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// 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) (r 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 rune(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 {
                r = rune(c0&mask2)<<6 | rune(c1&maskx)
                if r <= rune1Max {
                        return RuneError, 1, false
                }
                return r, 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 {
                r = rune(c0&mask3)<<12 | rune(c1&maskx)<<6 | rune(c2&maskx)
                if r <= rune2Max {
                        return RuneError, 1, false
                }
                return r, 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 {
                r = rune(c0&mask4)<<18 | rune(c1&maskx)<<12 | rune(c2&maskx)<<6 | rune(c3&maskx)
                if r <= rune3Max {
                        return RuneError, 1, false
                }
                return r, 4, false
        }

        // error
        return RuneError, 1, false
}

func decodeRuneInStringInternal(s string) (r 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 rune(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 {
                r = rune(c0&mask2)<<6 | rune(c1&maskx)
                if r <= rune1Max {
                        return RuneError, 1, false
                }
                return r, 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 {
                r = rune(c0&mask3)<<12 | rune(c1&maskx)<<6 | rune(c2&maskx)
                if r <= rune2Max {
                        return RuneError, 1, false
                }
                return r, 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 {
                r = rune(c0&mask4)<<18 | rune(c1&maskx)<<12 | rune(c2&maskx)<<6 | rune(c3&maskx)
                if r <= rune3Max {
                        return RuneError, 1, false
                }
                return r, 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) (r rune, size int) {
        r, size, _ = decodeRuneInternal(p)
        return
}

// DecodeRuneInString is like DecodeRune but its input is a string.
func DecodeRuneInString(s string) (r rune, size int) {
        r, 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) (r rune, size int) {
        end := len(p)
        if end == 0 {
                return RuneError, 0
        }
        start := end - 1
        r = rune(p[start])
        if r < RuneSelf {
                return r, 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
        }
        r, size = DecodeRune(p[start:end])
        if start+size != end {
                return RuneError, 1
        }
        return r, size
}

// DecodeLastRuneInString is like DecodeLastRune but its input is a string.
func DecodeLastRuneInString(s string) (r rune, size int) {
        end := len(s)
        if end == 0 {
                return RuneError, 0
        }
        start := end - 1
        r = rune(s[start])
        if r < RuneSelf {
                return r, 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
        }
        r, size = DecodeRuneInString(s[start:end])
        if start+size != end {
                return RuneError, 1
        }
        return r, size
}

// RuneLen returns the number of bytes required to encode the rune.
func RuneLen(r rune) int {
        switch {
        case r <= rune1Max:
                return 1
        case r <= rune2Max:
                return 2
        case r <= rune3Max:
                return 3
        case r <= 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, r rune) int {
        // Negative values are erroneous.  Making it unsigned addresses the problem.
        if uint32(r) <= rune1Max {
                p[0] = byte(r)
                return 1
        }

        if uint32(r) <= rune2Max {
                p[0] = t2 | byte(r>>6)
                p[1] = tx | byte(r)&maskx
                return 2
        }

        if uint32(r) > unicode.MaxRune {
                r = RuneError
        }

        if uint32(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 }

// Valid reports whether p consists entirely of valid UTF-8-encoded runes.
func Valid(p []byte) bool {
        i := 0
        for i < len(p) {
                if p[i] < RuneSelf {
                        i++
                } else {
                        _, size := DecodeRune(p[i:])
                        if size == 1 {
                                // All valid runes of size of 1 (those
                                // below RuneSelf) were handled above.
                                // This must be a RuneError.
                                return false
                        }
                        i += size
                }
        }
        return true
}

// ValidString reports whether s consists entirely of valid UTF-8-encoded runes.
func ValidString(s string) bool {
        for i, r := range s {
                if r == RuneError {
                        // The RuneError value can be an error
                        // sentinel value (if it's size 1) or the same
                        // value encoded properly. Decode it to see if
                        // it's the 1 byte sentinel value.
                        _, size := DecodeRuneInString(s[i:])
                        if size == 1 {
                                return false
                        }
                }
        }
        return true
}