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// Copyright 2010 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 utf16 implements encoding and decoding of UTF-16 sequences.
package utf16

import "unicode"

const (
        // 0xd800-0xdc00 encodes the high 10 bits of a pair.
        // 0xdc00-0xe000 encodes the low 10 bits of a pair.
        // the value is those 20 bits plus 0x10000.
        surr1 = 0xd800
        surr2 = 0xdc00
        surr3 = 0xe000

        surrSelf = 0x10000
)

// IsSurrogate returns true if the specified Unicode code point
// can appear in a surrogate pair.
func IsSurrogate(r rune) bool {
        return surr1 <= r && r < surr3
}

// DecodeRune returns the UTF-16 decoding of a surrogate pair.
// If the pair is not a valid UTF-16 surrogate pair, DecodeRune returns
// the Unicode replacement code point U+FFFD.
func DecodeRune(r1, r2 rune) rune {
        if surr1 <= r1 && r1 < surr2 && surr2 <= r2 && r2 < surr3 {
                return (rune(r1)-surr1)<<10 | (rune(r2) - surr2) + 0x10000
        }
        return unicode.ReplacementChar
}

// EncodeRune returns the UTF-16 surrogate pair r1, r2 for the given rune.
// If the rune is not a valid Unicode code point or does not need encoding,
// EncodeRune returns U+FFFD, U+FFFD.
func EncodeRune(r rune) (r1, r2 rune) {
        if r < surrSelf || r > unicode.MaxRune || IsSurrogate(r) {
                return unicode.ReplacementChar, unicode.ReplacementChar
        }
        r -= surrSelf
        return surr1 + (r>>10)&0x3ff, surr2 + r&0x3ff
}

// Encode returns the UTF-16 encoding of the Unicode code point sequence s.
func Encode(s []rune) []uint16 {
        n := len(s)
        for _, v := range s {
                if v >= surrSelf {
                        n++
                }
        }

        a := make([]uint16, n)
        n = 0
        for _, v := range s {
                switch {
                case v < 0, surr1 <= v && v < surr3, v > unicode.MaxRune:
                        v = unicode.ReplacementChar
                        fallthrough
                case v < surrSelf:
                        a[n] = uint16(v)
                        n++
                default:
                        r1, r2 := EncodeRune(v)
                        a[n] = uint16(r1)
                        a[n+1] = uint16(r2)
                        n += 2
                }
        }
        return a[0:n]
}

// Decode returns the Unicode code point sequence represented
// by the UTF-16 encoding s.
func Decode(s []uint16) []rune {
        a := make([]rune, len(s))
        n := 0
        for i := 0; i < len(s); i++ {
                switch r := s[i]; {
                case surr1 <= r && r < surr2 && i+1 < len(s) &&
                        surr2 <= s[i+1] && s[i+1] < surr3:
                        // valid surrogate sequence
                        a[n] = DecodeRune(rune(r), rune(s[i+1]))
                        i++
                        n++
                case surr1 <= r && r < surr3:
                        // invalid surrogate sequence
                        a[n] = unicode.ReplacementChar
                        n++
                default:
                        // normal rune
                        a[n] = rune(r)
                        n++
                }
        }
        return a[0:n]
}