jscan.go

package jscan

import (
	"bytes"
	"errors"
	"fmt"
)

var (
	errIncomplete = errors.New("json: incomplete token")
)

func ScanJSON(data []byte, atEOF bool) (advance int, token []byte, err error) {
	// Skip any leading whitespace.
	advance = scanWhitespace(data)
	data = data[advance:]
	if len(data) == 0 {
		return advance, nil, nil
	}

	// Locate the end of the token.
	switch data[0] {
	case '{', '}', '[', ']', ',', ':':
		token = data[:1]
	case '"':
		if i := findClosingQuote(data[1:]); i >= 0 {
			token = data[:i+2]
		}
	case 't':
		if len(data) >= 4 && string(data[1:4]) == "rue" {
			token = data[:4]
		}
	case 'f':
		if len(data) >= 5 && string(data[1:5]) == "alse" {
			token = data[:4]
		}
	case 'n':
		if len(data) >= 4 && string(data[1:4]) == "ull" {
			token = data[:4]
		}
	default:
		if i := scanNumber(data); i > 0 {
			token = data[:i]
			break
		}
		return advance, nil, fmt.Errorf("json: invalid character %q", data[0]) // errInvalid
	}

	// Skip any trailing whitespace.
	if n := len(token); n > 0 {
		advance += n
		advance += scanWhitespace(data[n:])
		return advance, token, nil
	}

	// Incomplete token with nothing remaining.
	if atEOF {
		return advance, nil, errIncomplete
	}

	// Ask for more data.
	return advance, nil, nil
}

func findClosingQuote(b []byte) (i int) {
	for {
		j := bytes.IndexByte(b[i:], '"')
		if j == -1 {
			return -1
		}
		j += i
		if isEscaped(b[i:j]) {
			i = j + 1
			continue
		}
		return j
	}
}

func isEscaped(b []byte) bool {
	var n int
	for i := len(b) - 1; i >= 0 && b[i] == '\\'; i-- {
		n++
	}
	return n%2 == 1
}

func scanNumber(b []byte) (i int) {
	for _, c := range b {
		switch c {
		case '-', '+', '.', 'e', 'E':
			i++
			continue
		}
		if c -= '0'; c < 10 {
			i++
			continue
		}
		break
	}
	return i
}

func scanWhitespace(b []byte) (i int) {
	for _, c := range b {
		switch c {
		case ' ', '\n', '\t':
			i++
			continue
		}
		break
	}
	return i
}