package bytes

import (
	"internal/bytealg"
	"unicode"
	"unicode/utf8"
)

// It splits the slice s around each instance of one or more consecutive white space
// characters, as defined by [unicode.IsSpace], returning a slice of subslices of s or an
// empty slice if s contains only white space. Every element of the returned slice is
// non-empty. Unlike [Split], leading and trailing runs of white space characters
// are discarded.
func Fields(s []byte) [][]byte {
	asciiSpace := _asciiSpace()
	n := 0
	wasSpace := 1
	setBits := uint8(0)
	for i := 0; i < len(s); i++ {
		r := s[i]
		setBits |= r
		isSpace := int32(asciiSpace[r])
		n += wasSpace & ^isSpace
		wasSpace = isSpace
	}

	if setBits >= utf8.RuneSelf {
		t := unicode.NewTables()
		return FieldsFunc(s, t.IsSpace)
	}

	a := [][]byte{:n}
	na := 0
	fieldStart := 0
	i := 0
	for i < len(s) && asciiSpace[s[i]] != 0 {
		i++
	}
	fieldStart = i
	for i < len(s) {
		if asciiSpace[s[i]] == 0 {
			i++
			continue
		}
		a[na] = s[fieldStart:i:i]
		na++
		i++
		for i < len(s) && asciiSpace[s[i]] != 0 {
			i++
		}
		fieldStart = i
	}
	if fieldStart < len(s) { // Last field might end at EOF.
		a[na] = s[fieldStart:len(s):len(s)]
	}
	return a
}

// FieldsFunc interprets s as a sequence of UTF-8-encoded code points.
// It splits the slice s at each run of code points c satisfying f(c) and
// returns a slice of subslices of s. If all code points in s satisfy f(c), or
// len(s) == 0, an empty slice is returned. Every element of the returned slice is
// non-empty. Unlike [SplitFunc], leading and trailing runs of code points
// satisfying f(c) are discarded.
//
// FieldsFunc makes no guarantees about the order in which it calls f(c)
// and assumes that f always returns the same value for a given c.
func FieldsFunc(s []byte, f func(rune) bool) [][]byte {
	// A span is used to record a slice of s of the form s[start:end].
	// The start index is inclusive and the end index is exclusive.
	type span struct {
		start int32
		end   int32
	}
	spans := []span{:0:32}

	// Find the field start and end indices.
	// Doing this in a separate pass (rather than slicing the string s
	// and collecting the result substrings right away) is significantly
	// more efficient, possibly due to cache effects.
	start := -1 // valid span start if >= 0
	for i := 0; i < len(s); {
		size := 1
		r := rune(s[i])
		if r >= utf8.RuneSelf {
			r, size = utf8.DecodeRune(s[i:])
		}
		if f(r) {
			if start >= 0 {
				spans = append(spans, span{start, i})
				start = -1
			}
		} else {
			if start < 0 {
				start = i
			}
		}
		i += size
	}

	// Last field might end at EOF.
	if start >= 0 {
		spans = append(spans, span{start, len(s)})
	}

	// Create subslices from recorded field indices.
	a := [][]byte{:len(spans)}
	for i, span := range spans {
		a[i] = s[span.start:span.end:span.end]
	}

	return a
}

// Join concatenates the elements of s to create a new byte slice. The separator
// sep is placed between elements in the resulting slice.
func Join(s [][]byte, sep []byte) []byte {
	if len(s) == 0 {
		return []byte{}
	}
	if len(s) == 1 {
		// Just return a copy.
		return append([]byte(nil), s[0]...)
	}

	var n int32
	if len(sep) > 0 {
		if len(sep) >= maxInt/(len(s)-1) {
			panic("bytes: Join output length overflow")
		}
		n += len(sep) * (len(s) - 1)
	}
	for _, v := range s {
		if len(v) > maxInt-n {
			panic("bytes: Join output length overflow")
		}
		n += len(v)
	}

	b := bytealg.MakeNoZero(n)[:n:n]
	bp := copy(b, s[0])
	for _, v := range s[1:] {
		bp += copy(b[bp:], sep)
		bp += copy(b[bp:], v)
	}
	return b
}

// HasPrefix reports whether the byte slice s begins with prefix.
func HasPrefix(s, prefix []byte) bool {
	return len(s) >= len(prefix) && Equal(s[:len(prefix)], prefix)
}

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

// Map returns a copy of the byte slice s with all its characters modified
// according to the mapping function. If mapping returns a negative value, the character is
// dropped from the byte slice with no replacement. The characters in s and the
// output are interpreted as UTF-8-encoded code points.
func Map(mapping func(r rune) rune, s []byte) []byte {
	// In the worst case, the slice 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.
	b := []byte{:0:len(s)}
	for i := 0; i < len(s); {
		wid := 1
		r := rune(s[i])
		if r >= utf8.RuneSelf {
			r, wid = utf8.DecodeRune(s[i:])
		}
		r = mapping(r)
		if r >= 0 {
			b = utf8.AppendRune(b, r)
		}
		i += wid
	}
	return b
}

// Despite being an exported symbol,
// Repeat is linknamed by widely used packages.
// Notable members of the hall of shame include:
//   - gitee.com/quant1x/num
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
// Note that this comment is not part of the doc comment.
//
// linkname Repeat (stripped for moxie)

// ToUpper returns a copy of the byte slice s with all Unicode letters mapped to
// their upper case.
func ToUpper(s []byte) []byte {
	isASCII, hasLower := true, false
	for i := 0; i < len(s); i++ {
		c := s[i]
		if c >= utf8.RuneSelf {
			isASCII = false
			break
		}
		hasLower = hasLower || ('a' <= c && c <= 'z')
	}

	if isASCII { // optimize for ASCII-only byte slices.
		if !hasLower {
			// Just return a copy.
			return append([]byte(""), s...)
		}
		b := bytealg.MakeNoZero(len(s))[:len(s):len(s)]
		for i := 0; i < len(s); i++ {
			c := s[i]
			if 'a' <= c && c <= 'z' {
				c -= 'a' - 'A'
			}
			b[i] = c
		}
		return b
	}
	t := unicode.NewTables()
	return Map(t.ToUpper, s)
}

// ToLower returns a copy of the byte slice s with all Unicode letters mapped to
// their lower case.
func ToLower(s []byte) []byte {
	isASCII, hasUpper := true, false
	for i := 0; i < len(s); i++ {
		c := s[i]
		if c >= utf8.RuneSelf {
			isASCII = false
			break
		}
		hasUpper = hasUpper || ('A' <= c && c <= 'Z')
	}

	if isASCII { // optimize for ASCII-only byte slices.
		if !hasUpper {
			return append([]byte(""), s...)
		}
		b := bytealg.MakeNoZero(len(s))[:len(s):len(s)]
		for i := 0; i < len(s); i++ {
			c := s[i]
			if 'A' <= c && c <= 'Z' {
				c += 'a' - 'A'
			}
			b[i] = c
		}
		return b
	}
	t := unicode.NewTables()
	return Map(t.ToLower, s)
}

// ToTitle treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their title case.
func ToTitle(s []byte) []byte {
	t := unicode.NewTables()
	return Map(t.ToTitle, s)
}

// ToUpperSpecial treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their
// upper case, giving priority to the special casing rules.
func ToUpperSpecial(c unicode.SpecialCase, s []byte) []byte {
	return Map(c.ToUpper, s)
}

// ToLowerSpecial treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their
// lower case, giving priority to the special casing rules.
func ToLowerSpecial(c unicode.SpecialCase, s []byte) []byte {
	return Map(c.ToLower, s)
}

// ToTitleSpecial treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their
// title case, giving priority to the special casing rules.
func ToTitleSpecial(c unicode.SpecialCase, s []byte) []byte {
	return Map(c.ToTitle, s)
}

// ToValidUTF8 treats s as UTF-8-encoded bytes and returns a copy with each run of bytes
// representing invalid UTF-8 replaced with the bytes in replacement, which may be empty.
func ToValidUTF8(s, replacement []byte) []byte {
	b := []byte{:0:len(s)+len(replacement)}
	invalid := false // previous byte was from an invalid UTF-8 sequence
	for i := 0; i < len(s); {
		c := s[i]
		if c < utf8.RuneSelf {
			i++
			invalid = false
			b = append(b, c)
			continue
		}
		_, wid := utf8.DecodeRune(s[i:])
		if wid == 1 {
			i++
			if !invalid {
				invalid = true
				b = append(b, replacement...)
			}
			continue
		}
		invalid = false
		b = append(b, s[i:i+wid]...)
		i += wid
	}
	return b
}

// 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
	t := unicode.NewTables()
	if t.IsLetter(r) || t.IsDigit(r) {
		return false
	}
	// Otherwise, all we can do for now is treat spaces as separators.
	return t.IsSpace(r)
}

// Title treats s as UTF-8-encoded bytes and returns a copy with all Unicode letters that begin
// words mapped to their title case.
//
// Deprecated: The rule Title uses for word boundaries does not handle Unicode
// punctuation properly. Use golang.org/x/text/cases instead.
func Title(s []byte) []byte {
	// Use a closure here to remember state.
	// Hackish but effective. Depends on Map scanning in order and calling
	// the closure once per rune.
	t := unicode.NewTables()
	prev := ' '
	return Map(
		func(r rune) rune {
			if isSeparator(prev) {
				prev = r
				return t.ToTitle(r)
			}
			prev = r
			return r
		},
		s)
}

// TrimLeftFunc treats s as UTF-8-encoded bytes and returns a subslice of s by slicing off
// all leading UTF-8-encoded code points c that satisfy f(c).
func TrimLeftFunc(s []byte, f func(r rune) bool) []byte {
	i := indexFunc(s, f, false)
	if i == -1 {
		return nil
	}
	return s[i:]
}

// TrimRightFunc returns a subslice of s by slicing off all trailing
// UTF-8-encoded code points c that satisfy f(c).
func TrimRightFunc(s []byte, f func(r rune) bool) []byte {
	i := lastIndexFunc(s, f, false)
	if i >= 0 && s[i] >= utf8.RuneSelf {
		_, wid := utf8.DecodeRune(s[i:])
		i += wid
	} else {
		i++
	}
	return s[0:i]
}

// TrimFunc returns a subslice of s by slicing off all leading and trailing
// UTF-8-encoded code points c that satisfy f(c).
func TrimFunc(s []byte, f func(r rune) bool) []byte {
	return TrimRightFunc(TrimLeftFunc(s, f), f)
}

// TrimPrefix returns s without the provided leading prefix string.
// If s doesn't start with prefix, s is returned unchanged.
func TrimPrefix(s, prefix []byte) []byte {
	if HasPrefix(s, prefix) {
		return s[len(prefix):]
	}
	return s
}

// TrimSuffix returns s without the provided trailing suffix string.
// If s doesn't end with suffix, s is returned unchanged.
func TrimSuffix(s, suffix []byte) []byte {
	if HasSuffix(s, suffix) {
		return s[:len(s)-len(suffix)]
	}
	return s
}

// IndexFunc interprets s as a sequence of UTF-8-encoded code points.
// It returns the byte index in s of the first Unicode
// code point satisfying f(c), or -1 if none do.
func IndexFunc(s []byte, f func(r rune) bool) int32 {
	return indexFunc(s, f, true)
}

// LastIndexFunc interprets s as a sequence of UTF-8-encoded code points.
// It returns the byte index in s of the last Unicode
// code point satisfying f(c), or -1 if none do.
func LastIndexFunc(s []byte, f func(r rune) bool) int32 {
	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 []byte, f func(r rune) bool, truth bool) int32 {
	start := 0
	for start < len(s) {
		wid := 1
		r := rune(s[start])
		if r >= utf8.RuneSelf {
			r, wid = utf8.DecodeRune(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 []byte, f func(r rune) bool, truth bool) int32 {
	for i := len(s); i > 0; {
		r, size := rune(s[i-1]), 1
		if r >= utf8.RuneSelf {
			r, size = utf8.DecodeLastRune(s[0:i])
		}
		i -= size
		if f(r) == truth {
			return i
		}
	}
	return -1
}

// asciiSet is a 32-byte value, where each bit represents the presence of a
// given ASCII character in the set. The 128-bits of the lower 16 bytes,
// starting with the least-significant bit of the lowest word to the
// most-significant bit of the highest word, map to the full range of all
// 128 ASCII characters. The 128-bits of the upper 16 bytes will be zeroed,
// ensuring that any non-ASCII character will be reported as not in the set.
// This allocates a total of 32 bytes even though the upper half
// is unused to avoid bounds checks in asciiSet.contains.
type asciiSet [8]uint32

// makeASCIISet creates a set of ASCII characters and reports whether all
// characters in chars are ASCII.
func makeASCIISet(chars []byte) (as asciiSet, ok bool) {
	for i := 0; i < len(chars); i++ {
		c := chars[i]
		if c >= utf8.RuneSelf {
			return as, false
		}
		as[c/32] |= 1 << (c % 32)
	}
	return as, true
}

// contains reports whether c is inside the set.
func (as *asciiSet) contains(c byte) bool {
	return (as[c/32] & (1 << (c % 32))) != 0
}

// containsRune is a simplified version of strings.ContainsRune
// to avoid importing the strings package.
// We avoid bytes.ContainsRune to avoid allocating a temporary copy of s.
func containsRune(s []byte, r rune) bool {
	for i := 0; i < len(s); {
		c, w := utf8.DecodeRune(s[i:])
		if c == r {
			return true
		}
		i += w
	}
	return false
}

// Trim returns a subslice of s by slicing off all leading and
// trailing UTF-8-encoded code points contained in cutset.
func Trim(s []byte, cutset []byte) []byte {
	if len(s) == 0 {
		// This is what we've historically done.
		return nil
	}
	if cutset == "" {
		return s
	}
	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
		return trimLeftByte(trimRightByte(s, cutset[0]), cutset[0])
	}
	if as, ok := makeASCIISet(cutset); ok {
		return trimLeftASCII(trimRightASCII(s, &as), &as)
	}
	return trimLeftUnicode(trimRightUnicode(s, cutset), cutset)
}

// TrimLeft returns a subslice of s by slicing off all leading
// UTF-8-encoded code points contained in cutset.
func TrimLeft(s []byte, cutset []byte) []byte {
	if len(s) == 0 {
		// This is what we've historically done.
		return nil
	}
	if cutset == "" {
		return s
	}
	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
		return trimLeftByte(s, cutset[0])
	}
	if as, ok := makeASCIISet(cutset); ok {
		return trimLeftASCII(s, &as)
	}
	return trimLeftUnicode(s, cutset)
}

func trimLeftByte(s []byte, c byte) []byte {
	for len(s) > 0 && s[0] == c {
		s = s[1:]
	}
	if len(s) == 0 {
		// This is what we've historically done.
		return nil
	}
	return s
}

func trimLeftASCII(s []byte, as *asciiSet) []byte {
	for len(s) > 0 {
		if !as.contains(s[0]) {
			break
		}
		s = s[1:]
	}
	if len(s) == 0 {
		// This is what we've historically done.
		return nil
	}
	return s
}

func trimLeftUnicode(s []byte, cutset []byte) []byte {
	for len(s) > 0 {
		r, n := rune(s[0]), 1
		if r >= utf8.RuneSelf {
			r, n = utf8.DecodeRune(s)
		}
		if !containsRune(cutset, r) {
			break
		}
		s = s[n:]
	}
	if len(s) == 0 {
		// This is what we've historically done.
		return nil
	}
	return s
}

// TrimRight returns a subslice of s by slicing off all trailing
// UTF-8-encoded code points that are contained in cutset.
func TrimRight(s []byte, cutset []byte) []byte {
	if len(s) == 0 || cutset == "" {
		return s
	}
	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
		return trimRightByte(s, cutset[0])
	}
	if as, ok := makeASCIISet(cutset); ok {
		return trimRightASCII(s, &as)
	}
	return trimRightUnicode(s, cutset)
}

func trimRightByte(s []byte, c byte) []byte {
	for len(s) > 0 && s[len(s)-1] == c {
		s = s[:len(s)-1]
	}
	return s
}

func trimRightASCII(s []byte, as *asciiSet) []byte {
	for len(s) > 0 {
		if !as.contains(s[len(s)-1]) {
			break
		}
		s = s[:len(s)-1]
	}
	return s
}

func trimRightUnicode(s []byte, cutset []byte) []byte {
	for len(s) > 0 {
		r, n := rune(s[len(s)-1]), 1
		if r >= utf8.RuneSelf {
			r, n = utf8.DecodeLastRune(s)
		}
		if !containsRune(cutset, r) {
			break
		}
		s = s[:len(s)-n]
	}
	return s
}

// TrimSpace returns a subslice of s by slicing off all leading and
// trailing white space, as defined by Unicode.
func TrimSpace(s []byte) []byte {
	t := unicode.NewTables()
	asciiSpace := _asciiSpace()
	start := 0
	for ; start < len(s); start++ {
		c := s[start]
		if c >= utf8.RuneSelf {
			return TrimFunc(s[start:], t.IsSpace)
		}
		if asciiSpace[c] == 0 {
			break
		}
	}

	stop := len(s)
	for ; stop > start; stop-- {
		c := s[stop-1]
		if c >= utf8.RuneSelf {
			return TrimFunc(s[start:stop], t.IsSpace)
		}
		if asciiSpace[c] == 0 {
			break
		}
	}

	// At this point s[start:stop] starts and ends with an ASCII
	// non-space bytes, so we're done. Non-ASCII cases have already
	// been handled above.
	if start == stop {
		// Special case to preserve previous TrimLeftFunc behavior,
		// returning nil instead of empty slice if all spaces.
		return nil
	}
	return s[start:stop]
}

// Runes interprets s as a sequence of UTF-8-encoded code points.
// It returns a slice of runes (Unicode code points) equivalent to s.
func Runes(s []byte) []rune {
	t := []rune{:utf8.RuneCount(s)}
	i := 0
	for len(s) > 0 {
		r, l := utf8.DecodeRune(s)
		t[i] = r
		i++
		s = s[l:]
	}
	return t
}

// Replace returns a copy of the slice s with the first n
// non-overlapping instances of old replaced by new.
// If old is empty, it matches at the beginning of the slice
// and after each UTF-8 sequence, yielding up to k+1 replacements
// for a k-rune slice.
// If n < 0, there is no limit on the number of replacements.
func Replace(s, old, new []byte, n int32) []byte {
	m := 0
	if n != 0 {
		// Compute number of replacements.
		m = Count(s, old)
	}
	if m == 0 {
		// Just return a copy.
		return append([]byte(nil), s...)
	}
	if n < 0 || m < n {
		n = m
	}

	// Apply replacements to buffer.
	t := []byte{:len(s)+n*(len(new)-len(old))}
	w := 0
	start := 0
	if len(old) > 0 {
		for range n {
			j := start + Index(s[start:], old)
			w += copy(t[w:], s[start:j])
			w += copy(t[w:], new)
			start = j + len(old)
		}
	} else { // len(old) == 0
		w += copy(t[w:], new)
		for range n - 1 {
			_, wid := utf8.DecodeRune(s[start:])
			j := start + wid
			w += copy(t[w:], s[start:j])
			w += copy(t[w:], new)
			start = j
		}
	}
	w += copy(t[w:], s[start:])
	return t[0:w]
}

// ReplaceAll returns a copy of the slice s with all
// non-overlapping instances of old replaced by new.
// If old is empty, it matches at the beginning of the slice
// and after each UTF-8 sequence, yielding up to k+1 replacements
// for a k-rune slice.
func ReplaceAll(s, old, new []byte) []byte {
	return Replace(s, old, new, -1)
}

// EqualFold reports whether s and t, interpreted as UTF-8 strings,
// are equal under simple Unicode case-folding, which is a more general
// form of case-insensitivity.
func EqualFold(s, t []byte) bool {
	// ASCII fast path
	i := 0
	for n := min(len(s), len(t)); i < n; i++ {
		sr := s[i]
		tr := t[i]
		if sr|tr >= utf8.RuneSelf {
			goto hasUnicode
		}

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

		// Make sr < tr to simplify what follows.
		if tr < sr {
			tr, sr = sr, tr
		}
		// ASCII only, sr/tr must be upper/lower case
		if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
			continue
		}
		return false
	}
	// Check if we've exhausted both strings.
	return len(s) == len(t)

hasUnicode:
	s = s[i:]
	t = t[i:]
	for len(s) != 0 && len(t) != 0 {
		// Extract first rune from each.
		var sr, tr rune
		if s[0] < utf8.RuneSelf {
			sr, s = rune(s[0]), s[1:]
		} else {
			r, size := utf8.DecodeRune(s)
			sr, s = r, s[size:]
		}
		if t[0] < utf8.RuneSelf {
			tr, t = rune(t[0]), t[1:]
		} else {
			r, size := utf8.DecodeRune(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 {
			// ASCII only, sr/tr must be upper/lower case
			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
				continue
			}
			return false
		}

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

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

// 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 []byte) int32 {
	n := len(sep)
	if n == 0 {
		return 0
	}
	if n == 1 {
		return IndexByte(s, sep[0])
	}
	if n == len(s) {
		if Equal(sep, s) {
			return 0
		}
		return -1
	}
	if n > len(s) {
		return -1
	}
	return indexBrute(s, sep, n)
}

func indexBrute(s, sep []byte, n int32) int32 {
	c0 := sep[0]
	c1 := sep[1]
	i := int32(0)
	fails := int32(0)
	t := len(s) - n + 1
	for i < t {
		if s[i] != c0 {
			o := IndexByte(s[i+1:t], c0)
			if o < 0 {
				break
			}
			i += o + 1
		}
		if s[i+1] == c1 && Equal(s[i:i+n], sep) {
			return i
		}
		i++
		fails++
		if fails >= 4+i>>4 && i < t {
			j := bytealg.IndexRabinKarp(s[i:], sep)
			if j < 0 {
				return -1
			}
			return i + j
		}
	}
	return -1
}

// Cut slices s around the first instance of sep,
// returning the text before and after sep.
// The found result reports whether sep appears in s.
// If sep does not appear in s, cut returns s, nil, false.
//
// Cut returns slices of the original slice s, not copies.
func Cut(s, sep []byte) (before, after []byte, found bool) {
	if i := Index(s, sep); i >= 0 {
		return s[:i], s[i+len(sep):], true
	}
	return s, nil, false
}

// Clone returns a copy of b[:len(b)].
// The result may have additional unused capacity.
// Clone(nil) returns nil.
func Clone(b []byte) []byte {
	if b == nil {
		return nil
	}
	return append([]byte{}, b...)
}

// CutPrefix returns s without the provided leading prefix byte slice
// and reports whether it found the prefix.
// If s doesn't start with prefix, CutPrefix returns s, false.
// If prefix is the empty byte slice, CutPrefix returns s, true.
//
// CutPrefix returns slices of the original slice s, not copies.
func CutPrefix(s, prefix []byte) (after []byte, found bool) {
	if !HasPrefix(s, prefix) {
		return s, false
	}
	return s[len(prefix):], true
}

// CutSuffix returns s without the provided ending suffix byte slice
// and reports whether it found the suffix.
// If s doesn't end with suffix, CutSuffix returns s, false.
// If suffix is the empty byte slice, CutSuffix returns s, true.
//
// CutSuffix returns slices of the original slice s, not copies.
func CutSuffix(s, suffix []byte) (before []byte, found bool) {
	if !HasSuffix(s, suffix) {
		return s, false
	}
	return s[:len(s)-len(suffix)], true
}