// reflectwalk is a package that allows you to "walk" complex structures
// similar to how you may "walk" a filesystem: visiting every element one
// by one and calling callback functions allowing you to handle and manipulate
// those elements.
package reflectwalk

import "reflect"

// PrimitiveWalker implementations are able to handle primitive values
// within complex structures. Primitive values are numbers, strings,
// booleans, funcs, chans.
//
// These primitive values are often members of more complex
// structures (slices, maps, etc.) that are walkable by other interfaces.
type PrimitiveWalker interface {
	Primitive(reflect.Value) error
}

// MapWalker implementations are able to handle individual elements
// found within a map structure.
type MapWalker interface {
	Map(m reflect.Value) error
	MapElem(m, k, v reflect.Value) error
}

// SliceWalker implementations are able to handle slice elements found
// within complex structures.
type SliceWalker interface {
	Slice(reflect.Value) error
	SliceElem(int, reflect.Value) error
}

// StructWalker is an interface that has methods that are called for
// structs when a Walk is done.
type StructWalker interface {
	Struct(reflect.Value) error
	StructField(reflect.StructField, reflect.Value) error
}

// EnterExitWalker implementations are notified before and after
// they walk deeper into complex structures (into struct fields,
// into slice elements, etc.)
type EnterExitWalker interface {
	Enter(Location) error
	Exit(Location) error
}

// PointerWalker implementations are notified when the value they're
// walking is a pointer or not. Pointer is called for _every_ value whether
// it is a pointer or not.
type PointerWalker interface {
	PointerEnter(bool) error
	PointerExit(bool) error
}

// Walk takes an arbitrary value and an interface and traverses the
// value, calling callbacks on the interface if they are supported.
// The interface should implement one or more of the walker interfaces
// in this package, such as PrimitiveWalker, StructWalker, etc.
func Walk(data, walker interface{}) (err error) {
	v := reflect.ValueOf(data)
	ew, ok := walker.(EnterExitWalker)
	if ok {
		err = ew.Enter(WalkLoc)
	}

	if err == nil {
		err = walk(v, walker)
	}

	if ok && err == nil {
		err = ew.Exit(WalkLoc)
	}

	return
}

func walk(v reflect.Value, w interface{}) (err error) {
	// Determine if we're receiving a pointer and if so notify the walker.
	// The logic here is convoluted but very important (tests will fail if
	// almost any part is changed). I will try to explain here.
	//
	// First, we check if the value is an interface, if so, we really need
	// to check the interface's VALUE to see whether it is a pointer (pointers
	// to interfaces are not allowed).
	//
	// Check whether the value is then an interface. If so, then set pointer
	// to true to notify the user.
	//
	// At this time, we also set "v" to be the dereferenced value. This is
	// because once we've unwrapped the pointer we want to use that value.
	pointer := false
	pointerV := v
	if pointerV.Kind() == reflect.Interface {
		pointerV = pointerV.Elem()
	}
	if pointerV.Kind() == reflect.Ptr {
		pointer = true
		v = reflect.Indirect(pointerV)
	}
	if pw, ok := w.(PointerWalker); ok {
		if err = pw.PointerEnter(pointer); err != nil {
			return
		}

		defer func() {
			if err != nil {
				return
			}

			err = pw.PointerExit(pointer)
		}()
	}

	// We preserve the original value here because if it is an interface
	// type, we want to pass that directly into the walkPrimitive, so that
	// we can set it.
	originalV := v
	if v.Kind() == reflect.Interface {
		v = v.Elem()
	}

	k := v.Kind()
	if k >= reflect.Int && k <= reflect.Complex128 {
		k = reflect.Int
	}

	switch k {
	// Primitives
	case reflect.Bool, reflect.Chan, reflect.Func, reflect.Int, reflect.String, reflect.Invalid:
		err = walkPrimitive(originalV, w)
		return
	case reflect.Map:
		err = walkMap(v, w)
		return
	case reflect.Slice:
		err = walkSlice(v, w)
		return
	case reflect.Struct:
		err = walkStruct(v, w)
		return
	default:
		panic("unsupported type: " + k.String())
	}
}

func walkMap(v reflect.Value, w interface{}) error {
	ew, ewok := w.(EnterExitWalker)
	if ewok {
		ew.Enter(Map)
	}

	if mw, ok := w.(MapWalker); ok {
		if err := mw.Map(v); err != nil {
			return err
		}
	}

	for _, k := range v.MapKeys() {
		kv := v.MapIndex(k)

		if mw, ok := w.(MapWalker); ok {
			if err := mw.MapElem(v, k, kv); err != nil {
				return err
			}
		}

		ew, ok := w.(EnterExitWalker)
		if ok {
			ew.Enter(MapKey)
		}

		if err := walk(k, w); err != nil {
			return err
		}

		if ok {
			ew.Exit(MapKey)
			ew.Enter(MapValue)
		}

		if err := walk(kv, w); err != nil {
			return err
		}

		if ok {
			ew.Exit(MapValue)
		}
	}

	if ewok {
		ew.Exit(Map)
	}

	return nil
}

func walkPrimitive(v reflect.Value, w interface{}) error {
	if pw, ok := w.(PrimitiveWalker); ok {
		return pw.Primitive(v)
	}

	return nil
}

func walkSlice(v reflect.Value, w interface{}) (err error) {
	ew, ok := w.(EnterExitWalker)
	if ok {
		ew.Enter(Slice)
	}

	if sw, ok := w.(SliceWalker); ok {
		if err := sw.Slice(v); err != nil {
			return err
		}
	}

	for i := 0; i < v.Len(); i++ {
		elem := v.Index(i)

		if sw, ok := w.(SliceWalker); ok {
			if err := sw.SliceElem(i, elem); err != nil {
				return err
			}
		}

		ew, ok := w.(EnterExitWalker)
		if ok {
			ew.Enter(SliceElem)
		}

		if err := walk(elem, w); err != nil {
			return err
		}

		if ok {
			ew.Exit(SliceElem)
		}
	}

	ew, ok = w.(EnterExitWalker)
	if ok {
		ew.Exit(Slice)
	}

	return nil
}

func walkStruct(v reflect.Value, w interface{}) (err error) {
	ew, ewok := w.(EnterExitWalker)
	if ewok {
		ew.Enter(Struct)
	}

	if sw, ok := w.(StructWalker); ok {
		if err = sw.Struct(v); err != nil {
			return
		}
	}

	vt := v.Type()
	for i := 0; i < vt.NumField(); i++ {
		sf := vt.Field(i)
		f := v.FieldByIndex([]int{i})

		if sw, ok := w.(StructWalker); ok {
			err = sw.StructField(sf, f)
			if err != nil {
				return
			}
		}

		ew, ok := w.(EnterExitWalker)
		if ok {
			ew.Enter(StructField)
		}

		err = walk(f, w)
		if err != nil {
			return
		}

		if ok {
			ew.Exit(StructField)
		}
	}

	if ewok {
		ew.Exit(Struct)
	}

	return nil
}