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

// Bytes returns a new slice containing the bytes in s.
func Bytes(s string) []byte {
	b := make([]byte, len(s))
	for i := 0; i < len(s); i++ {
		b[i] = s[i]
	}
	return b
}

// Runes returns a slice of runes (Unicode code points) equivalent to the string s.
func Runes(s string) []int {
	t := make([]int, utf8.RuneCountInString(s))
	i := 0
	for _, r := range s {
		t[i] = r
		i++
	}
	return t
}