Pretty JSON syntax errors in Go

json.go

package main

import (
	"bytes"
	"encoding/json"
	"io"
	"strconv"
)

type JSONSyntaxError struct {
	Msg    string
	Line   []byte
	LineNo int
	CharNo int
}

func (e *JSONSyntaxError) Error() string {
	return e.Msg + " at line " + strconv.Itoa(e.LineNo) + ", char " + strconv.Itoa(e.CharNo) + "."
}

func newJSONSyntaxError(err *json.SyntaxError, data []byte) *JSONSyntaxError {
	newline := []byte{'\n'}
	start := bytes.LastIndex(data[:err.Offset], newline) + 1
	end := len(data)
	if idx := bytes.Index(data[start:], newline); idx >= 0 {
		end = start + idx
	}
	return &JSONSyntaxError{
		LineNo: bytes.Count(data[:start], newline) + 1,
		CharNo: int(err.Offset) - start - 1,
		Line:   append([]byte{}, data[start:end]...),
		Msg:    err.Error(),
	}
}

func DecodeJSON(data io.Reader, dest interface{}) error {
	buf := &bytes.Buffer{}
	err := json.NewDecoder(io.TeeReader(data, buf)).Decode(dest)
	if syntaxErr, ok := err.(*json.SyntaxError); ok {
		return newJSONSyntaxError(syntaxErr, buf.Bytes())
	}
	return err
}