converter.go

/*
Copyright 2014 The Kubernetes Authors.

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
}

type NameFunc func(t reflect.Type) string

var DefaultNameFunc = func(t reflect.Type) string { return t.Name() }

// ConversionFunc converts the object a into the object b, reusing arrays or objects
// or pointers if necessary. It should return an error if the object cannot be converted
// or if some data is invalid. If you do not wish a and b to share fields or nested
// objects, you must copy a before calling this function.
type ConversionFunc func(a, b interface{}, scope Scope) error

// 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.
	conversionFuncs          ConversionFuncs
	generatedConversionFuncs ConversionFuncs

	// Set of conversions that should be treated as a no-op
	ignoredUntypedConversions map[typePair]struct{}
}

// NewConverter creates a new Converter object.
// Arg NameFunc is just for backward compatibility.
func NewConverter(NameFunc) *Converter {
	c := &Converter{
		conversionFuncs:           NewConversionFuncs(),
		generatedConversionFuncs:  NewConversionFuncs(),
		ignoredUntypedConversions: make(map[typePair]struct{}),
	}
	c.RegisterUntypedConversionFunc(
		(*[]byte)(nil), (*[]byte)(nil),
		func(a, b interface{}, s Scope) error {
			return Convert_Slice_byte_To_Slice_byte(a.(*[]byte), b.(*[]byte), s)
		},
	)
	return c
}

// WithConversions returns a Converter that is a copy of c but with the additional
// fns merged on top.
func (c *Converter) WithConversions(fns ConversionFuncs) *Converter {
	copied := *c
	copied.conversionFuncs = c.conversionFuncs.Merge(fns)
	return &copied
}

// DefaultMeta returns meta for a given type.
func (c *Converter) DefaultMeta(t reflect.Type) *Meta {
	return &Meta{}
}

// Convert_Slice_byte_To_Slice_byte prevents recursing into every byte
func Convert_Slice_byte_To_Slice_byte(in *[]byte, out *[]byte, s Scope) error {
	if *in == nil {
		*out = nil
		return nil
	}
	*out = make([]byte, len(*in))
	copy(*out, *in)
	return nil
}

// 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{}) error

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

func NewConversionFuncs() ConversionFuncs {
	return ConversionFuncs{
		untyped: make(map[typePair]ConversionFunc),
	}
}

type ConversionFuncs struct {
	untyped map[typePair]ConversionFunc
}

// AddUntyped adds the provided conversion function to the lookup table for the types that are
// supplied as a and b. a and b must be pointers or an error is returned. This method overwrites
// previously defined functions.
func (c ConversionFuncs) AddUntyped(a, b interface{}, fn ConversionFunc) error {
	tA, tB := reflect.TypeOf(a), reflect.TypeOf(b)
	if tA.Kind() != reflect.Pointer {
		return fmt.Errorf("the type %T must be a pointer to register as an untyped conversion", a)
	}
	if tB.Kind() != reflect.Pointer {
		return fmt.Errorf("the type %T must be a pointer to register as an untyped conversion", b)
	}
	c.untyped[typePair{tA, tB}] = fn
	return nil
}

// Merge returns a new ConversionFuncs that contains all conversions from
// both other and c, with other conversions taking precedence.
func (c ConversionFuncs) Merge(other ConversionFuncs) ConversionFuncs {
	merged := NewConversionFuncs()
	for k, v := range c.untyped {
		merged.untyped[k] = v
	}
	for k, v := range other.untyped {
		merged.untyped[k] = v
	}
	return merged
}

// Meta is supplied by Scheme, when it calls Convert.
type Meta struct {
	// Context is an optional field that callers may use to pass info to conversion functions.
	Context interface{}
}

// scope contains information about an ongoing conversion.
type scope struct {
	converter *Converter
	meta      *Meta
}

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

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

// RegisterUntypedConversionFunc registers a function that converts between a and b by passing objects of those
// types to the provided function. The function *must* accept objects of a and b - this machinery will not enforce
// any other guarantee.
func (c *Converter) RegisterUntypedConversionFunc(a, b interface{}, fn ConversionFunc) error {
	return c.conversionFuncs.AddUntyped(a, b, fn)
}

// RegisterGeneratedUntypedConversionFunc registers a function that converts between a and b by passing objects of those
// types to the provided function. The function *must* accept objects of a and b - this machinery will not enforce
// any other guarantee.
func (c *Converter) RegisterGeneratedUntypedConversionFunc(a, b interface{}, fn ConversionFunc) error {
	return c.generatedConversionFuncs.AddUntyped(a, b, fn)
}

// RegisterIgnoredConversion registers a "no-op" for conversion, where any requested
// conversion between from and to is ignored.
func (c *Converter) RegisterIgnoredConversion(from, to interface{}) error {
	typeFrom := reflect.TypeOf(from)
	typeTo := reflect.TypeOf(to)
	if typeFrom.Kind() != reflect.Pointer {
		return fmt.Errorf("expected pointer arg for 'from' param 0, got: %v", typeFrom)
	}
	if typeTo.Kind() != reflect.Pointer {
		return fmt.Errorf("expected pointer arg for 'to' param 1, got: %v", typeTo)
	}
	c.ignoredUntypedConversions[typePair{typeFrom, typeTo}] = struct{}{}
	return nil
}

// 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.
// '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{}, meta *Meta) error {
	pair := typePair{reflect.TypeOf(src), reflect.TypeOf(dest)}
	scope := &scope{
		converter: c,
		meta:      meta,
	}

	// ignore conversions of this type
	if _, ok := c.ignoredUntypedConversions[pair]; ok {
		return nil
	}
	if fn, ok := c.conversionFuncs.untyped[pair]; ok {
		return fn(src, dest, scope)
	}
	if fn, ok := c.generatedConversionFuncs.untyped[pair]; ok {
		return fn(src, dest, scope)
	}

	dv, err := EnforcePtr(dest)
	if err != nil {
		return err
	}
	sv, err := EnforcePtr(src)
	if err != nil {
		return err
	}
	return fmt.Errorf("converting (%s) to (%s): unknown conversion", sv.Type(), dv.Type())
}