j2n.go

/*
Copyright (c) 2013 Shark Tank. All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

   * Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
   * Redistributions in binary form must reproduce the above
copyright notice, this list of conditions and the following disclaimer
in the documentation and/or other materials provided with the
distribution.
   * Neither the name of Shark Tank nor the names of its
contributors may be used to endorse or promote products derived from
this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

// Edited by Brawaru to use a field type instead of name

// Package j2n allows arbitrary JSON to be marshaled into structs. Any JSON
// fields that are not marshaled directly into the fields of the struct are put
// into a field with type UnknownFields
//
// 	map[string]*json_helpers.RawMessage
//
// This means that fields that are not explicitly named in the struct will
// survive an Unmarshal/Marshal round trip.
//
// To avoid recursive calls to MarshalJSON/UnmarshalJSON, use the following
// pattern:
//
//  type CatData struct {
//  	Name string        `json_helpers:"name"`
//  	Rest UnknownFields `json_helpers:"-"`
//  }
//
//  type Cat struct {
//  	CatData
//  }
//
//  func (c *Cat) UnmarshalJSON(data []byte) error {
//  	return j2n.UnmarshalJSON(data, &c.CatData)
//  }
//
//  func (c Cat) MarshalJSON() ([]byte, error) {
//  	return j2n.MarshalJSON(c.CatData)
//  }
package j2n

import (
	"encoding/json"
	"errors"
	"fmt"
	"reflect"
)

type UnknownFields map[string]*json.RawMessage

var unknownFieldsType = reflect.TypeOf((UnknownFields)(nil))

// UnmarshalJSON parses the JSON-encoded data into the struct pointed to by v.
//
// This behaves exactly like json.Unmarshal, but any extra JSON fields that
// are not explicitly named in the struct are unmarshalled in the 'Overflow'
// field.
//
// The struct v must contain a field 'Overflow' of type
//
//	map[string]*json.RawMessage
//
func UnmarshalJSON(data []byte, v interface{}) error {
	overflow, err := resetOverflowMap(v)
	if err != nil {
		return err
	}

	if err := json.Unmarshal(data, &overflow); err != nil {
		return err
	}

	if err := json.Unmarshal(data, v); err != nil {
		return err
	}

	namedFieldsJSON, err := json.Marshal(v)
	if err != nil {
		return err
	}

	namedFieldsMap := make(UnknownFields)
	if err := json.Unmarshal(namedFieldsJSON, &namedFieldsMap); err != nil {
		return err
	}

	for k := range namedFieldsMap {
		delete(overflow, k)
	}

	return nil
}

// MarshalJSON returns the JSON encoding of v, which must be a struct.
//
// This behaves exactly like json.Marshal, but ensures that any extra fields
// mentioned in v.Overflow are output alongside the explicitly named struct
// fields.
//
// It expects v to contain a field named 'Overflow' of type
//
// 	map[string]*json.RawMessage
//
func MarshalJSON(v interface{}) ([]byte, error) {
	result := make(map[string]*json.RawMessage)

	// Do a round trip of the named fields into a map[string]*json_helpers.RawMessage
	namedFieldsJSON, err := json.Marshal(v)
	if err != nil {
		return nil, err
	}

	err = json.Unmarshal(namedFieldsJSON, &result)
	if err != nil {
		return nil, err
	}

	overflow, err := getOverflowMap(v)
	if err != nil {
		return nil, err
	}

	for k, v := range overflow {
		if _, ok := result[k]; ok {
			errorText := fmt.Sprintf("named field present in overflow: '%s'", k)
			return nil, errors.New(errorText)
		}
		result[k] = v
	}

	resultJSON, err := json.Marshal(result)
	if err != nil {
		return nil, err
	}

	return resultJSON, nil
}

func resetOverflowMap(v interface{}) (map[string]*json.RawMessage, error) {
	if value, err := getOverflowFieldValue(v); err != nil {
		return nil, err
	} else {
		overflow := make(UnknownFields)
		value.Set(reflect.ValueOf(overflow))
		return overflow, nil
	}
}

func getOverflowMap(v interface{}) (UnknownFields, error) {
	if value, err := getOverflowFieldValue(v); err != nil {
		return nil, err
	} else {
		return value.Interface().(UnknownFields), nil
	}
}

func getOverflowFieldValue(v interface{}) (reflect.Value, error) {
	value := reflect.ValueOf(v)

	// Unwrap the pointer if necessary
	if value.Type().Kind() == reflect.Ptr {
		value = value.Elem()
	}

	// Check that we're dealing with a struct
	if value.Type().Kind() != reflect.Struct {
		errText := fmt.Sprintf("expected struct, got %s", value.Type().Kind())
		return reflect.Value{}, errors.New(errText)
	}

	var field reflect.Value
	var fieldIndex = -1
	for i := 0; i < value.NumField(); i++ {
		f := value.Field(i)

		if f.Type() == unknownFieldsType {
			if fieldIndex == -1 {
				field = f
				fieldIndex = i
			} else {
				return reflect.Value{}, errors.New("multiple unknown fields")
			}
		}
	}

	// Check that we actually found the field
	if fieldIndex == -1 {
		return reflect.Value{}, errors.New("field is not defined")
	}

	// And that it has a tag ensuring that it is omitted from the JSON output
	if val, ok := value.Type().Field(fieldIndex).Tag.Lookup("json"); ok {
		if val != "-" {
			return reflect.Value{}, errors.New("unknown fields must be ignored by the standard marshaller (use `json:\"-\"`)")
		}
	}

	return field, nil
}