src/internal/stringslite/strings.go

// Copyright 2024 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 stringslite implements a subset of strings,
// only using packages that may be imported by "os".
//
// Tests for these functions are in the strings package.
package stringslite

import "internal/bytealg"

func HasPrefix(s, prefix string) bool {
	return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
}

func HasSuffix(s, suffix string) bool {
	return len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix
}

func IndexByte(s string, c byte) int {
	return bytealg.IndexByteString(s, c)
}

func Index(s, substr string) int {
	n := len(substr)
	switch {
	case n == 0:
		return 0
	case n == 1:
		return IndexByte(s, substr[0])
	case n == len(s):
		if substr == s {
			return 0
		}
		return -1
	case n > len(s):
		return -1
	case n <= bytealg.MaxLen:
		// Use brute force when s and substr both are small
		if len(s) <= bytealg.MaxBruteForce {
			return bytealg.IndexString(s, substr)
		}
		c0 := substr[0]
		c1 := substr[1]
		i := 0
		t := len(s) - n + 1
		fails := 0
		for i < t {
			if s[i] != c0 {
				// IndexByte is faster than bytealg.IndexString, so use it as long as
				// we're not getting lots of false positives.
				o := IndexByte(s[i+1:t], c0)
				if o < 0 {
					return -1
				}
				i += o + 1
			}
			if s[i+1] == c1 && s[i:i+n] == substr {
				return i
			}
			fails++
			i++
			// Switch to bytealg.IndexString when IndexByte produces too many false positives.
			if fails > bytealg.Cutover(i) {
				r := bytealg.IndexString(s[i:], substr)
				if r >= 0 {
					return r + i
				}
				return -1
			}
		}
		return -1
	}
	c0 := substr[0]
	c1 := substr[1]
	i := 0
	t := len(s) - n + 1
	fails := 0
	for i < t {
		if s[i] != c0 {
			o := IndexByte(s[i+1:t], c0)
			if o < 0 {
				return -1
			}
			i += o + 1
		}
		if s[i+1] == c1 && s[i:i+n] == substr {
			return i
		}
		i++
		fails++
		if fails >= 4+i>>4 && i < t {
			// See comment in ../bytes/bytes.go.
			j := bytealg.IndexRabinKarp(s[i:], substr)
			if j < 0 {
				return -1
			}
			return i + j
		}
	}
	return -1
}

func Cut(s, sep string) (before, after string, found bool) {
	if i := Index(s, sep); i >= 0 {
		return s[:i], s[i+len(sep):], true
	}
	return s, "", false
}

func CutPrefix(s, prefix string) (after string, found bool) {
	if !HasPrefix(s, prefix) {
		return s, false
	}
	return s[len(prefix):], true
}

func CutSuffix(s, suffix string) (before string, found bool) {
	if !HasSuffix(s, suffix) {
		return s, false
	}
	return s[:len(s)-len(suffix)], true
}