converter.go

/*
Copyright 2014 Google Inc. All rights reserved.

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 conversion

import (
	"fmt"
	"reflect"
)

type typePair struct {
	source reflect.Type
	dest   reflect.Type
}

// DebugLogger allows you to get debugging messages if necessary.
type DebugLogger interface {
	Logf(format string, args ...interface{})
}

// Converter knows how to convert one type to another.
type Converter struct {
	// Map from the conversion pair to a function which can
	// do the conversion.
	funcs map[typePair]reflect.Value

	// If non-nil, will be called to print helpful debugging info. Quite verbose.
	Debug DebugLogger

	// NameFunc is called to retrieve the name of a type; this name is used for the
	// purpose of deciding whether two types match or not (i.e., will we attempt to
	// do a conversion). The default returns the go type name.
	NameFunc func(t reflect.Type) string
}

// NewConverter creates a new Converter object.
func NewConverter() *Converter {
	return &Converter{
		funcs:    map[typePair]reflect.Value{},
		NameFunc: func(t reflect.Type) string { return t.Name() },
	}
}

// Scope is passed to conversion funcs to allow them to continue an ongoing conversion.
// If multiple converters exist in the system, Scope will allow you to use the correct one
// from a conversion function--that is, the one your conversion function was called by.
type Scope interface {
	// Call Convert to convert sub-objects. Note that if you call it with your own exact
	// parameters, you'll run out of stack space before anything useful happens.
	Convert(src, dest interface{}, flags FieldMatchingFlags) error

	// SrcTags and DestTags contain the struct tags that src and dest had, respectively.
	// If the enclosing object was not a struct, then these will contain no tags, of course.
	SrcTag() reflect.StructTag
	DestTag() reflect.StructTag

	// Flags returns the flags with which the conversion was started.
	Flags() FieldMatchingFlags

	// Meta returns any information originally passed to Convert.
	Meta() *Meta
}

// Meta is supplied by Scheme, when it calls Convert.
type Meta struct {
	SrcVersion  string
	DestVersion string

	// TODO: If needed, add a user data field here.
}

// scope contains information about an ongoing conversion.
type scope struct {
	converter    *Converter
	meta         *Meta
	flags        FieldMatchingFlags
	srcTagStack  []reflect.StructTag
	destTagStack []reflect.StructTag
}

// push adds a level to the src/dest tag stacks.
func (s *scope) push() {
	s.srcTagStack = append(s.srcTagStack, "")
	s.destTagStack = append(s.destTagStack, "")
}

// pop removes a level to the src/dest tag stacks.
func (s *scope) pop() {
	n := len(s.srcTagStack)
	s.srcTagStack = s.srcTagStack[:n-1]
	s.destTagStack = s.destTagStack[:n-1]
}

func (s *scope) setSrcTag(tag reflect.StructTag) {
	s.srcTagStack[len(s.srcTagStack)-1] = tag
}

func (s *scope) setDestTag(tag reflect.StructTag) {
	s.destTagStack[len(s.destTagStack)-1] = tag
}

// Convert continues a conversion.
func (s *scope) Convert(src, dest interface{}, flags FieldMatchingFlags) error {
	return s.converter.Convert(src, dest, flags, s.meta)
}

// SrcTag returns the tag of the struct containing the current source item, if any.
func (s *scope) SrcTag() reflect.StructTag {
	return s.srcTagStack[len(s.srcTagStack)-1]
}

// DestTag returns the tag of the struct containing the current dest item, if any.
func (s *scope) DestTag() reflect.StructTag {
	return s.destTagStack[len(s.destTagStack)-1]
}

// Flags returns the flags with which the current conversion was started.
func (s *scope) Flags() FieldMatchingFlags {
	return s.flags
}

// Meta returns the meta object that was originally passed to Convert.
func (s *scope) Meta() *Meta {
	return s.meta
}

// Register registers a conversion func with the Converter. conversionFunc must take
// three parameters: a pointer to the input type, a pointer to the output type, and
// a conversion.Scope (which should be used if recursive conversion calls are desired).
// It must return an error.
//
// Example:
// c.Register(func(in *Pod, out *v1beta1.Pod, s Scope) error { ... return nil })
func (c *Converter) Register(conversionFunc interface{}) error {
	fv := reflect.ValueOf(conversionFunc)
	ft := fv.Type()
	if ft.Kind() != reflect.Func {
		return fmt.Errorf("expected func, got: %v", ft)
	}
	if ft.NumIn() != 3 {
		return fmt.Errorf("expected three 'in' params, got: %v", ft)
	}
	if ft.NumOut() != 1 {
		return fmt.Errorf("expected one 'out' param, got: %v", ft)
	}
	if ft.In(0).Kind() != reflect.Ptr {
		return fmt.Errorf("expected pointer arg for 'in' param 0, got: %v", ft)
	}
	if ft.In(1).Kind() != reflect.Ptr {
		return fmt.Errorf("expected pointer arg for 'in' param 1, got: %v", ft)
	}
	scopeType := Scope(nil)
	if e, a := reflect.TypeOf(&scopeType).Elem(), ft.In(2); e != a {
		return fmt.Errorf("expected '%v' arg for 'in' param 2, got '%v' (%v)", e, a, ft)
	}
	var forErrorType error
	// This convolution is necessary, otherwise TypeOf picks up on the fact
	// that forErrorType is nil.
	errorType := reflect.TypeOf(&forErrorType).Elem()
	if ft.Out(0) != errorType {
		return fmt.Errorf("expected error return, got: %v", ft)
	}
	c.funcs[typePair{ft.In(0).Elem(), ft.In(1).Elem()}] = fv
	return nil
}

// FieldMatchingFlags contains a list of ways in which struct fields could be
// copied. These constants may be | combined.
type FieldMatchingFlags int

const (
	// Loop through destination fields, search for matching source
	// field to copy it from. Source fields with no corresponding
	// destination field will be ignored. If SourceToDest is
	// specified, this flag is ignored. If niether is specified,
	// or no flags are passed, this flag is the default.
	DestFromSource FieldMatchingFlags = 0
	// Loop through source fields, search for matching dest field
	// to copy it into. Destination fields with no corresponding
	// source field will be ignored.
	SourceToDest FieldMatchingFlags = 1 << iota
	// Don't treat it as an error if the corresponding source or
	// dest field can't be found.
	IgnoreMissingFields
	// Don't require type names to match.
	AllowDifferentFieldTypeNames
)

// IsSet returns true if the given flag or combination of flags is set.
func (f FieldMatchingFlags) IsSet(flag FieldMatchingFlags) bool {
	return f&flag == flag
}

// Convert will translate src to dest if it knows how. Both must be pointers.
// If no conversion func is registered and the default copying mechanism
// doesn't work on this type pair, an error will be returned.
// Read the comments on the various FieldMatchingFlags constants to understand
// what the 'flags' parameter does.
// 'meta' is given to allow you to pass information to conversion functions,
// it is not used by Convert() other than storing it in the scope.
// Not safe for objects with cyclic references!
func (c *Converter) Convert(src, dest interface{}, flags FieldMatchingFlags, meta *Meta) error {
	dv, sv := reflect.ValueOf(dest), reflect.ValueOf(src)
	if dv.Kind() != reflect.Ptr {
		return fmt.Errorf("Need pointer, but got %#v", dest)
	}
	if sv.Kind() != reflect.Ptr {
		return fmt.Errorf("Need pointer, but got %#v", src)
	}
	dv = dv.Elem()
	sv = sv.Elem()
	if !dv.CanAddr() {
		return fmt.Errorf("Can't write to dest")
	}
	s := &scope{
		converter: c,
		flags:     flags,
		meta:      meta,
	}
	s.push() // Easy way to make SrcTag and DestTag never fail
	return c.convert(sv, dv, s)
}

// convert recursively copies sv into dv, calling an appropriate conversion function if
// one is registered.
func (c *Converter) convert(sv, dv reflect.Value, scope *scope) error {
	dt, st := dv.Type(), sv.Type()
	if fv, ok := c.funcs[typePair{st, dt}]; ok {
		if c.Debug != nil {
			c.Debug.Logf("Calling custom conversion of '%v' to '%v'", st, dt)
		}
		args := []reflect.Value{sv.Addr(), dv.Addr(), reflect.ValueOf(scope)}
		ret := fv.Call(args)[0].Interface()
		// This convolution is necssary because nil interfaces won't convert
		// to errors.
		if ret == nil {
			return nil
		}
		return ret.(error)
	}

	if !scope.flags.IsSet(AllowDifferentFieldTypeNames) && c.NameFunc(dt) != c.NameFunc(st) {
		return fmt.Errorf("Can't convert %v to %v because type names don't match (%v, %v).", st, dt, c.NameFunc(st), c.NameFunc(dt))
	}

	// This should handle all simple types.
	if st.AssignableTo(dt) {
		dv.Set(sv)
		return nil
	}
	if st.ConvertibleTo(dt) {
		dv.Set(sv.Convert(dt))
		return nil
	}

	if c.Debug != nil {
		c.Debug.Logf("Trying to convert '%v' to '%v'", st, dt)
	}

	scope.push()
	defer scope.pop()

	switch dv.Kind() {
	case reflect.Struct:
		listType := dt
		if scope.flags.IsSet(SourceToDest) {
			listType = st
		}
		for i := 0; i < listType.NumField(); i++ {
			f := listType.Field(i)
			df := dv.FieldByName(f.Name)
			sf := sv.FieldByName(f.Name)
			if sf.IsValid() {
				// No need to check error, since we know it's valid.
				field, _ := st.FieldByName(f.Name)
				scope.setSrcTag(field.Tag)
			}
			if df.IsValid() {
				field, _ := dt.FieldByName(f.Name)
				scope.setDestTag(field.Tag)
			}
			// TODO: set top level of scope.src/destTagStack with these field tags here.
			if !df.IsValid() || !sf.IsValid() {
				switch {
				case scope.flags.IsSet(IgnoreMissingFields):
					// No error.
				case scope.flags.IsSet(SourceToDest):
					return fmt.Errorf("%v not present in dest (%v to %v)", f.Name, st, dt)
				default:
					return fmt.Errorf("%v not present in src (%v to %v)", f.Name, st, dt)
				}
				continue
			}
			if err := c.convert(sf, df, scope); err != nil {
				return err
			}
		}
	case reflect.Slice:
		if sv.IsNil() {
			// Don't make a zero-length slice.
			dv.Set(reflect.Zero(dt))
			return nil
		}
		dv.Set(reflect.MakeSlice(dt, sv.Len(), sv.Cap()))
		for i := 0; i < sv.Len(); i++ {
			if err := c.convert(sv.Index(i), dv.Index(i), scope); err != nil {
				return err
			}
		}
	case reflect.Ptr:
		if sv.IsNil() {
			// Don't copy a nil ptr!
			dv.Set(reflect.Zero(dt))
			return nil
		}
		dv.Set(reflect.New(dt.Elem()))
		return c.convert(sv.Elem(), dv.Elem(), scope)
	case reflect.Map:
		if sv.IsNil() {
			// Don't copy a nil ptr!
			dv.Set(reflect.Zero(dt))
			return nil
		}
		dv.Set(reflect.MakeMap(dt))
		for _, sk := range sv.MapKeys() {
			dk := reflect.New(dt.Key()).Elem()
			if err := c.convert(sk, dk, scope); err != nil {
				return err
			}
			dkv := reflect.New(dt.Elem()).Elem()
			if err := c.convert(sv.MapIndex(sk), dkv, scope); err != nil {
				return err
			}
			dv.SetMapIndex(dk, dkv)
		}
	default:
		return fmt.Errorf("Couldn't copy '%v' into '%v'", st, dt)
	}
	return nil
}