You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: mapstructure.go
    path of file: ./repos/goldfish/vendor/github.com/mitchellh/mapstructure
    
    the code of the file until where you have to start completion: // Package mapstructure exposes functionality to convert an arbitrary
// map[string]interface{} into a native Go structure.
//
// The Go structure can be arbitrarily complex, containing slices,
// other structs, etc. and the decoder will properly decode nested
// maps and so on into the proper structures in the native Go struct.
// See the examples to see what the decoder is capable of.
package mapstructure

import (
	"encoding/json"
	"errors"
	"fmt"
	"reflect"
	"sort"
	"strconv"
	"strings"
)

// DecodeHookFunc is the callback function that can be used for
// data transformations. See "DecodeHook" in the DecoderConfig
// struct.
//
// The type should be DecodeHookFuncType or DecodeHookFuncKind.
// Either is accepted. Types are a superset of Kinds (Types can return
// Kinds) and are generally a richer thing to use, but Kinds are simpler
// if you only need those.
//
// The reason DecodeHookFunc is multi-typed is for backwards compatibility:
// we started with Kinds and then realized Types were the better solution,
// but have a promise to not break backwards compat so we now support
// both.
type DecodeHookFunc interface{}

// DecodeHookFuncType is a DecodeHookFunc which has complete information about
// the source and target types.
type DecodeHookFuncType func(reflect.Type, reflect.Type, interface{}) (interface{}, error)

// DecodeHookFuncKind is a DecodeHookFunc which knows only the Kinds of the
// source and target types.
type DecodeHookFuncKind func(reflect.Kind, reflect.Kind, interface{}) (interface{}, error)

// DecoderConfig is the configuration that is used to create a new decoder
// and allows customization of various aspects of decoding.
type DecoderConfig struct {
	// DecodeHook, if set, will be called before any decoding and any
	// typ
    
func (d *Decoder) decodeStruct(name string, data interface{}, val reflect.Value) error {
	dataVal := reflect.Indirect(reflect.ValueOf(data))

	// If the type of the value to write to and the data match directly,
	// then we just set it directly instead of recursing into the structure.
	if dataVal.Type() == val.Type() {
		val.Set(dataVal)
		return nil
	}

	dataValKind := dataVal.Kind()
	if dataValKind != reflect.Map {
		return fmt.Errorf("'%s' expected a map, got '%s'", name, dataValKind)
	}

	dataValType := dataVal.Type()
	if kind := dataValType.Key().Kind(); kind != reflect.String && kind != reflect.Interface {
		return fmt.Errorf(
			"'%s' needs a map with string keys, has '%s' keys",
			name, dataValType.Key().Kind())
	}

	dataValKeys := make(map[reflect.Value]struct{})
	dataValKeysUnused := make(map[interface{}]struct{})
	for _, dataValKey := range dataVal.MapKeys() {
		dataValKeys[dataValKey] = struct{}{}
		dataValKeysUnused[dataValKey.Interface()] = struct{}{}
	}

	errors := make([]string, 0)

	// This slice will keep track of all the structs we'll be decoding.
	// There can be more than one struct if there are embedded structs
	// that are squashed.
	structs := make([]reflect.Value, 1, 5)
	structs[0] = val

	// Compile the list of all the fields that we're going to be decoding
	// from all the structs.
	fields := make(map[*reflect.StructField]reflect.Value)
	for len(structs) > 0 {
		structVal := structs[0]
		structs = structs[1:]

		structType := structVal.Type()

		for i := 0; i < structType.NumField(); i++ {
			fieldType := structType.Field(i)
			fieldKind := fieldType.Type.Kind()

			// If "squash" is specified in the tag, we squash the field down.
			squash := false
			tagParts := strings.Split(fieldType.Tag.Get(d.config.TagName), ",")
			for _, tag := range tagParts[1:] {
				if tag == "squash" {
					squash = true
					break
				}
			}

			if squash {
				if fieldKind != reflect.Struct {
					errors = appendErrors(errors,
						fmt.Errorf("%s: unsupported type for squash: %s", fieldType.Name, fieldKind))
				} else {
					structs = append(structs, val.FieldByName(fieldType.Name))
				}
				continue
			}

			// Normal struct field, store it away
			fields[&fieldType] = structVal.Field(i)
		}
	}

	for fieldType, field := range fields {
		fieldName := fieldType.Name

		tagValue := fieldType.Tag.Get(d.config.TagName)
		tagValue = strings.SplitN(tagValue, ",", 2)[0]
		if tagValue != "" {
			fieldName = tagValue
		}

		rawMapKey := reflect.ValueOf(fieldName)
		rawMapVal := dataVal.MapIndex(rawMapKey)
		if !rawMapVal.IsValid() {
			// Do a slower search by iterating over each key and
			// doing case-insensitive search.
			for dataValKey := range dataValKeys {
				mK, ok := dataValKey.Interface().(string)
				if !ok {
					// Not a string key
					continue
				}

				if strings.EqualFold(mK, fieldName) {
					rawMapKey = dataValKey
					rawMapVal = dataVal.MapIndex(dataValKey)
					break
				}
			}

			if !rawMapVal.IsValid() {
				// There was no matching key in the map for the value in
				// the struct. Just ignore.
				continue
			}
		}

		// Delete the key we're using from the unused map so we stop tracking
		delete(dataValKeysUnused, rawMapKey.Interface())

		if !field.IsValid() {
			// This should never happen
			panic("field is not valid")
		}

		// If we can't set the field, then it is unexported or something,
		// and we just continue onwards.
		if !field.CanSet() {
			continue
		}

		// If the name is empty string, then we're at the root, and we
		// don't dot-join the fields.
		if name != "" {
			fieldName = fmt.Sprintf("%s.%s", name, fieldName)
		}

		if err := d.decode(fieldName, rawMapVal.Interface(), field); err != nil {
			errors = appendErrors(errors, err)
		}
	}

	if d.config.ErrorUnused && len(dataValKeysUnused) > 0 {
		keys := make([]string, 0, len(dataValKeysUnused))
		for rawKey := range dataValKeysUnused {
			keys = append(keys, rawKey.(string))
		}
		sort.Strings(keys)

		err := fmt.Errorf("'%s' has invalid keys: %s", name, strings.Join(keys, ", "))
		errors = appendErrors(errors, err)
	}

	if len(errors) > 0 {
		return &Error{errors}
	}

	// Add the unused keys to the list of unused keys if we're tracking metadata
	if d.config.Metadata != nil {
		for rawKey := range dataValKeysUnused {
			key := rawKey.(string)
			if name != "" {
				key = fmt.Sprintf("%s.%s", name, key)
			}

			d.config.Metadata.Unused = append(d.config.Metadata.Unused, key)
		}
	}

	return nil
}