src/pkg/utf8/utf8.go

// Copyright 2009 The Go 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 utf8 implements functions and constants to support text encoded in
// UTF-8. This package calls a Unicode character a rune for brevity.
package utf8

import "unicode" // only needed for a couple of constants

// Numbers fundamental to the encoding.
const (
	RuneError = unicode.ReplacementChar // the "error" Rune or "replacement character".
	RuneSelf  = 0x80                    // characters below Runeself are represented as themselves in a single byte.
	UTFMax    = 4                       // maximum number of bytes of a UTF-8 encoded Unicode character.
)

const (
	_T1 = 0x00 // 0000 0000
	_Tx = 0x80 // 1000 0000
	_T2 = 0xC0 // 1100 0000
	_T3 = 0xE0 // 1110 0000
	_T4 = 0xF0 // 1111 0000
	_T5 = 0xF8 // 1111 1000

	_Maskx = 0x3F // 0011 1111
	_Mask2 = 0x1F // 0001 1111
	_Mask3 = 0x0F // 0000 1111
	_Mask4 = 0x07 // 0000 0111

	_Rune1Max = 1<<7 - 1
	_Rune2Max = 1<<11 - 1
	_Rune3Max = 1<<16 - 1
	_Rune4Max = 1<<21 - 1
)

func decodeRuneInternal(p []byte) (rune, size int, short bool) {
	n := len(p)
	if n < 1 {
		return RuneError, 0, true
	}
	c0 := p[0]

	// 1-byte, 7-bit sequence?
	if c0 < _Tx {
		return int(c0), 1, false
	}

	// unexpected continuation byte?
	if c0 < _T2 {
		return RuneError, 1, false
	}

	// need first continuation byte
	if n < 2 {
		return RuneError, 1, true
	}
	c1 := p[1]
	if c1 < _Tx || _T2 <= c1 {
		return RuneError, 1, false
	}

	// 2-byte, 11-bit sequence?
	if c0 < _T3 {
		rune = int(c0&_Mask2)<<6 | int(c1&_Maskx)
		if rune <= _Rune1Max {
			return RuneError, 1, false
		}
		return rune, 2, false
	}

	// need second continuation byte
	if n < 3 {
		return RuneError, 1, true
	}
	c2 := p[2]
	if c2 < _Tx || _T2 <= c2 {
		return RuneError, 1, false
	}

	// 3-byte, 16-bit sequence?
	if c0 < _T4 {
		rune = int(c0&_Mask3)<<12 | int(c1&_Maskx)<<6 | int(c2&_Maskx)
		if rune <= _Rune2Max {
			return RuneError, 1, false
		}
		return rune, 3, false
	}

	// need third continuation byte
	if n < 4 {
		return RuneError, 1, true
	}
	c3 := p[3]
	if c3 < _Tx || _T2 <= c3 {
		return RuneError, 1, false
	}

	// 4-byte, 21-bit sequence?
	if c0 < _T5 {
		rune = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
		if rune <= _Rune3Max {
			return RuneError, 1, false
		}
		return rune, 4, false
	}

	// error
	return RuneError, 1, false
}

func decodeRuneInStringInternal(s string) (rune, size int, short bool) {
	n := len(s)
	if n < 1 {
		return RuneError, 0, true
	}
	c0 := s[0]

	// 1-byte, 7-bit sequence?
	if c0 < _Tx {
		return int(c0), 1, false
	}

	// unexpected continuation byte?
	if c0 < _T2 {
		return RuneError, 1, false
	}

	// need first continuation byte
	if n < 2 {
		return RuneError, 1, true
	}
	c1 := s[1]
	if c1 < _Tx || _T2 <= c1 {
		return RuneError, 1, false
	}

	// 2-byte, 11-bit sequence?
	if c0 < _T3 {
		rune = int(c0&_Mask2)<<6 | int(c1&_Maskx)
		if rune <= _Rune1Max {
			return RuneError, 1, false
		}
		return rune, 2, false
	}

	// need second continuation byte
	if n < 3 {
		return RuneError, 1, true
	}
	c2 := s[2]
	if c2 < _Tx || _T2 <= c2 {
		return RuneError, 1, false
	}

	// 3-byte, 16-bit sequence?
	if c0 < _T4 {
		rune = int(c0&_Mask3)<<12 | int(c1&_Maskx)<<6 | int(c2&_Maskx)
		if rune <= _Rune2Max {
			return RuneError, 1, false
		}
		return rune, 3, false
	}

	// need third continuation byte
	if n < 4 {
		return RuneError, 1, true
	}
	c3 := s[3]
	if c3 < _Tx || _T2 <= c3 {
		return RuneError, 1, false
	}

	// 4-byte, 21-bit sequence?
	if c0 < _T5 {
		rune = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
		if rune <= _Rune3Max {
			return RuneError, 1, false
		}
		return rune, 4, false
	}

	// error
	return RuneError, 1, false
}

// FullRune reports whether the bytes in p begin with a full UTF-8 encoding of a rune.
// An invalid encoding is considered a full Rune since it will convert as a width-1 error rune.
func FullRune(p []byte) bool {
	_, _, short := decodeRuneInternal(p)
	return !short
}

// FullRuneInString is like FullRune but its input is a string.
func FullRuneInString(s string) bool {
	_, _, short := decodeRuneInStringInternal(s)
	return !short
}

// DecodeRune unpacks the first UTF-8 encoding in p and returns the rune and its width in bytes.
func DecodeRune(p []byte) (rune, size int) {
	rune, size, _ = decodeRuneInternal(p)
	return
}

// DecodeRuneInString is like DecodeRune but its input is a string.
func DecodeRuneInString(s string) (rune, size int) {
	rune, size, _ = decodeRuneInStringInternal(s)
	return
}

// DecodeLastRune unpacks the last UTF-8 encoding in p
// and returns the rune and its width in bytes.
func DecodeLastRune(p []byte) (rune, size int) {
	end := len(p)
	if end == 0 {
		return RuneError, 0
	}
	start := end - 1
	rune = int(p[start])
	if rune < RuneSelf {
		return rune, 1
	}
	// guard against O(n^2) behavior when traversing
	// backwards through strings with long sequences of
	// invalid UTF-8.
	lim := end - UTFMax
	if lim < 0 {
		lim = 0
	}
	for start--; start >= lim; start-- {
		if RuneStart(p[start]) {
			break
		}
	}
	if start < 0 {
		start = 0
	}
	rune, size = DecodeRune(p[start:end])
	if start+size != end {
		return RuneError, 1
	}
	return rune, size
}

// DecodeLastRuneInString is like DecodeLastRune but its input is a string.
func DecodeLastRuneInString(s string) (rune, size int) {
	end := len(s)
	if end == 0 {
		return RuneError, 0
	}
	start := end - 1
	rune = int(s[start])
	if rune < RuneSelf {
		return rune, 1
	}
	// guard against O(n^2) behavior when traversing
	// backwards through strings with long sequences of
	// invalid UTF-8.
	lim := end - UTFMax
	if lim < 0 {
		lim = 0
	}
	for start--; start >= lim; start-- {
		if RuneStart(s[start]) {
			break
		}
	}
	if start < 0 {
		start = 0
	}
	rune, size = DecodeRuneInString(s[start:end])
	if start+size != end {
		return RuneError, 1
	}
	return rune, size
}

// RuneLen returns the number of bytes required to encode the rune.
func RuneLen(rune int) int {
	switch {
	case rune <= _Rune1Max:
		return 1
	case rune <= _Rune2Max:
		return 2
	case rune <= _Rune3Max:
		return 3
	case rune <= _Rune4Max:
		return 4
	}
	return -1
}

// EncodeRune writes into p (which must be large enough) the UTF-8 encoding of the rune.
// It returns the number of bytes written.
func EncodeRune(p []byte, rune int) int {
	// Negative values are erroneous.  Making it unsigned addresses the problem.
	r := uint(rune)

	if r <= _Rune1Max {
		p[0] = byte(r)
		return 1
	}

	if r <= _Rune2Max {
		p[0] = _T2 | byte(r>>6)
		p[1] = _Tx | byte(r)&_Maskx
		return 2
	}

	if r > unicode.MaxRune {
		r = RuneError
	}

	if r <= _Rune3Max {
		p[0] = _T3 | byte(r>>12)
		p[1] = _Tx | byte(r>>6)&_Maskx
		p[2] = _Tx | byte(r)&_Maskx
		return 3
	}

	p[0] = _T4 | byte(r>>18)
	p[1] = _Tx | byte(r>>12)&_Maskx
	p[2] = _Tx | byte(r>>6)&_Maskx
	p[3] = _Tx | byte(r)&_Maskx
	return 4
}

// RuneCount returns the number of runes in p.  Erroneous and short
// encodings are treated as single runes of width 1 byte.
func RuneCount(p []byte) int {
	i := 0
	var n int
	for n = 0; i < len(p); n++ {
		if p[i] < RuneSelf {
			i++
		} else {
			_, size := DecodeRune(p[i:])
			i += size
		}
	}
	return n
}

// RuneCountInString is like RuneCount but its input is a string.
func RuneCountInString(s string) (n int) {
	for _ = range s {
		n++
	}
	return
}

// RuneStart reports whether the byte could be the first byte of
// an encoded rune.  Second and subsequent bytes always have the top
// two bits set to 10.
func RuneStart(b byte) bool { return b&0xC0 != 0x80 }