Source file src/pkg/strings/strings.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.

// A package of simple functions to manipulate strings.
package strings

import (
    "unicode"
    "utf8"
)

// explode splits s into an array of UTF-8 sequences, one per Unicode character (still strings) up to a maximum of n (n < 0 means no limit).
// Invalid UTF-8 sequences become correct encodings of U+FFF8.
func explode(s string, n int) []string {
    if n == 0 {
        return nil
    }
    l := utf8.RuneCountInString(s)
    if n <= 0 || n > l {
        n = l
    }
    a := make([]string, n)
    var size, rune int
    i, cur := 0, 0
    for ; i+1 < n; i++ {
        rune, size = utf8.DecodeRuneInString(s[cur:])
        a[i] = string(rune)
        cur += size
    }
    // add the rest, if there is any
    if cur < len(s) {
        a[i] = s[cur:]
    }
    return a
}

// Count counts the number of non-overlapping instances of sep in s.
func Count(s, sep string) int {
    if sep == "" {
        return utf8.RuneCountInString(s) + 1
    }
    c := sep[0]
    l := len(sep)
    n := 0
    if l == 1 {
        // special case worth making fast
        for i := 0; i < len(s); i++ {
            if s[i] == c {
                n++
            }
        }
        return n
    }
    for i := 0; i+l <= len(s); i++ {
        if s[i] == c && s[i:i+l] == sep {
            n++
            i += l - 1
        }
    }
    return n
}

// Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
func Index(s, sep string) int {
    n := len(sep)
    if n == 0 {
        return 0
    }
    c := sep[0]
    if n == 1 {
        // special case worth making fast
        for i := 0; i < len(s); i++ {
            if s[i] == c {
                return i
            }
        }
        return -1
    }
    // n > 1
    for i := 0; i+n <= len(s); i++ {
        if s[i] == c && s[i:i+n] == sep {
            return i
        }
    }
    return -1
}

// LastIndex returns the index of the last instance of sep in s, or -1 if sep is not present in s.
func LastIndex(s, sep string) int {
    n := len(sep)
    if n == 0 {
        return len(s)
    }
    c := sep[0]
    if n == 1 {
        // special case worth making fast
        for i := len(s) - 1; i >= 0; i-- {
            if s[i] == c {
                return i
            }
        }
        return -1
    }
    // n > 1
    for i := len(s) - n; i >= 0; i-- {
        if s[i] == c && s[i:i+n] == sep {
            return i
        }
    }
    return -1
}

// IndexRune returns the index of the first instance of the Unicode code point
// rune, or -1 if rune is not present in s.
func IndexRune(s string, rune int) int {
    for i, c := range s {
        if c == rune {
            return i
        }
    }
    return -1
}

// IndexAny returns the index of the first instance of any Unicode code point
// from chars in s, or -1 if no Unicode code point from chars is present in s.
func IndexAny(s, chars string) int {
    if len(chars) > 0 {
        for i, c := range s {
            for _, m := range chars {
                if c == m {
                    return i
                }
            }
        }
    }
    return -1
}

// Generic split: splits after each instance of sep,
// including sepSave bytes of sep in the subarrays.
func genSplit(s, sep string, sepSave, n int) []string {
    if n == 0 {
        return nil
    }
    if sep == "" {
        return explode(s, n)
    }
    if n < 0 {
        n = Count(s, sep) + 1
    }
    c := sep[0]
    start := 0
    a := make([]string, n)
    na := 0
    for i := 0; i+len(sep) <= len(s) && na+1 < n; i++ {
        if s[i] == c && (len(sep) == 1 || s[i:i+len(sep)] == sep) {
            a[na] = s[start : i+sepSave]
            na++
            start = i + len(sep)
            i += len(sep) - 1
        }
    }
    a[na] = s[start:]
    return a[0 : na+1]
}

// Split slices s into substrings separated by sep and returns a slice of
// the substrings between those separators.
// If sep is empty, Split splits after each UTF-8 sequence.
// The count determines the number of substrings to return:
//   n > 0: at most n substrings; the last substring will be the unsplit remainder.
//   n == 0: the result is nil (zero substrings)
//   n < 0: all substrings
func Split(s, sep string, n int) []string { return genSplit(s, sep, 0, n) }

// SplitAfter slices s into substrings after each instance of sep and
// returns a slice of those substrings.
// If sep is empty, Split splits after each UTF-8 sequence.
// The count determines the number of substrings to return:
//   n > 0: at most n substrings; the last substring will be the unsplit remainder.
//   n == 0: the result is nil (zero substrings)
//   n < 0: all substrings
func SplitAfter(s, sep string, n int) []string {
    return genSplit(s, sep, len(sep), n)
}

// Fields splits the string s around each instance of one or more consecutive white space
// characters, returning an array of substrings of s or an empty list if s contains only white space.
func Fields(s string) []string {
    return FieldsFunc(s, unicode.IsSpace)
}

// FieldsFunc splits the string s at each run of Unicode code points c satisfying f(c)
// and returns an array of slices of s. If no code points in s satisfy f(c), an empty slice
// is returned.
func FieldsFunc(s string, f func(int) bool) []string {
    // First count the fields.
    n := 0
    inField := false
    for _, rune := range s {
        wasInField := inField
        inField = !f(rune)
        if inField && !wasInField {
            n++
        }
    }

    // Now create them.
    a := make([]string, n)
    na := 0
    fieldStart := -1 // Set to -1 when looking for start of field.
    for i, rune := range s {
        if f(rune) {
            if fieldStart >= 0 {
                a[na] = s[fieldStart:i]
                na++
                fieldStart = -1
            }
        } else if fieldStart == -1 {
            fieldStart = i
        }
    }
    if fieldStart != -1 { // Last field might end at EOF.
        a[na] = s[fieldStart:]
    }
    return a
}

// Join concatenates the elements of a to create a single string.   The separator string
// sep is placed between elements in the resulting string.
func Join(a []string, sep string) string {
    if len(a) == 0 {
        return ""
    }
    if len(a) == 1 {
        return a[0]
    }
    n := len(sep) * (len(a) - 1)
    for i := 0; i < len(a); i++ {
        n += len(a[i])
    }

    b := make([]byte, n)
    bp := 0
    for i := 0; i < len(a); i++ {
        s := a[i]
        for j := 0; j < len(s); j++ {
            b[bp] = s[j]
            bp++
        }
        if i+1 < len(a) {
            s = sep
            for j := 0; j < len(s); j++ {
                b[bp] = s[j]
                bp++
            }
        }
    }
    return string(b)
}

// HasPrefix tests whether the string s begins with prefix.
func HasPrefix(s, prefix string) bool {
    return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
}

// HasSuffix tests whether the string s ends with suffix.
func HasSuffix(s, suffix string) bool {
    return len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix
}

// Map returns a copy of the string s with all its characters modified
// according to the mapping function. If mapping returns a negative value, the character is
// dropped from the string with no replacement.
func Map(mapping func(rune int) int, s string) string {
    // In the worst case, the string can grow when mapped, making
    // things unpleasant.  But it's so rare we barge in assuming it's
    // fine.  It could also shrink but that falls out naturally.
    maxbytes := len(s) // length of b
    nbytes := 0        // number of bytes encoded in b
    b := make([]byte, maxbytes)
    for _, c := range s {
        rune := mapping(c)
        if rune >= 0 {
            wid := 1
            if rune >= utf8.RuneSelf {
                wid = utf8.RuneLen(rune)
            }
            if nbytes+wid > maxbytes {
                // Grow the buffer.
                maxbytes = maxbytes*2 + utf8.UTFMax
                nb := make([]byte, maxbytes)
                copy(nb, b[0:nbytes])
                b = nb
            }
            nbytes += utf8.EncodeRune(rune, b[nbytes:maxbytes])
        }
    }
    return string(b[0:nbytes])
}

// Repeat returns a new string consisting of count copies of the string s.
func Repeat(s string, count int) string {
    b := make([]byte, len(s)*count)
    bp := 0
    for i := 0; i < count; i++ {
        for j := 0; j < len(s); j++ {
            b[bp] = s[j]
            bp++
        }
    }
    return string(b)
}


// ToUpper returns a copy of the string s with all Unicode letters mapped to their upper case.
func ToUpper(s string) string { return Map(unicode.ToUpper, s) }

// ToLower returns a copy of the string s with all Unicode letters mapped to their lower case.
func ToLower(s string) string { return Map(unicode.ToLower, s) }

// ToTitle returns a copy of the string s with all Unicode letters mapped to their title case.
func ToTitle(s string) string { return Map(unicode.ToTitle, s) }

// ToUpperSpecial returns a copy of the string s with all Unicode letters mapped to their
// upper case, giving priority to the special casing rules.
func ToUpperSpecial(_case unicode.SpecialCase, s string) string {
    return Map(func(r int) int { return _case.ToUpper(r) }, s)
}

// ToLowerSpecial returns a copy of the string s with all Unicode letters mapped to their
// lower case, giving priority to the special casing rules.
func ToLowerSpecial(_case unicode.SpecialCase, s string) string {
    return Map(func(r int) int { return _case.ToLower(r) }, s)
}

// ToTitleSpecial returns a copy of the string s with all Unicode letters mapped to their
// title case, giving priority to the special casing rules.
func ToTitleSpecial(_case unicode.SpecialCase, s string) string {
    return Map(func(r int) int { return _case.ToTitle(r) }, s)
}

// isSeparator reports whether the rune could mark a word boundary.
// TODO: update when package unicode captures more of the properties.
func isSeparator(rune int) bool {
    // ASCII alphanumerics and underscore are not separators
    if rune <= 0x7F {
        switch {
        case '0' <= rune && rune <= '9':
            return false
        case 'a' <= rune && rune <= 'z':
            return false
        case 'A' <= rune && rune <= 'Z':
            return false
        case rune == '_':
            return false
        }
        return true
    }
    // Letters and digits are not separators
    if unicode.IsLetter(rune) || unicode.IsDigit(rune) {
        return false
    }
    // Otherwise, all we can do for now is treat spaces as separators.
    return unicode.IsSpace(rune)
}

// BUG(r): The rule Title uses for word boundaries does not handle Unicode punctuation properly.

// Title returns a copy of the string s with all Unicode letters that begin words
// mapped to their title case.
func Title(s string) string {
    // Use a closure here to remember state.
    // Hackish but effective. Depends on Map scanning in order and calling
    // the closure once per rune.
    prev := ' '
    return Map(
        func(r int) int {
            if isSeparator(prev) {
                prev = r
                return unicode.ToTitle(r)
            }
            prev = r
            return r
        },
        s)
}

// TrimLeftFunc returns a slice of the string s with all leading
// Unicode code points c satisfying f(c) removed.
func TrimLeftFunc(s string, f func(r int) bool) string {
    i := indexFunc(s, f, false)
    if i == -1 {
        return ""
    }
    return s[i:]
}

// TrimRightFunc returns a slice of the string s with all trailing
// Unicode code points c satisfying f(c) removed.
func TrimRightFunc(s string, f func(r int) bool) string {
    i := lastIndexFunc(s, f, false)
    if i >= 0 && s[i] >= utf8.RuneSelf {
        _, wid := utf8.DecodeRuneInString(s[i:])
        i += wid
    } else {
        i++
    }
    return s[0:i]
}

// TrimFunc returns a slice of the string s with all leading
// and trailing Unicode code points c satisfying f(c) removed.
func TrimFunc(s string, f func(r int) bool) string {
    return TrimRightFunc(TrimLeftFunc(s, f), f)
}

// IndexFunc returns the index into s of the first Unicode
// code point satisfying f(c), or -1 if none do.
func IndexFunc(s string, f func(r int) bool) int {
    return indexFunc(s, f, true)
}

// LastIndexFunc returns the index into s of the last
// Unicode code point satisfying f(c), or -1 if none do.
func LastIndexFunc(s string, f func(r int) bool) int {
    return lastIndexFunc(s, f, true)
}

// indexFunc is the same as IndexFunc except that if
// truth==false, the sense of the predicate function is
// inverted.
func indexFunc(s string, f func(r int) bool, truth bool) int {
    start := 0
    for start < len(s) {
        wid := 1
        rune := int(s[start])
        if rune >= utf8.RuneSelf {
            rune, wid = utf8.DecodeRuneInString(s[start:])
        }
        if f(rune) == truth {
            return start
        }
        start += wid
    }
    return -1
}

// lastIndexFunc is the same as LastIndexFunc except that if
// truth==false, the sense of the predicate function is
// inverted.
func lastIndexFunc(s string, f func(r int) bool, truth bool) int {
    end := len(s)
    for end > 0 {
        start := end - 1
        rune := int(s[start])
        if rune >= utf8.RuneSelf {
            // Back up & look for beginning of rune. Mustn't pass start.
            for start--; start >= 0; start-- {
                if utf8.RuneStart(s[start]) {
                    break
                }
            }
            if start < 0 {
                return -1
            }
            var wid int
            rune, wid = utf8.DecodeRuneInString(s[start:end])

            // If we've decoded fewer bytes than we expected,
            // we've got some invalid UTF-8, so make sure we return
            // the last possible index in s.
            if start+wid < end && f(utf8.RuneError) == truth {
                return end - 1
            }
        }
        if f(rune) == truth {
            return start
        }
        end = start
    }
    return -1
}

func makeCutsetFunc(cutset string) func(rune int) bool {
    return func(rune int) bool { return IndexRune(cutset, rune) != -1 }
}

// Trim returns a slice of the string s with all leading and
// trailing Unicode code points contained in cutset removed.
func Trim(s string, cutset string) string {
    if s == "" || cutset == "" {
        return s
    }
    return TrimFunc(s, makeCutsetFunc(cutset))
}

// TrimLeft returns a slice of the string s with all leading
// Unicode code points contained in cutset removed.
func TrimLeft(s string, cutset string) string {
    if s == "" || cutset == "" {
        return s
    }
    return TrimLeftFunc(s, makeCutsetFunc(cutset))
}

// TrimRight returns a slice of the string s, with all trailing
// Unicode code points contained in cutset removed.
func TrimRight(s string, cutset string) string {
    if s == "" || cutset == "" {
        return s
    }
    return TrimRightFunc(s, makeCutsetFunc(cutset))
}

// TrimSpace returns a slice of the string s, with all leading
// and trailing white space removed, as defined by Unicode.
func TrimSpace(s string) string {
    return TrimFunc(s, unicode.IsSpace)
}

// Replace returns a copy of the string s with the first n
// non-overlapping instances of old replaced by new.
// If n < 0, there is no limit on the number of replacements.
func Replace(s, old, new string, n int) string {
    if old == new || n == 0 {
        return s // avoid allocation
    }

    // Compute number of replacements.
    if m := Count(s, old); m == 0 {
        return s // avoid allocation
    } else if n < 0 || m < n {
        n = m
    }

    // Apply replacements to buffer.
    t := make([]byte, len(s)+n*(len(new)-len(old)))
    w := 0
    start := 0
    for i := 0; i < n; i++ {
        j := start
        if len(old) == 0 {
            if i > 0 {
                _, wid := utf8.DecodeRuneInString(s[start:])
                j += wid
            }
        } else {
            j += Index(s[start:], old)
        }
        w += copyString(t[w:], s[start:j])
        w += copyString(t[w:], new)
        start = j + len(old)
    }
    w += copyString(t[w:], s[start:])
    return string(t[0:w])
}

func copyString(dst []byte, src string) int {
    n := len(dst)
    if n > len(src) {
        n = len(src)
    }
    for i := 0; i < n; i++ {
        dst[i] = src[i]
    }
    return n
}