whatcanGOwrong
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// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package utf8string provides an efficient way to index strings by rune rather than by byte.
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package utf8string // import "golang.org/x/exp/utf8string"
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import (
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"errors"
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"unicode/utf8"
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)
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// String wraps a regular string with a small structure that provides more
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// efficient indexing by code point index, as opposed to byte index.
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// Scanning incrementally forwards or backwards is O(1) per index operation
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// (although not as fast a range clause going forwards). Random access is
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// O(N) in the length of the string, but the overhead is less than always
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// scanning from the beginning.
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// If the string is ASCII, random access is O(1).
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// Unlike the built-in string type, String has internal mutable state and
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// is not thread-safe.
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type String struct {
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str string
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numRunes int
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// If width > 0, the rune at runePos starts at bytePos and has the specified width.
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width int
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bytePos int
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runePos int
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nonASCII int // byte index of the first non-ASCII rune.
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}
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// NewString returns a new UTF-8 string with the provided contents.
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func NewString(contents string) *String {
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return new(String).Init(contents)
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}
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// Init initializes an existing String to hold the provided contents.
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// It returns a pointer to the initialized String.
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func (s *String) Init(contents string) *String {
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s.str = contents
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s.bytePos = 0
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s.runePos = 0
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for i := 0; i < len(contents); i++ {
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if contents[i] >= utf8.RuneSelf {
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// Not ASCII.
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s.numRunes = utf8.RuneCountInString(contents)
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_, s.width = utf8.DecodeRuneInString(contents)
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s.nonASCII = i
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return s
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}
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}
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// ASCII is simple. Also, the empty string is ASCII.
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s.numRunes = len(contents)
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s.width = 0
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s.nonASCII = len(contents)
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return s
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}
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// String returns the contents of the String. This method also means the
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// String is directly printable by fmt.Print.
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func (s *String) String() string {
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return s.str
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}
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// RuneCount returns the number of runes (Unicode code points) in the String.
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func (s *String) RuneCount() int {
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return s.numRunes
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}
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// IsASCII returns a boolean indicating whether the String contains only ASCII bytes.
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func (s *String) IsASCII() bool {
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return s.width == 0
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}
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// Slice returns the string sliced at rune positions [i:j].
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func (s *String) Slice(i, j int) string {
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// ASCII is easy. Let the compiler catch the indexing error if there is one.
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if j < s.nonASCII {
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return s.str[i:j]
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}
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if i < 0 || j > s.numRunes || i > j {
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panic(sliceOutOfRange)
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}
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if i == j {
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return ""
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}
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// For non-ASCII, after At(i), bytePos is always the position of the indexed character.
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var low, high int
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switch {
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case i < s.nonASCII:
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low = i
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case i == s.numRunes:
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low = len(s.str)
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default:
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s.At(i)
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low = s.bytePos
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}
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switch {
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case j == s.numRunes:
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high = len(s.str)
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default:
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s.At(j)
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high = s.bytePos
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}
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return s.str[low:high]
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}
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// At returns the rune with index i in the String. The sequence of runes is the same
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// as iterating over the contents with a "for range" clause.
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func (s *String) At(i int) rune {
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// ASCII is easy. Let the compiler catch the indexing error if there is one.
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if i < s.nonASCII {
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return rune(s.str[i])
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}
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// Now we do need to know the index is valid.
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if i < 0 || i >= s.numRunes {
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panic(outOfRange)
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}
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var r rune
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// Five easy common cases: within 1 spot of bytePos/runePos, or the beginning, or the end.
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// With these cases, all scans from beginning or end work in O(1) time per rune.
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switch {
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case i == s.runePos-1: // backing up one rune
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r, s.width = utf8.DecodeLastRuneInString(s.str[0:s.bytePos])
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s.runePos = i
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s.bytePos -= s.width
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return r
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case i == s.runePos+1: // moving ahead one rune
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s.runePos = i
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s.bytePos += s.width
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fallthrough
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case i == s.runePos:
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r, s.width = utf8.DecodeRuneInString(s.str[s.bytePos:])
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return r
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case i == 0: // start of string
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r, s.width = utf8.DecodeRuneInString(s.str)
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s.runePos = 0
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s.bytePos = 0
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return r
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case i == s.numRunes-1: // last rune in string
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r, s.width = utf8.DecodeLastRuneInString(s.str)
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s.runePos = i
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s.bytePos = len(s.str) - s.width
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return r
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}
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// We need to do a linear scan. There are three places to start from:
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// 1) The beginning
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// 2) bytePos/runePos.
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// 3) The end
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// Choose the closest in rune count, scanning backwards if necessary.
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forward := true
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if i < s.runePos {
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// Between beginning and pos. Which is closer?
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// Since both i and runePos are guaranteed >= nonASCII, that's the
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// lowest location we need to start from.
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if i < (s.runePos-s.nonASCII)/2 {
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// Scan forward from beginning
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s.bytePos, s.runePos = s.nonASCII, s.nonASCII
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} else {
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// Scan backwards from where we are
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forward = false
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}
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} else {
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// Between pos and end. Which is closer?
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if i-s.runePos < (s.numRunes-s.runePos)/2 {
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// Scan forward from pos
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} else {
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// Scan backwards from end
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s.bytePos, s.runePos = len(s.str), s.numRunes
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forward = false
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}
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}
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if forward {
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// TODO: Is it much faster to use a range loop for this scan?
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for {
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r, s.width = utf8.DecodeRuneInString(s.str[s.bytePos:])
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if s.runePos == i {
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break
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}
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s.runePos++
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s.bytePos += s.width
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}
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} else {
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for {
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r, s.width = utf8.DecodeLastRuneInString(s.str[0:s.bytePos])
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s.runePos--
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s.bytePos -= s.width
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if s.runePos == i {
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break
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}
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}
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}
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return r
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}
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var outOfRange = errors.New("utf8string: index out of range")
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var sliceOutOfRange = errors.New("utf8string: slice index out of range")
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