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 {
        n = len(s)
    }
    a := make([]string, n)
    var size, rune int
    na := 0
    for len(s) > 0 {
        if na+1 >= n {
            a[na] = s
            na++
            break
        }
        rune, size = utf8.DecodeRuneInString(s)
        s = s[size:]
        a[na] = string(rune)
        na++
    }
    return a[0:na]
}

// 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]
    n := 0
    for i := 0; i+len(sep) <= len(s); i++ {
        if s[i] == c && (len(sep) == 1 || s[i:i+len(sep)] == sep) {
            n++
            i += len(sep) - 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
}

// 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 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 splits the string s around each instance of sep, returning an array of substrings of s.
// If sep is empty, Split splits s after each UTF-8 sequence.
// If n > 0, Split splits s into at most n substrings; the last substring will be the unsplit remainder.
func Split(s, sep string, n int) []string { return genSplit(s, sep, 0, n) }

// SplitAfter splits the string s after each instance of sep, returning an array of substrings of s.
// If sep is empty, SplitAfter splits s after each UTF-8 sequence.
// If n > 0, SplitAfter splits s into at most n substrings; the last substring will be the unsplit remainder.
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 {
    n := 0
    inField := false
    for _, rune := range s {
        wasInField := inField
        inField = !unicode.IsSpace(rune)
        if inField && !wasInField {
            n++
        }
    }

    a := make([]string, n)
    na := 0
    fieldStart := -1
    for i, rune := range s {
        if unicode.IsSpace(rune) {
            if fieldStart >= 0 {
                a[na] = s[fieldStart:i]
                na++
                fieldStart = -1
            }
        } else if fieldStart == -1 {
            fieldStart = i
        }
    }
    if fieldStart != -1 {
        a[na] = s[fieldStart:]
        na++
    }
    return a[0:na]
}

// 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)
                for i, c := range b[0:nbytes] {
                    nb[i] = c
                }
                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) }

// Trim returns a slice of the string s, with all leading and trailing white space
// removed, as defined by Unicode.
func TrimSpace(s string) string {
    start, end := 0, len(s)
    for start < end {
        wid := 1
        rune := int(s[start])
        if rune >= utf8.RuneSelf {
            rune, wid = utf8.DecodeRuneInString(s[start:end])
        }
        if !unicode.IsSpace(rune) {
            break
        }
        start += wid
    }
    for start < end {
        wid := 1
        rune := int(s[end-1])
        if rune >= utf8.RuneSelf {
            // Back up carefully looking for beginning of rune. Mustn't pass start.
            for wid = 2; start <= end-wid && !utf8.RuneStart(s[end-wid]); wid++ {
            }
            if start > end-wid { // invalid UTF-8 sequence; stop processing
                return s[start:end]
            }
            rune, wid = utf8.DecodeRuneInString(s[end-wid : end])
        }
        if !unicode.IsSpace(rune) {
            break
        }
        end -= wid
    }
    return s[start:end]
}