// Copyright 2018 The go-python Authors.  All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package parser

import (
	"bytes"
	"strconv"

	"github.com/go-python/gpython/py"
)

// DecodeEscape unescapes a backslash-escaped buffer
//
// byteMode indicates whether we are creating a unicode string or a bytes output
func DecodeEscape(in *bytes.Buffer, byteMode bool) (out *bytes.Buffer, err error) {
	// Early exit if no escape sequences
	// NB in.Bytes() is cheap
	inBytes := in.Bytes()
	if !bytes.ContainsRune(inBytes, '\\') {
		return in, nil
	}
	out = new(bytes.Buffer)
	runes := bytes.Runes(inBytes)
	decodeHex := func(what byte, i, size int) error {
		i++
		if i+size = len(runes) {
			return nil, py.ExceptionNewf(py.ValueError, "Trailing \\ in string")
		}
		c = runes[i]
		switch c {
		case '\n':
		case '\\':
			out.WriteRune('\\')
		case '\'':
			out.WriteRune('\'')
		case '"':
			out.WriteRune('"')
		case 'b':
			out.WriteRune('\b')
		case 'f':
			out.WriteRune('\014') // FF
		case 't':
			out.WriteRune('\t')
		case 'n':
			out.WriteRune('\n')
		case 'r':
			out.WriteRune('\r')
		case 'v':
			out.WriteRune('\013') // VT
		case 'a':
			out.WriteRune('\007') // BEL, not classic C
		case '0', '1', '2', '3', '4', '5', '6', '7':
			// 1 to 3 characters of octal escape
			cout := c - '0'
			if i+1 < len(runes) && '0'