package main

import (
	"encoding/json"
	"log"
)

type Foo struct {
	ID uint `json:"id"`
}

const jsonStr = `{"id":2,"ID":5}`

func main() {
	a := &Foo{}
	b := make(map[string]interface{})
	json.Unmarshal([]byte(jsonStr), a)
	json.Unmarshal([]byte(jsonStr), &b)

	log.Println(a)
	// output &{5}

	log.Println(b)
	// output map[ID:5 id:2]
}

type structFields struct {
	list      []field
	nameIndex map[string]int
}

// A field represents a single field found in a struct.
type field struct {
	...

        isMatched bool // add a flag when nameIndex matched
}

	if i, ok := fields.nameIndex[string(key)]; ok {
		// Found an exact name match.
		f = &fields.list[i]
+               f.isMatched = true
	} else {
		// Fall back to the expensive case-insensitive
		// linear search.
		for i := range fields.list {
			ff := &fields.list[i]
+			if !ff.isMatched && ff.equalFold(ff.nameBytes, key) {
				f = ff
				break
			}
		}
	}