function getDiagnosticsHelper(
            sourceFile: SourceFile,
            getDiagnostics: (sourceFile: SourceFile, cancellationToken: CancellationToken) => Diagnostic[],
            cancellationToken: CancellationToken): Diagnostic[] {
            if (sourceFile) {
                return getDiagnostics(sourceFile, cancellationToken);
            }

            const allDiagnostics: Diagnostic[] = [];
            forEach(program.getSourceFiles(), sourceFile => {
                if (cancellationToken) {
                    cancellationToken.throwIfCancellationRequested();
                }
                addRange(allDiagnostics, getDiagnostics(sourceFile, cancellationToken));
            });

            return sortAndDeduplicateDiagnostics(allDiagnostics);
        }

        function getSyntacticDiagnostics(sourceFile: SourceFile, cancellationToken: CancellationToken): Diagnostic[] {
            return getDiagnosticsHelper(sourceFile, getSyntacticDiagnosticsForFile, cancellationToken);
        }

        function getSemanticDiagnostics(sourceFile: SourceFile, cancellationToken: CancellationToken): Diagnostic[] {
            return getDiagnosticsHelper(sourceFile, getSemanticDiagnosticsForFile, cancellationToken);
        }

        function getDeclarationDiagnostics(sourceFile: SourceFile, cancellationToken: CancellationToken): Diagnostic[] {
            const options = program.getCompilerOptions();
            // collect diagnostics from the program only once if either no source file was specified or out/outFile is set (bundled emit)
            if (!sourceFile || options.out || options.outFile) {
                return getDeclarationDiagnosticsWorker(sourceFile, cancellationToken);
            }
            else {
                return getDiagnosticsHelper(sourceFile, getDeclarationDiagnosticsForFile, cancellationToken);
            }
        }

        function getSyntacticDiagnosticsForFile(sourceFile: SourceFile): Diagnostic[] {
            // For JavaScript files, we report semantic errors for using TypeScript-only
            // constructs from within a JavaScript file as syntactic errors.
            if (isSourceFileJavaScript(sourceFile)) {
                if (!sourceFile.additionalSyntacticDiagnostics) {
                    sourceFile.additionalSyntacticDiagnostics = getJavaScriptSyntacticDiagnosticsForFile(sourceFile);
                }
                return concatenate(sourceFile.additionalSyntacticDiagnostics, sourceFile.parseDiagnostics);
            }
            return sourceFile.parseDiagnostics;
        }

        function runWithCancellationToken<T>(func: () => T): T {
            try {
                return func();
            }
            catch (e) {
                if (e instanceof OperationCanceledException) {
                    // We were canceled while performing the operation.  Because our type checker
                    // might be a bad state, we need to throw it away.
                    //
                    // Note: we are overly aggressive here.  We do not actually *have* to throw away
                    // the "noDiagnosticsTypeChecker".  However, for simplicity, i'd like to keep
                    // the lifetimes of these two TypeCheckers the same.  Also, we generally only
                    // cancel when the user has made a change anyways.  And, in that case, we (the
                    // program instance) will get thrown away anyways.  So trying to keep one of
                    // these type checkers alive doesn't serve much purpose.
                    noDiagnosticsTypeChecker = undefined;
                    diagnosticsProducingTypeChecker = undefined;
                }

                throw e;
            }
        }

        function getSemanticDiagnosticsForFile(sourceFile: SourceFile, cancellationToken: CancellationToken): Diagnostic[] {
            return runWithCancellationToken(() => {
                const typeChecker = getDiagnosticsProducingTypeChecker();

                Debug.assert(!!sourceFile.bindDiagnostics);
                const bindDiagnostics = sourceFile.bindDiagnostics;
                // For JavaScript files, we don't want to report semantic errors.
                // Instead, we'll report errors for using TypeScript-only constructs from within a
                // JavaScript file when we get syntactic diagnostics for the file.
                const checkDiagnostics = isSourceFileJavaScript(sourceFile) ? [] : typeChecker.getDiagnostics(sourceFile, cancellationToken);
                const fileProcessingDiagnosticsInFile = fileProcessingDiagnostics.getDiagnostics(sourceFile.fileName);
                const programDiagnosticsInFile = programDiagnostics.getDiagnostics(sourceFile.fileName);

                return bindDiagnostics.concat(checkDiagnostics, fileProcessingDiagnosticsInFile, programDiagnosticsInFile);
            });
        }

        function getJavaScriptSyntacticDiagnosticsForFile(sourceFile: SourceFile): Diagnostic[] {
            return runWithCancellationToken(() => {
                const diagnostics: Diagnostic[] = [];
                let parent: Node = sourceFile;
                walk(sourceFile);

                return diagnostics;

                function walk(node: Node) {
                    // Return directly from the case if the given node doesnt want to visit each child
                    // Otherwise break to visit each child

                    switch (parent.kind) {
                        case SyntaxKind.Parameter:
                        case SyntaxKind.PropertyDeclaration:
                            if ((<ParameterDeclaration | PropertyDeclaration>parent).questionToken === node) {
                                diagnostics.push(createDiagnosticForNode(node, Diagnostics._0_can_only_be_used_in_a_ts_file, "?"));
                                return;
                            }

                        // Pass through
                        case SyntaxKind.MethodDeclaration:
                        case SyntaxKind.MethodSignature:
                        case SyntaxKind.Constructor:
                        case SyntaxKind.GetAccessor:
                        case SyntaxKind.SetAccessor:
                        case SyntaxKind.FunctionExpression:
                        case SyntaxKind.FunctionDeclaration:
                        case SyntaxKind.ArrowFunction:
                        case SyntaxKind.FunctionDeclaration:
                        case SyntaxKind.VariableDeclaration:
                            // type annotation
                            if ((<FunctionLikeDeclaration | VariableDeclaration | ParameterDeclaration | PropertyDeclaration>parent).type === node) {
                                diagnostics.push(createDiagnosticForNode(node, Diagnostics.types_can_only_be_used_in_a_ts_file));
                                return;
                            }
                    }

                    switch (node.kind) {
                        case SyntaxKind.ImportEqualsDeclaration:
                            diagnostics.push(createDiagnosticForNode(node, Diagnostics.import_can_only_be_used_in_a_ts_file));
                            return;
                        case SyntaxKind.ExportAssignment:
                            if ((<ExportAssignment>node).isExportEquals) {
                                diagnostics.push(createDiagnosticForNode(node, Diagnostics.export_can_only_be_used_in_a_ts_file));
                                return;
                            }
                            break;
                        case SyntaxKind.HeritageClause:
                            let heritageClause = <HeritageClause>node;
                            if (heritageClause.token === SyntaxKind.ImplementsKeyword) {
                                diagnostics.push(createDiagnosticForNode(node, Diagnostics.implements_clauses_can_only_be_used_in_a_ts_file));
                                return;
                            }
                            break;
                        case SyntaxKind.InterfaceDeclaration:
                            diagnostics.push(createDiagnosticForNode(node, Diagnostics.interface_declarations_can_only_be_used_in_a_ts_file));
                            return;
                        case SyntaxKind.ModuleDeclaration:
                            diagnostics.push(createDiagnosticForNode(node, Diagnostics.module_declarations_can_only_be_used_in_a_ts_file));
                            return;
                        case SyntaxKind.TypeAliasDeclaration:
                            diagnostics.push(createDiagnosticForNode(node, Diagnostics.type_aliases_can_only_be_used_in_a_ts_file));
                            return;
                        case SyntaxKind.EnumDeclaration:
                            diagnostics.push(createDiagnosticForNode(node, Diagnostics.enum_declarations_can_only_be_used_in_a_ts_file));
                            return;
                        case SyntaxKind.TypeAssertionExpression:
                            let typeAssertionExpression = <TypeAssertion>node;
                            diagnostics.push(createDiagnosticForNode(typeAssertionExpression.type, Diagnostics.type_assertion_expressions_can_only_be_used_in_a_ts_file));
                            return;
                    }

                    const prevParent = parent;
                    parent = node;
                    forEachChild(node, walk, walkArray);
                    parent = prevParent;
                }

                function walkArray(nodes: NodeArray<Node>) {
                    if (parent.decorators === nodes && !options.experimentalDecorators) {
                        diagnostics.push(createDiagnosticForNode(parent, Diagnostics.Experimental_support_for_decorators_is_a_feature_that_is_subject_to_change_in_a_future_release_Set_the_experimentalDecorators_option_to_remove_this_warning));
                    }

                    switch (parent.kind) {
                        case SyntaxKind.ClassDeclaration:
                        case SyntaxKind.MethodDeclaration:
                        case SyntaxKind.MethodSignature:
                        case SyntaxKind.Constructor:
                        case SyntaxKind.GetAccessor:
                        case SyntaxKind.SetAccessor:
                        case SyntaxKind.FunctionExpression:
                        case SyntaxKind.FunctionDeclaration:
                        case SyntaxKind.ArrowFunction:
                        case SyntaxKind.FunctionDeclaration:
                            // Check type parameters
                            if (nodes === (<ClassDeclaration | FunctionLikeDeclaration>parent).typeParameters) {
                                diagnostics.push(createDiagnosticForNodeArray(nodes, Diagnostics.type_parameter_declarations_can_only_be_used_in_a_ts_file));
                                return;
                            }
                            // pass through
                        case SyntaxKind.VariableStatement:
                            // Check modifiers
                            if (nodes === (<ClassDeclaration | FunctionLikeDeclaration | VariableStatement>parent).modifiers) {
                                return checkModifiers(<NodeArray<Modifier>>nodes, parent.kind === SyntaxKind.VariableStatement);
                            }
                            break;
                        case SyntaxKind.PropertyDeclaration:
                            // Check modifiers of property declaration
                            if (nodes === (<PropertyDeclaration>parent).modifiers) {
                                for (const modifier of <NodeArray<Modifier>>nodes) {
                                    if (modifier.kind !== SyntaxKind.StaticKeyword) {
                                        diagnostics.push(createDiagnosticForNode(modifier, Diagnostics._0_can_only_be_used_in_a_ts_file, tokenToString(modifier.kind)));
                                    }
                                }
                                return;
                            }
                            break;
                        case SyntaxKind.Parameter:
                            // Check modifiers of parameter declaration
                            if (nodes === (<ParameterDeclaration>parent).modifiers) {
                                diagnostics.push(createDiagnosticForNodeArray(nodes, Diagnostics.parameter_modifiers_can_only_be_used_in_a_ts_file));
                                return;
                            }
                            break;
                        case SyntaxKind.CallExpression:
                        case SyntaxKind.NewExpression:
                        case SyntaxKind.ExpressionWithTypeArguments:
                            // Check type arguments
                            if (nodes === (<CallExpression | NewExpression | ExpressionWithTypeArguments>parent).typeArguments) {
                                diagnostics.push(createDiagnosticForNodeArray(nodes, Diagnostics.type_arguments_can_only_be_used_in_a_ts_file));
                                return;
                            }
                            break;
                    }

                    for (const node of nodes) {
                        walk(node);
                    }
                }

                function checkModifiers(modifiers: NodeArray<Modifier>, isConstValid: boolean) {
                    for (const modifier of modifiers) {
                        switch (modifier.kind) {
                            case SyntaxKind.ConstKeyword:
                                if (isConstValid) {
                                    continue;
                                }
                                // Fallthrough to report error
                            case SyntaxKind.PublicKeyword:
                            case SyntaxKind.PrivateKeyword:
                            case SyntaxKind.ProtectedKeyword:
                            case SyntaxKind.ReadonlyKeyword:
                            case SyntaxKind.DeclareKeyword:
                            case SyntaxKind.AbstractKeyword:
                                diagnostics.push(createDiagnosticForNode(modifier, Diagnostics._0_can_only_be_used_in_a_ts_file, tokenToString(modifier.kind)));
                                break;

                            // These are all legal modifiers.
                            case SyntaxKind.StaticKeyword:
                            case SyntaxKind.ExportKeyword:
                            case SyntaxKind.DefaultKeyword:
                        }
                    }
                }

                function createDiagnosticForNodeArray(nodes: NodeArray<Node>, message: DiagnosticMessage, arg0?: string | number, arg1?: string | number, arg2?: string | number): Diagnostic {
                    const start = nodes.pos;
                    return createFileDiagnostic(sourceFile, start, nodes.end - start, message, arg0, arg1, arg2);
                }

                // Since these are syntactic diagnostics, parent might not have been set
                // this means the sourceFile cannot be infered from the node
                function createDiagnosticForNode(node: Node, message: DiagnosticMessage, arg0?: string | number, arg1?: string | number, arg2?: string | number): Diagnostic {
                    return createDiagnosticForNodeInSourceFile(sourceFile, node, message, arg0, arg1, arg2);
                }
            });
        }

        function getDeclarationDiagnosticsWorker(sourceFile: SourceFile, cancellationToken: CancellationToken): Diagnostic[] {
            return runWithCancellationToken(() => {
                const resolver = getDiagnosticsProducingTypeChecker().getEmitResolver(sourceFile, cancellationToken);
                // Don't actually write any files since we're just getting diagnostics.
                return ts.getDeclarationDiagnostics(getEmitHost(noop), resolver, sourceFile);
            });
        }

        function getDeclarationDiagnosticsForFile(sourceFile: SourceFile, cancellationToken: CancellationToken): Diagnostic[] {
            return isDeclarationFile(sourceFile) ? [] : getDeclarationDiagnosticsWorker(sourceFile, cancellationToken);
        }

        function getOptionsDiagnostics(): Diagnostic[] {
            const allDiagnostics: Diagnostic[] = [];
            addRange(allDiagnostics, fileProcessingDiagnostics.getGlobalDiagnostics());
            addRange(allDiagnostics, programDiagnostics.getGlobalDiagnostics());
            return sortAndDeduplicateDiagnostics(allDiagnostics);
        }

        function getGlobalDiagnostics(): Diagnostic[] {
            const allDiagnostics: Diagnostic[] = [];
            addRange(allDiagnostics, getDiagnosticsProducingTypeChecker().getGlobalDiagnostics());
            return sortAndDeduplicateDiagnostics(allDiagnostics);
        }

        function processRootFile(fileName: string, isDefaultLib: boolean) {
            processSourceFile(normalizePath(fileName), isDefaultLib);
        }

        function fileReferenceIsEqualTo(a: FileReference, b: FileReference): boolean {
            return a.fileName === b.fileName;
        }

        function moduleNameIsEqualTo(a: LiteralExpression, b: LiteralExpression): boolean {
            return a.text === b.text;
        }

        function getTextOfLiteral(literal: LiteralExpression): string {
            return literal.text;
        }

        function collectExternalModuleReferences(file: SourceFile): void {
            if (file.imports) {
                return;
            }

            const isJavaScriptFile = isSourceFileJavaScript(file);
            const isExternalModuleFile = isExternalModule(file);
            const isDtsFile = isDeclarationFile(file);

            let imports: LiteralExpression[];
            let moduleAugmentations: LiteralExpression[];
            let ambientModules: string[];

            // If we are importing helpers, we need to add a synthetic reference to resolve the
            // helpers library.
            if (options.importHelpers
                && (options.isolatedModules || isExternalModuleFile)
                && !file.isDeclarationFile) {
                // synthesize 'import "tslib"' declaration
                const externalHelpersModuleReference = <StringLiteral>createSynthesizedNode(SyntaxKind.StringLiteral);
                externalHelpersModuleReference.text = externalHelpersModuleNameText;
                const importDecl = createSynthesizedNode(SyntaxKind.ImportDeclaration);

                importDecl.parent = file;
                externalHelpersModuleReference.parent = importDecl;

                imports = [externalHelpersModuleReference];
            }

            for (const node of file.statements) {
                collectModuleReferences(node, /*inAmbientModule*/ false);
                if (isJavaScriptFile) {
                    collectRequireCalls(node);
                }
            }

            file.imports = imports || emptyArray;
            file.moduleAugmentations = moduleAugmentations || emptyArray;
            file.ambientModuleNames = ambientModules || emptyArray;

            return;

            function collectModuleReferences(node: Node, inAmbientModule: boolean): void {
                switch (node.kind) {
                    case SyntaxKind.ImportDeclaration:
                    case SyntaxKind.ImportEqualsDeclaration:
                    case SyntaxKind.ExportDeclaration:
                        let moduleNameExpr = getExternalModuleName(node);
                        if (!moduleNameExpr || moduleNameExpr.kind !== SyntaxKind.StringLiteral) {
                            break;
                        }
                        if (!(<LiteralExpression>moduleNameExpr).text) {
                            break;
                        }

                        // TypeScript 1.0 spec (April 2014): 12.1.6
                        // An ExternalImportDeclaration in an AmbientExternalModuleDeclaration may reference other external modules
                        // only through top - level external module names. Relative external module names are not permitted.
                        if (!inAmbientModule || !isExternalModuleNameRelative((<LiteralExpression>moduleNameExpr).text)) {
                            (imports || (imports = [])).push(<LiteralExpression>moduleNameExpr);
                        }
                        break;
                    case SyntaxKind.ModuleDeclaration:
                        if (isAmbientModule(<ModuleDeclaration>node) && (inAmbientModule || hasModifier(node, ModifierFlags.Ambient) || isDeclarationFile(file))) {
                            const moduleName = <LiteralExpression>(<ModuleDeclaration>node).name;
                            // Ambient module declarations can be interpreted as augmentations for some existing external modules.
                            // This will happen in two cases:
                            // - if current file is external module then module augmentation is a ambient module declaration defined in the top level scope
                            // - if current file is not external module then module augmentation is an ambient module declaration with non-relative module name
                            //   immediately nested in top level ambient module declaration .
                            if (isExternalModuleFile || (inAmbientModule && !isExternalModuleNameRelative(moduleName.text))) {
                                (moduleAugmentations || (moduleAugmentations = [])).push(moduleName);
                            }
                            else if (!inAmbientModule) {
                                if (isDtsFile) {
                                    // for global .d.ts files record name of ambient module
                                    (ambientModules || (ambientModules = [])).push(moduleName.text);
                                }
                                // An AmbientExternalModuleDeclaration declares an external module.
                                // This type of declaration is permitted only in the global module.
                                // The StringLiteral must specify a top - level external module name.
                                // Relative external module names are not permitted

                                // NOTE: body of ambient module is always a module block, if it exists
                                const body = <ModuleBlock>(<ModuleDeclaration>node).body;
                                if (body) {
                                    for (const statement of body.statements) {
                                        collectModuleReferences(statement, /*inAmbientModule*/ true);
                                    }
                                }
                            }
                        }
                }
            }

            function collectRequireCalls(node: Node): void {
                if (isRequireCall(node, /*checkArgumentIsStringLiteral*/true)) {
                    (imports || (imports = [])).push(<StringLiteral>(<CallExpression>node).arguments[0]);
                }
                else {
                    forEachChild(node, collectRequireCalls);
                }
            }
        }

        function processSourceFile(fileName: string, isDefaultLib: boolean, refFile?: SourceFile, refPos?: number, refEnd?: number) {
            let diagnosticArgument: string[];
            let diagnostic: DiagnosticMessage;
            if (hasExtension(fileName)) {
                if (!options.allowNonTsExtensions && !forEach(supportedExtensions, extension => fileExtensionIs(host.getCanonicalFileName(fileName), extension))) {
                    diagnostic = Diagnostics.File_0_has_unsupported_extension_The_only_supported_extensions_are_1;
                    diagnosticArgument = [fileName, "'" + supportedExtensions.join("', '") + "'"];
                }
                else if (!findSourceFile(fileName, toPath(fileName, currentDirectory, getCanonicalFileName), isDefaultLib, refFile, refPos, refEnd)) {
                    diagnostic = Diagnostics.File_0_not_found;
                    diagnosticArgument = [fileName];
                }
                else if (refFile && host.getCanonicalFileName(fileName) === host.getCanonicalFileName(refFile.fileName)) {
                    diagnostic = Diagnostics.A_file_cannot_have_a_reference_to_itself;
                    diagnosticArgument = [fileName];
                }
            }
            else {
                const nonTsFile: SourceFile = options.allowNonTsExtensions && findSourceFile(fileName, toPath(fileName, currentDirectory, getCanonicalFileName), isDefaultLib, refFile, refPos, refEnd);
                if (!nonTsFile) {
                    if (options.allowNonTsExtensions) {
                        diagnostic = Diagnostics.File_0_not_found;
                        diagnosticArgument = [fileName];
                    }
                    else if (!forEach(supportedExtensions, extension => findSourceFile(fileName + extension, toPath(fileName + extension, currentDirectory, getCanonicalFileName), isDefaultLib, refFile, refPos, refEnd))) {
                        diagnostic = Diagnostics.File_0_not_found;
                        fileName += ".ts";
                        diagnosticArgument = [fileName];
                    }
                }
            }

            if (diagnostic) {
                if (refFile !== undefined && refEnd !== undefined && refPos !== undefined) {
                    fileProcessingDiagnostics.add(createFileDiagnostic(refFile, refPos, refEnd - refPos, diagnostic, ...diagnosticArgument));
                }
                else {
                    fileProcessingDiagnostics.add(createCompilerDiagnostic(diagnostic, ...diagnosticArgument));
                }
            }
        }

        function reportFileNamesDifferOnlyInCasingError(fileName: string, existingFileName: string, refFile: SourceFile, refPos: number, refEnd: number): void {
            if (refFile !== undefined && refPos !== undefined && refEnd !== undefined) {
                fileProcessingDiagnostics.add(createFileDiagnostic(refFile, refPos, refEnd - refPos,
                    Diagnostics.File_name_0_differs_from_already_included_file_name_1_only_in_casing, fileName, existingFileName));
            }
            else {
                fileProcessingDiagnostics.add(createCompilerDiagnostic(Diagnostics.File_name_0_differs_from_already_included_file_name_1_only_in_casing, fileName, existingFileName));
            }
        }

        // Get source file from normalized fileName
        function findSourceFile(fileName: string, path: Path, isDefaultLib: boolean, refFile?: SourceFile, refPos?: number, refEnd?: number): SourceFile {
            if (filesByName.contains(path)) {
                const file = filesByName.get(path);
                // try to check if we've already seen this file but with a different casing in path
                // NOTE: this only makes sense for case-insensitive file systems
                if (file && options.forceConsistentCasingInFileNames && getNormalizedAbsolutePath(file.fileName, currentDirectory) !== getNormalizedAbsolutePath(fileName, currentDirectory)) {
                    reportFileNamesDifferOnlyInCasingError(fileName, file.fileName, refFile, refPos, refEnd);
                }

                // If the file was previously found via a node_modules search, but is now being processed as a root file,
                // then everything it sucks in may also be marked incorrectly, and needs to be checked again.
                if (file && sourceFilesFoundSearchingNodeModules[file.path] && currentNodeModulesDepth == 0) {
                    sourceFilesFoundSearchingNodeModules[file.path] = false;
                    if (!options.noResolve) {
                        processReferencedFiles(file, isDefaultLib);
                        processTypeReferenceDirectives(file);
                    }

                    modulesWithElidedImports[file.path] = false;
                    processImportedModules(file);
                }
                // See if we need to reprocess the imports due to prior skipped imports
                else if (file && modulesWithElidedImports[file.path]) {
                    if (currentNodeModulesDepth < maxNodeModuleJsDepth) {
                        modulesWithElidedImports[file.path] = false;
                        processImportedModules(file);
                    }
                }

                return file;
            }

            // We haven't looked for this file, do so now and cache result
            const file = host.getSourceFile(fileName, options.target, hostErrorMessage => {
                if (refFile !== undefined && refPos !== undefined && refEnd !== undefined) {
                    fileProcessingDiagnostics.add(createFileDiagnostic(refFile, refPos, refEnd - refPos,
                        Diagnostics.Cannot_read_file_0_Colon_1, fileName, hostErrorMessage));
                }
                else {
                    fileProcessingDiagnostics.add(createCompilerDiagnostic(Diagnostics.Cannot_read_file_0_Colon_1, fileName, hostErrorMessage));
                }
            });

            filesByName.set(path, file);
            if (file) {
                sourceFilesFoundSearchingNodeModules[path] = (currentNodeModulesDepth > 0);
                file.path = path;

                if (host.useCaseSensitiveFileNames()) {
                    // for case-sensitive file systems check if we've already seen some file with similar filename ignoring case
                    const existingFile = filesByNameIgnoreCase.get(path);
                    if (existingFile) {
                        reportFileNamesDifferOnlyInCasingError(fileName, existingFile.fileName, refFile, refPos, refEnd);
                    }
                    else {
                        filesByNameIgnoreCase.set(path, file);
                    }
                }

                skipDefaultLib = skipDefaultLib || file.hasNoDefaultLib;

                if (!options.noResolve) {
                    processReferencedFiles(file, isDefaultLib);
                    processTypeReferenceDirectives(file);
                }

                // always process imported modules to record module name resolutions
                processImportedModules(file);

                if (isDefaultLib) {
                    files.unshift(file);
                }
                else {
                    files.push(file);
                }
            }

            return file;
        }

        function processReferencedFiles(file: SourceFile, isDefaultLib: boolean) {
            forEach(file.referencedFiles, ref => {
                const referencedFileName = resolveTripleslashReference(ref.fileName, file.fileName);
                processSourceFile(referencedFileName, isDefaultLib, file, ref.pos, ref.end);
            });
        }

        function processTypeReferenceDirectives(file: SourceFile) {
            // We lower-case all type references because npm automatically lowercases all packages. See GH#9824.
            const typeDirectives = map(file.typeReferenceDirectives, ref => ref.fileName.toLocaleLowerCase());
            const resolutions = resolveTypeReferenceDirectiveNamesWorker(typeDirectives, file.fileName);

            for (let i = 0; i < typeDirectives.length; i++) {
                const ref = file.typeReferenceDirectives[i];
                const resolvedTypeReferenceDirective = resolutions[i];
                // store resolved type directive on the file
                const fileName = ref.fileName.toLocaleLowerCase();
                setResolvedTypeReferenceDirective(file, fileName, resolvedTypeReferenceDirective);
                processTypeReferenceDirective(fileName, resolvedTypeReferenceDirective, file, ref.pos, ref.end);
            }
        }

        function processTypeReferenceDirective(typeReferenceDirective: string, resolvedTypeReferenceDirective: ResolvedTypeReferenceDirective,
            refFile?: SourceFile, refPos?: number, refEnd?: number): void {

            // If we already found this library as a primary reference - nothing to do
            const previousResolution = resolvedTypeReferenceDirectives[typeReferenceDirective];
            if (previousResolution && previousResolution.primary) {
                return;
            }
            let saveResolution = true;
            if (resolvedTypeReferenceDirective) {
                if (resolvedTypeReferenceDirective.primary) {
                    // resolved from the primary path
                    processSourceFile(resolvedTypeReferenceDirective.resolvedFileName, /*isDefaultLib*/ false, refFile, refPos, refEnd);
                }
                else {
                    // If we already resolved to this file, it must have been a secondary reference. Check file contents
                    // for sameness and possibly issue an error
                    if (previousResolution) {
                        const otherFileText = host.readFile(resolvedTypeReferenceDirective.resolvedFileName);
                        if (otherFileText !== getSourceFile(previousResolution.resolvedFileName).text) {
                            fileProcessingDiagnostics.add(createDiagnostic(refFile, refPos, refEnd,
                                Diagnostics.Conflicting_definitions_for_0_found_at_1_and_2_Consider_installing_a_specific_version_of_this_library_to_resolve_the_conflict,
                                typeReferenceDirective,
                                resolvedTypeReferenceDirective.resolvedFileName,
                                previousResolution.resolvedFileName
                            ));
                        }
                        // don't overwrite previous resolution result
                        saveResolution = false;
                    }
                    else {
                        // First resolution of this library
                        processSourceFile(resolvedTypeReferenceDirective.resolvedFileName, /*isDefaultLib*/ false, refFile, refPos, refEnd);
                    }
                }
            }
            else {
                fileProcessingDiagnostics.add(createDiagnostic(refFile, refPos, refEnd, Diagnostics.Cannot_find_type_definition_file_for_0, typeReferenceDirective));
            }

            if (saveResolution) {
                resolvedTypeReferenceDirectives[typeReferenceDirective] = resolvedTypeReferenceDirective;
            }
        }

        function createDiagnostic(refFile: SourceFile, refPos: number, refEnd: number, message: DiagnosticMessage, ...args: any[]): Diagnostic {
            if (refFile === undefined || refPos === undefined || refEnd === undefined) {
                return createCompilerDiagnostic(message, ...args);
            }
            else {
                return createFileDiagnostic(refFile, refPos, refEnd - refPos, message, ...args);
            }
        }

        function getCanonicalFileName(fileName: string): string {
            return host.getCanonicalFileName(fileName);
        }

        function processImportedModules(file: SourceFile) {
            collectExternalModuleReferences(file);
            if (file.imports.length || file.moduleAugmentations.length) {
                file.resolvedModules = createMap<ResolvedModuleFull>();
                const moduleNames = map(concatenate(file.imports, file.moduleAugmentations), getTextOfLiteral);
                const resolutions = resolveModuleNamesReusingOldState(moduleNames, getNormalizedAbsolutePath(file.fileName, currentDirectory), file);
                Debug.assert(resolutions.length === moduleNames.length);
                for (let i = 0; i < moduleNames.length; i++) {
                    const resolution = resolutions[i];
                    setResolvedModule(file, moduleNames[i], resolution);

                    if (!resolution) {
                        continue;
                    }

                    const isFromNodeModulesSearch = resolution.isExternalLibraryImport;
                    const isJsFileFromNodeModules = isFromNodeModulesSearch && !extensionIsTypeScript(resolution.extension);
                    const resolvedFileName = resolution.resolvedFileName;

                    if (isFromNodeModulesSearch) {
                        currentNodeModulesDepth++;
                    }

                    // add file to program only if:
                    // - resolution was successful
                    // - noResolve is falsy
                    // - module name comes from the list of imports
                    // - it's not a top level JavaScript module that exceeded the search max
                    const elideImport = isJsFileFromNodeModules && currentNodeModulesDepth > maxNodeModuleJsDepth;
                    // Don't add the file if it has a bad extension (e.g. 'tsx' if we don't have '--allowJs')
                    // This may still end up being an untyped module -- the file won't be included but imports will be allowed.
                    const shouldAddFile = resolvedFileName && !getResolutionDiagnostic(options, resolution) && !options.noResolve && i < file.imports.length && !elideImport;

                    if (elideImport) {
                        modulesWithElidedImports[file.path] = true;
                    }
                    else if (shouldAddFile) {
                        const path = toPath(resolvedFileName, currentDirectory, getCanonicalFileName);
                        const pos = skipTrivia(file.text, file.imports[i].pos);
                        findSourceFile(resolvedFileName, path, /*isDefaultLib*/ false, file, pos, file.imports[i].end);
                    }

                    if (isFromNodeModulesSearch) {
                        currentNodeModulesDepth--;
                    }
                }
            }
            else {
                // no imports - drop cached module resolutions
                file.resolvedModules = undefined;
            }
        }

        function computeCommonSourceDirectory(sourceFiles: SourceFile[]): string {
            const fileNames: string[] = [];
            for (const file of sourceFiles) {
                if (!file.isDeclarationFile) {
                    fileNames.push(file.fileName);
                }
            }
            return computeCommonSourceDirectoryOfFilenames(fileNames, currentDirectory, getCanonicalFileName);
        }

        function checkSourceFilesBelongToPath(sourceFiles: SourceFile[], rootDirectory: string): boolean {
            let allFilesBelongToPath = true;
            if (sourceFiles) {
                const absoluteRootDirectoryPath = host.getCanonicalFileName(getNormalizedAbsolutePath(rootDirectory, currentDirectory));

                for (const sourceFile of sourceFiles) {
                    if (!isDeclarationFile(sourceFile)) {
                        const absoluteSourceFilePath = host.getCanonicalFileName(getNormalizedAbsolutePath(sourceFile.fileName, currentDirectory));
                        if (absoluteSourceFilePath.indexOf(absoluteRootDirectoryPath) !== 0) {
                            programDiagnostics.add(createCompilerDiagnostic(Diagnostics.File_0_is_not_under_rootDir_1_rootDir_is_expected_to_contain_all_source_files, sourceFile.fileName, options.rootDir));
                            allFilesBelongToPath = false;
                        }
                    }
                }
            }

            return allFilesBelongToPath;
        }

        function verifyCompilerOptions() {
            if (options.isolatedModules) {
                if (options.declaration) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_with_option_1, "declaration", "isolatedModules"));
                }

                if (options.noEmitOnError) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_with_option_1, "noEmitOnError", "isolatedModules"));
                }

                if (options.out) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_with_option_1, "out", "isolatedModules"));
                }

                if (options.outFile) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_with_option_1, "outFile", "isolatedModules"));
                }
            }

            if (options.inlineSourceMap) {
                if (options.sourceMap) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_with_option_1, "sourceMap", "inlineSourceMap"));
                }
                if (options.mapRoot) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_with_option_1, "mapRoot", "inlineSourceMap"));
                }
            }

            if (options.paths && options.baseUrl === undefined) {
                programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_paths_cannot_be_used_without_specifying_baseUrl_option));
            }

            if (options.paths) {
                for (const key in options.paths) {
                    if (!hasProperty(options.paths, key)) {
                        continue;
                    }
                    if (!hasZeroOrOneAsteriskCharacter(key)) {
                        programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Pattern_0_can_have_at_most_one_Asterisk_character, key));
                    }
                    if (isArray(options.paths[key])) {
                        if (options.paths[key].length === 0) {
                            programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Substitutions_for_pattern_0_shouldn_t_be_an_empty_array, key));
                        }
                        for (const subst of options.paths[key]) {
                            const typeOfSubst = typeof subst;
                            if (typeOfSubst === "string") {
                                if (!hasZeroOrOneAsteriskCharacter(subst)) {
                                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Substitution_0_in_pattern_1_in_can_have_at_most_one_Asterisk_character, subst, key));
                                }
                            }
                            else {
                                programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Substitution_0_for_pattern_1_has_incorrect_type_expected_string_got_2, subst, key, typeOfSubst));
                            }
                        }
                    }
                    else {
                        programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Substitutions_for_pattern_0_should_be_an_array, key));
                    }
                }
            }

            if (!options.sourceMap && !options.inlineSourceMap) {
                if (options.inlineSources) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_can_only_be_used_when_either_option_inlineSourceMap_or_option_sourceMap_is_provided, "inlineSources"));
                }
                if (options.sourceRoot) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_can_only_be_used_when_either_option_inlineSourceMap_or_option_sourceMap_is_provided, "sourceRoot"));
                }
            }

            if (options.out && options.outFile) {
                programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_with_option_1, "out", "outFile"));
            }

            if (options.mapRoot && !options.sourceMap) {
                // Error to specify --mapRoot without --sourcemap
                programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_without_specifying_option_1, "mapRoot", "sourceMap"));
            }

            if (options.declarationDir) {
                if (!options.declaration) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_without_specifying_option_1, "declarationDir", "declaration"));
                }
                if (options.out || options.outFile) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_with_option_1, "declarationDir", options.out ? "out" : "outFile"));
                }
            }

            if (options.lib && options.noLib) {
                programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_with_option_1, "lib", "noLib"));
            }

            if (options.noImplicitUseStrict && options.alwaysStrict) {
                programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_with_option_1, "noImplicitUseStrict", "alwaysStrict"));
            }

            const languageVersion = options.target || ScriptTarget.ES3;
            const outFile = options.outFile || options.out;

            const firstNonAmbientExternalModuleSourceFile = forEach(files, f => isExternalModule(f) && !isDeclarationFile(f) ? f : undefined);
            if (options.isolatedModules) {
                if (options.module === ModuleKind.None && languageVersion < ScriptTarget.ES2015) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_isolatedModules_can_only_be_used_when_either_option_module_is_provided_or_option_target_is_ES2015_or_higher));
                }

                const firstNonExternalModuleSourceFile = forEach(files, f => !isExternalModule(f) && !isDeclarationFile(f) ? f : undefined);
                if (firstNonExternalModuleSourceFile) {
                    const span = getErrorSpanForNode(firstNonExternalModuleSourceFile, firstNonExternalModuleSourceFile);
                    programDiagnostics.add(createFileDiagnostic(firstNonExternalModuleSourceFile, span.start, span.length, Diagnostics.Cannot_compile_namespaces_when_the_isolatedModules_flag_is_provided));
                }
            }
            else if (firstNonAmbientExternalModuleSourceFile && languageVersion < ScriptTarget.ES2015 && options.module === ModuleKind.None) {
                // We cannot use createDiagnosticFromNode because nodes do not have parents yet
                const span = getErrorSpanForNode(firstNonAmbientExternalModuleSourceFile, firstNonAmbientExternalModuleSourceFile.externalModuleIndicator);
                programDiagnostics.add(createFileDiagnostic(firstNonAmbientExternalModuleSourceFile, span.start, span.length, Diagnostics.Cannot_use_imports_exports_or_module_augmentations_when_module_is_none));
            }

            // Cannot specify module gen that isn't amd or system with --out
            if (outFile) {
                if (options.module && !(options.module === ModuleKind.AMD || options.module === ModuleKind.System)) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Only_amd_and_system_modules_are_supported_alongside_0, options.out ? "out" : "outFile"));
                }
                else if (options.module === undefined && firstNonAmbientExternalModuleSourceFile) {
                    const span = getErrorSpanForNode(firstNonAmbientExternalModuleSourceFile, firstNonAmbientExternalModuleSourceFile.externalModuleIndicator);
                    programDiagnostics.add(createFileDiagnostic(firstNonAmbientExternalModuleSourceFile, span.start, span.length, Diagnostics.Cannot_compile_modules_using_option_0_unless_the_module_flag_is_amd_or_system, options.out ? "out" : "outFile"));
                }
            }

            // there has to be common source directory if user specified --outdir || --sourceRoot
            // if user specified --mapRoot, there needs to be common source directory if there would be multiple files being emitted
            if (options.outDir || // there is --outDir specified
                options.sourceRoot || // there is --sourceRoot specified
                options.mapRoot) { // there is --mapRoot specified

                // Precalculate and cache the common source directory
                const dir = getCommonSourceDirectory();

                // If we failed to find a good common directory, but outDir is specified and at least one of our files is on a windows drive/URL/other resource, add a failure
                if (options.outDir && dir === "" && forEach(files, file => getRootLength(file.fileName) > 1)) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Cannot_find_the_common_subdirectory_path_for_the_input_files));
                }
            }

            if (!options.noEmit && options.allowJs && options.declaration) {
                programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_with_option_1, "allowJs", "declaration"));
            }

            if (options.emitDecoratorMetadata &&
                !options.experimentalDecorators) {
                programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_without_specifying_option_1, "emitDecoratorMetadata", "experimentalDecorators"));
            }

            if (options.jsxFactory) {
                if (options.reactNamespace) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_0_cannot_be_specified_with_option_1, "reactNamespace", "jsxFactory"));
                }
                if (!parseIsolatedEntityName(options.jsxFactory, languageVersion)) {
                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Invalid_value_for_jsxFactory_0_is_not_a_valid_identifier_or_qualified_name, options.jsxFactory));
                }
            }
            else if (options.reactNamespace && !isIdentifierText(options.reactNamespace, languageVersion)) {
                programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Invalid_value_for_reactNamespace_0_is_not_a_valid_identifier, options.reactNamespace));
            }

            // If the emit is enabled make sure that every output file is unique and not overwriting any of the input files
            if (!options.noEmit && !options.suppressOutputPathCheck) {
                const emitHost = getEmitHost();
                const emitFilesSeen = createFileMap<boolean>(!host.useCaseSensitiveFileNames() ? key => key.toLocaleLowerCase() : undefined);
                forEachExpectedEmitFile(emitHost, (emitFileNames) => {
                    verifyEmitFilePath(emitFileNames.jsFilePath, emitFilesSeen);
                    verifyEmitFilePath(emitFileNames.declarationFilePath, emitFilesSeen);
                });
            }

            // Verify that all the emit files are unique and don't overwrite input files
            function verifyEmitFilePath(emitFileName: string, emitFilesSeen: FileMap<boolean>) {
                if (emitFileName) {
                    const emitFilePath = toPath(emitFileName, currentDirectory, getCanonicalFileName);
                    // Report error if the output overwrites input file
                    if (filesByName.contains(emitFilePath)) {
                        let chain: DiagnosticMessageChain;
                        if (!options.configFilePath) {
                            // The program is from either an inferred project or an external project
                            chain = chainDiagnosticMessages(/*details*/ undefined, Diagnostics.Adding_a_tsconfig_json_file_will_help_organize_projects_that_contain_both_TypeScript_and_JavaScript_files_Learn_more_at_https_Colon_Slash_Slashaka_ms_Slashtsconfig);
                        }
                        chain = chainDiagnosticMessages(chain, Diagnostics.Cannot_write_file_0_because_it_would_overwrite_input_file, emitFileName);
                        blockEmittingOfFile(emitFileName, createCompilerDiagnosticFromMessageChain(chain));
                    }

                    // Report error if multiple files write into same file
                    if (emitFilesSeen.contains(emitFilePath)) {
                        // Already seen the same emit file - report error
                        blockEmittingOfFile(emitFileName, createCompilerDiagnostic(Diagnostics.Cannot_write_file_0_because_it_would_be_overwritten_by_multiple_input_files, emitFileName));
                    }
                    else {
                        emitFilesSeen.set(emitFilePath, true);
                    }
                }
            }
        }

        function blockEmittingOfFile(emitFileName: string, diag: Diagnostic) {
            hasEmitBlockingDiagnostics.set(toPath(emitFileName, currentDirectory, getCanonicalFileName), true);
            programDiagnostics.add(diag);
        }
    }

    /* @internal */
    /**
     * Returns a DiagnosticMessage if we won't include a resolved module due to its extension.
     * The DiagnosticMessage's parameters are the imported module name, and the filename it resolved to.
     * This returns a diagnostic even if the module will be an untyped module.
     */
    export function getResolutionDiagnostic(options: CompilerOptions, { extension }: ResolvedModuleFull): DiagnosticMessage | undefined {
        switch (extension) {
            case Extension.Ts:
            case Extension.Dts:
                // These are always allowed.
                return undefined;
            case Extension.Tsx:
                return needJsx();
            case Extension.Jsx:
                return needJsx() || needAllowJs();
            case Extension.Js:
                return needAllowJs();
        }

        function needJsx() {
            return options.jsx ? undefined : Diagnostics.Module_0_was_resolved_to_1_but_jsx_is_not_set;
        }
        function needAllowJs() {
            return options.allowJs ? undefined : Diagnostics.Module_0_was_resolved_to_1_but_allowJs_is_not_set;
        }
    }
}