/*
 * Minio Cloud Storage, (C) 2016 Minio, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package main

import (
	"bytes"
	"errors"
	"io"
)

// ReadFile - decoded erasure coded file.
func (e erasure) ReadFile(volume, path string, startOffset int64, totalSize int64) (io.Reader, error) {
	var totalLeft = totalSize
	var bufWriter = new(bytes.Buffer)
	for totalLeft > 0 {
		// Figure out the right blockSize as it was encoded before.
		var curBlockSize int64
		if erasureBlockSize < totalLeft {
			curBlockSize = erasureBlockSize
		} else {
			curBlockSize = totalLeft
		}

		// Calculate the current encoded block size.
		curEncBlockSize := getEncodedBlockLen(curBlockSize, e.DataBlocks)

		// Allocate encoded blocks up to storage disks.
		enBlocks := make([][]byte, len(e.storageDisks))

		// Counter to keep success data blocks.
		var successDataBlocksCount = 0
		var noReconstruct bool // Set for no reconstruction.

		// Read from all the disks.
		for index, disk := range e.storageDisks {
			blockIndex := e.distribution[index] - 1
			// Initialize shard slice and fill the data from each parts.
			enBlocks[blockIndex] = make([]byte, curEncBlockSize)
			if disk == nil {
				enBlocks[blockIndex] = nil
			} else {
				var offset = int64(0)
				// Read the necessary blocks.
				_, err := disk.ReadFile(volume, path, offset, enBlocks[blockIndex])
				if err != nil {
					enBlocks[blockIndex] = nil
				}
			}
			// Verify if we have successfully read all the data blocks.
			if blockIndex < e.DataBlocks && enBlocks[blockIndex] != nil {
				successDataBlocksCount++
				// Set when we have all the data blocks and no
				// reconstruction is needed, so that we can avoid
				// erasure reconstruction.
				noReconstruct = successDataBlocksCount == e.DataBlocks
				if noReconstruct {
					// Break out we have read all the data blocks.
					break
				}
			}
		}

		// Check blocks if they are all zero in length, we have corruption return error.
		if checkBlockSize(enBlocks) == 0 {
			return nil, errDataCorrupt
		}

		// Verify if reconstruction is needed, proceed with reconstruction.
		if !noReconstruct {
			err := e.ReedSolomon.Reconstruct(enBlocks)
			if err != nil {
				return nil, err
			}
			// Verify reconstructed blocks (parity).
			ok, err := e.ReedSolomon.Verify(enBlocks)
			if err != nil {
				return nil, err
			}
			if !ok {
				// Blocks cannot be reconstructed, corrupted data.
				err = errors.New("Verification failed after reconstruction, data likely corrupted.")
				return nil, err
			}
		}

		// Get data blocks from encoded blocks.
		dataBlocks := getDataBlocks(enBlocks, e.DataBlocks, int(curBlockSize))

		// Verify if the offset is right for the block, if not move to
		// the next block.
		if startOffset > 0 {
			startOffset = startOffset - int64(len(dataBlocks))
			// Start offset is greater than or equal to zero, skip the dataBlocks.
			if startOffset >= 0 {
				totalLeft = totalLeft - erasureBlockSize
				continue
			}
			// Now get back the remaining offset if startOffset is negative.
			startOffset = startOffset + int64(len(dataBlocks))
		}

		// Copy data blocks.
		_, err := bufWriter.Write(dataBlocks[startOffset:])
		if err != nil {
			return nil, err
		}

		// Reset dataBlocks to relenquish memory.
		dataBlocks = nil

		// Save what's left after reading erasureBlockSize.
		totalLeft = totalLeft - erasureBlockSize
	}
	return bufWriter, nil
}