// Copyright 2016 Marapongo, Inc. All rights reserved.

package mapper

import (
	"fmt"
	"reflect"
	"strings"

	"github.com/marapongo/mu/pkg/util/contract"
)

type Mapper interface {
	Decode(tree Object, target interface{}) error
	DecodeField(tree Object, ty reflect.Type, key string, target interface{}, optional bool) error
}

func New(customDecoders map[reflect.Type]Decoder) Mapper {
	return &mapper{customDecoders}
}

type mapper struct {
	customDecoders Decoders
}

// Decoder is a func that knows how to decode into particular type.
type Decoder func(m Mapper, tree Object) (interface{}, error)

// Decoders is a map from type to a decoder func that understands how to decode that type.
type Decoders map[reflect.Type]Decoder

// Decode decodes an entire map into a target object, using tag-directed mappings.
func (md *mapper) Decode(tree Object, target interface{}) error {
	vdst := reflect.ValueOf(target)
	contract.Assertf(vdst.Kind() == reflect.Ptr && !vdst.IsNil() && vdst.Elem().CanSet(),
		"Target %v must be a non-nil, settable pointer", vdst.Type())
	vdstType := vdst.Type().Elem()
	contract.Assertf(vdstType.Kind() == reflect.Struct && !vdst.IsNil(),
		"Target %v must be a struct type with `json:\"x\"` tags to direct decoding", vdstType)

	// For each field in the struct that has a `json:"name"`, look it up in the map by that `name`, issuing an error if
	// it is missing or of the wrong type.  For each field that has a tag with omitempty specified, i.e.,
	// `json:"name,omitempty"`, do the same, but permit it to be missing without issuing an error.

	// We need to pass over the struct first to build up the set of infos, so we can dig into embedded structs.
	fldtypes := []reflect.Type{vdstType}
	var fldinfos []reflect.StructField
	for len(fldtypes) > 0 {
		fldtype := fldtypes[0]
		fldtypes = fldtypes[1:]
		for i := 0; i < fldtype.NumField(); i++ {
			fldinfo := fldtype.Field(i)
			if fldinfo.Anonymous {
				// If an embedded struct, push it onto the queue to visit.
				fldtypes = append(fldtypes, fldinfo.Type)
			} else {
				// Otherwise, we will go ahead and consider this field in our decoding.
				fldinfos = append(fldinfos, fldinfo)
			}
		}
	}

	// Now go through, read and parse the "json" tags, and actually perform the decoding for each one.
	flds := make(map[string]bool)
	for _, fldinfo := range fldinfos {
		if tag := fldinfo.Tag.Get("json"); tag != "" {
			var key string    // the JSON key name.
			var optional bool // true if this can be missing.
			var skip bool     // true if we should skip auto-marshaling.

			// Decode the tag.
			tagparts := strings.Split(tag, ",")
			contract.Assertf(len(tagparts) > 0,
				"Expected >0 tagparts on field %v.%v; got %v", vdstType.Name(), fldinfo.Name, len(tagparts))
			key = tagparts[0]
			if key == "-" {
				skip = true // a name of "-" means skip
			}
			for i := 1; i < len(tagparts); i++ {
				switch tagparts[i] {
				case "omitempty":
					optional = true
				case "skip":
					skip = true
				default:
					contract.Failf("Unrecognized tagpart on field %v.%v: %v", vdstType.Name(), fldinfo.Name, tagparts[i])
				}
			}

			// Now use the tag to direct unmarshaling.
			fld := vdst.Elem().FieldByName(fldinfo.Name)
			if !skip {
				if err := md.DecodeField(tree, vdstType, key, fld.Addr().Interface(), optional); err != nil {
					return err
				}
			}

			// Remember this key so we can be sure not to reject it later when checking for unrecognized fields.
			flds[key] = true
		}
	}

	// Afterwards, if there are any unrecognized fields, issue an error.
	for k := range tree {
		if !flds[k] {
			return fmt.Errorf("Unrecognized %v field `%v`", vdstType.Name(), k)
		}
	}

	return nil
}

// decodeField decodes primitive fields.  For fields of complex types, we use custom deserialization.
func (md *mapper) DecodeField(tree Object, ty reflect.Type, key string, target interface{}, optional bool) error {
	vdst := reflect.ValueOf(target)
	contract.Assertf(vdst.Kind() == reflect.Ptr && !vdst.IsNil() && vdst.Elem().CanSet(),
		"Target %v must be a non-nil, settable pointer", vdst.Type())
	if v, has := tree[key]; has {
		// The field exists; okay, try to map it to the right type.
		vsrc := reflect.ValueOf(v)
		// Ensure the source is valid; this is false if the value reflects the zero value.
		if vsrc.IsValid() {
			vdstType := vdst.Type().Elem()

			// So long as the target element is a pointer, we have a pointer to pointer; dig through until we bottom out
			// on the non-pointer type that matches the source.  This assumes the source isn't itself a pointer!
			contract.Assert(vsrc.Type().Kind() != reflect.Ptr)
			for vdstType.Kind() == reflect.Ptr {
				vdst = vdst.Elem()
				vdstType = vdstType.Elem()
				if !vdst.Elem().CanSet() {
					// If the pointer is nil, initialize it so we can set it below.
					contract.Assert(vdst.IsNil())
					vdst.Set(reflect.New(vdstType))
				}
			}

			// Adjust the value if necessary; this handles recursive struct marshaling, interface unboxing, and more.
			var err error
			if vsrc, err = md.adjustValue(vsrc, vdstType, ty, key); err != nil {
				return err
			}

			// Finally, provided everything is kosher, go ahead and store the value; otherwise, issue an error.
			if vsrc.Type().AssignableTo(vdstType) {
				vdst.Elem().Set(vsrc)
				return nil
			} else {
				return ErrWrongType(ty, key, vdstType, vsrc.Type())
			}
		}
	}
	if !optional {
		// The field doesn't exist and yet it is required; issue an error.
		return ErrMissing(ty, key)
	}
	return nil
}

var emptyObject map[string]interface{}

// adjustValue converts if possible to produce the target type.
func (md *mapper) adjustValue(val reflect.Value, to reflect.Type, ty reflect.Type, key string) (reflect.Value, error) {
	for !val.Type().AssignableTo(to) {
		// The source cannot be assigned directly to the destination.  Go through all known conversions.

		if val.Type().ConvertibleTo(to) {
			// A simple conversion exists to make this right.
			val = val.Convert(to)
		} else if to.Kind() == reflect.Ptr && val.Type().AssignableTo(to.Elem()) {
			// If the target is a pointer, turn the target into a pointer.  If it's not addressable, make a copy.
			if val.CanAddr() {
				val = val.Addr()
			} else {
				slot := reflect.New(val.Type().Elem())
				copy := reflect.ValueOf(val.Interface())
				contract.Assert(copy.CanAddr())
				slot.Set(copy)
				val = slot
			}
		} else if val.Kind() == reflect.Interface {
			// It could be that the source is an interface{} with the right element type (or the right element type
			// through a series of successive conversions); go ahead and give it a try.
			val = val.Elem()
		} else if val.Type().Kind() == reflect.Slice && to.Kind() == reflect.Slice {
			// If a slice, everything's ok so long as the elements are compatible.
			arr := reflect.New(to).Elem()
			for i := 0; i < val.Len(); i++ {
				elem := val.Index(i)
				if !elem.Type().AssignableTo(to.Elem()) {
					ekey := fmt.Sprintf("%v[%v]", key, i)
					var err error
					if elem, err = md.adjustValue(elem, to.Elem(), ty, ekey); err != nil {
						return val, err
					}
					if !elem.Type().AssignableTo(to.Elem()) {
						return val, ErrWrongType(ty, ekey, to.Elem(), elem.Type())
					}
				}
				arr = reflect.Append(arr, elem)
			}
			val = arr
		} else if val.Type().Kind() == reflect.Map && to.Kind() == reflect.Map {
			// Similarly, if a map, everything's ok so long as elements and keys are compatible.
			m := reflect.MakeMap(to)
			for _, k := range val.MapKeys() {
				entry := val.MapIndex(k)
				if !k.Type().AssignableTo(to.Key()) {
					kkey := fmt.Sprintf("%v[%v] key", key, k.Interface())
					var err error
					if k, err = md.adjustValue(k, to.Key(), ty, kkey); err != nil {
						return val, err
					}
					if !k.Type().AssignableTo(to.Key()) {
						return val, ErrWrongType(ty, kkey, to.Key(), k.Type())
					}
				}
				if !entry.Type().AssignableTo(to.Elem()) {
					ekey := fmt.Sprintf("%v[%v] value", key, k.Interface())
					var err error
					if entry, err = md.adjustValue(entry, to.Elem(), ty, ekey); err != nil {
						return val, err
					}
					if !entry.Type().AssignableTo(to.Elem()) {
						return val, ErrWrongType(ty, ekey, to.Elem(), entry.Type())
					}
				}
				m.SetMapIndex(k, entry)
			}
			val = m
		} else if val.Type() == reflect.TypeOf(Object{}) || val.Type() == reflect.TypeOf(emptyObject) {
			// The value is an object and needs to be decoded into a value.
			var tree map[string]interface{}
			mi := val.Interface()
			if ma, ok := mi.(Object); ok {
				tree = ma
			} else {
				tree = mi.(map[string]interface{})
			}

			if decode, has := md.customDecoders[to]; has {
				// A custom decoder exists; use it to unmarshal the type.
				target, err := decode(md, tree)
				if err != nil {
					return val, err
				}
				val = reflect.ValueOf(target)
			} else if to.Kind() == reflect.Struct || (to.Kind() == reflect.Ptr && to.Elem().Kind() == reflect.Struct) {
				// If the target is a struct, we can use the built-in decoding logic.
				var target interface{}
				if to.Kind() == reflect.Ptr {
					target = reflect.New(to.Elem()).Interface()
				} else {
					target = reflect.New(to).Interface()
				}
				if err := md.Decode(tree, target); err != nil {
					return val, err
				}
				val = reflect.ValueOf(target).Elem()
			} else {
				return val, fmt.Errorf(
					"Cannot decode Object to type %v; it isn't a struct, and no custom decoder exists", to)
			}
		} else {
			break
		}
	}
	return val, nil
}