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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 strings implements simple functions to manipulate strings.
package strings

import (
        "unicode"
        "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 int
        var ch rune
        i, cur := 0, 0
        for ; i+1 < n; i++ {
                ch, size = utf8.DecodeRuneInString(s[cur:])
                a[i] = string(ch)
                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
}

// Contains returns true if substr is within s.
func Contains(s, substr string) bool {
        return Index(s, substr) >= 0
}

// ContainsAny returns true if any Unicode code points in chars are within s.
func ContainsAny(s, chars string) bool {
        return IndexAny(s, chars) >= 0
}

// ContainsRune returns true if the Unicode code point r is within s.
func ContainsRune(s string, r rune) bool {
        return IndexRune(s, r) >= 0
}

// 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
// r, or -1 if rune is not present in s.
func IndexRune(s string, r rune) int {
        switch {
        case r < 0x80:
                b := byte(r)
                for i := 0; i < len(s); i++ {
                        if s[i] == b {
                                return i
                        }
                }
        default:
                for i, c := range s {
                        if c == r {
                                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
}

// LastIndexAny returns the index of the last instance of any Unicode code
// point from chars in s, or -1 if no Unicode code point from chars is
// present in s.
func LastIndexAny(s, chars string) int {
        if len(chars) > 0 {
                for i := len(s); i > 0; {
                        rune, size := utf8.DecodeLastRuneInString(s[0:i])
                        i -= size
                        for _, m := range chars {
                                if rune == 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]
}

// SplitN slices s into substrings separated by sep and returns a slice of
// the substrings between those separators.
// If sep is empty, SplitN 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 SplitN(s, sep string, n int) []string { return genSplit(s, sep, 0, n) }

// SplitAfterN slices s into substrings after each instance of sep and
// returns a slice of those substrings.
// If sep is empty, SplitAfterN 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 SplitAfterN(s, sep string, n int) []string {
        return genSplit(s, sep, len(sep), n)
}

// Split slices s into all 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.
// It is equivalent to SplitN with a count of -1.
func Split(s, sep string) []string { return genSplit(s, sep, 0, -1) }

// SplitAfter slices s into all substrings after each instance of sep and
// returns a slice of those substrings.
// If sep is empty, SplitAfter splits after each UTF-8 sequence.
// It is equivalent to SplitAfterN with a count of -1.
func SplitAfter(s, sep string) []string {
        return genSplit(s, sep, len(sep), -1)
}

// 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 all code points in s satisfy f(c) or the
// string is empty, an empty slice is returned.
func FieldsFunc(s string, f func(rune) 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 >= 0 { // 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 := copy(b, a[0])
        for _, s := range a[1:] {
                bp += copy(b[bp:], sep)
                bp += copy(b[bp:], s)
        }
        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) rune, 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
        // The output buffer b is initialized on demand, the first
        // time a character differs.
        var b []byte

        for i, c := range s {
                r := mapping(c)
                if b == nil {
                        if r == c {
                                continue
                        }
                        b = make([]byte, maxbytes)
                        nbytes = copy(b, s[:i])
                }
                if r >= 0 {
                        wid := 1
                        if r >= utf8.RuneSelf {
                                wid = utf8.RuneLen(r)
                        }
                        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(b[nbytes:maxbytes], r)
                }
        }
        if b == nil {
                return s
        }
        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 rune) rune { 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 rune) rune { 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 rune) rune { 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(r rune) bool {
        // ASCII alphanumerics and underscore are not separators
        if r <= 0x7F {
                switch {
                case '0' <= r && r <= '9':
                        return false
                case 'a' <= r && r <= 'z':
                        return false
                case 'A' <= r && r <= 'Z':
                        return false
                case r == '_':
                        return false
                }
                return true
        }
        // Letters and digits are not separators
        if unicode.IsLetter(r) || unicode.IsDigit(r) {
                return false
        }
        // Otherwise, all we can do for now is treat spaces as separators.
        return unicode.IsSpace(r)
}

// 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 rune) rune {
                        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(rune) 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(rune) 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(rune) 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(rune) 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(rune) 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(rune) bool, truth bool) int {
        start := 0
        for start < len(s) {
                wid := 1
                r := rune(s[start])
                if r >= utf8.RuneSelf {
                        r, wid = utf8.DecodeRuneInString(s[start:])
                }
                if f(r) == 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(rune) bool, truth bool) int {
        for i := len(s); i > 0; {
                r, size := utf8.DecodeLastRuneInString(s[0:i])
                i -= size
                if f(r) == truth {
                        return i
                }
        }
        return -1
}

func makeCutsetFunc(cutset string) func(rune) bool {
        return func(r rune) bool { return IndexRune(cutset, r) >= 0 }
}

// 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 += copy(t[w:], s[start:j])
                w += copy(t[w:], new)
                start = j + len(old)
        }
        w += copy(t[w:], s[start:])
        return string(t[0:w])
}

// EqualFold reports whether s and t, interpreted as UTF-8 strings,
// are equal under Unicode case-folding.
func EqualFold(s, t string) bool {
        for s != "" && t != "" {
                // Extract first rune from each string.
                var sr, tr rune
                if s[0] < utf8.RuneSelf {
                        sr, s = rune(s[0]), s[1:]
                } else {
                        r, size := utf8.DecodeRuneInString(s)
                        sr, s = r, s[size:]
                }
                if t[0] < utf8.RuneSelf {
                        tr, t = rune(t[0]), t[1:]
                } else {
                        r, size := utf8.DecodeRuneInString(t)
                        tr, t = r, t[size:]
                }

                // If they match, keep going; if not, return false.

                // Easy case.
                if tr == sr {
                        continue
                }

                // Make sr < tr to simplify what follows.
                if tr < sr {
                        tr, sr = sr, tr
                }
                // Fast check for ASCII.
                if tr < utf8.RuneSelf && 'A' <= sr && sr <= 'Z' {
                        // ASCII, and sr is upper case.  tr must be lower case.
                        if tr == sr+'a'-'A' {
                                continue
                        }
                        return false
                }

                // General case.  SimpleFold(x) returns the next equivalent rune > x
                // or wraps around to smaller values.
                r := unicode.SimpleFold(sr)
                for r != sr && r < tr {
                        r = unicode.SimpleFold(r)
                }
                if r == tr {
                        continue
                }
                return false
        }

        // One string is empty.  Are both?
        return s == t
}