src/cmd/go/internal/str/str.go

// Copyright 2017 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 str provides string manipulation utilities.
package str

import (
	"fmt"
	"strings"
	"unicode"
	"unicode/utf8"
)

// StringList flattens its arguments into a single []string.
// Each argument in args must have type string or []string.
func StringList(args ...any) []string {
	var x []string
	for _, arg := range args {
		switch arg := arg.(type) {
		case []string:
			x = append(x, arg...)
		case string:
			x = append(x, arg)
		default:
			panic("stringList: invalid argument of type " + fmt.Sprintf("%T", arg))
		}
	}
	return x
}

// ToFold returns a string with the property that
//
//	strings.EqualFold(s, t) iff ToFold(s) == ToFold(t)
//
// This lets us test a large set of strings for fold-equivalent
// duplicates without making a quadratic number of calls
// to EqualFold. Note that strings.ToUpper and strings.ToLower
// do not have the desired property in some corner cases.
func ToFold(s string) string {
	// Fast path: all ASCII, no upper case.
	// Most paths look like this already.
	for i := 0; i < len(s); i++ {
		c := s[i]
		if c >= utf8.RuneSelf || 'A' <= c && c <= 'Z' {
			goto Slow
		}
	}
	return s

Slow:
	var b strings.Builder
	for _, r := range s {
		// SimpleFold(x) cycles to the next equivalent rune > x
		// or wraps around to smaller values. Iterate until it wraps,
		// and we've found the minimum value.
		for {
			r0 := r
			r = unicode.SimpleFold(r0)
			if r <= r0 {
				break
			}
		}
		// Exception to allow fast path above: A-Z => a-z
		if 'A' <= r && r <= 'Z' {
			r += 'a' - 'A'
		}
		b.WriteRune(r)
	}
	return b.String()
}

// FoldDup reports a pair of strings from the list that are
// equal according to strings.EqualFold.
// It returns "", "" if there are no such strings.
func FoldDup(list []string) (string, string) {
	clash := map[string]string{}
	for _, s := range list {
		fold := ToFold(s)
		if t := clash[fold]; t != "" {
			if s > t {
				s, t = t, s
			}
			return s, t
		}
		clash[fold] = s
	}
	return "", ""
}

// Uniq removes consecutive duplicate strings from ss.
func Uniq(ss *[]string) {
	if len(*ss) <= 1 {
		return
	}
	uniq := (*ss)[:1]
	for _, s := range *ss {
		if s != uniq[len(uniq)-1] {
			uniq = append(uniq, s)
		}
	}
	*ss = uniq
}