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// Copyright 2014 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.
//go:generate go run gen.go gen_trieval.go
// Package cases provides general and language-specific case mappers.
package cases // import ""
import (
// References:
// - Unicode Reference Manual Chapter 3.13, 4.2, and 5.18.
// -
// -
// -
// -
// -
// -
// -
// TODO:
// - Case folding
// - Wide and Narrow?
// - Segmenter option for title casing.
// - ASCII fast paths
// - Encode Soft-Dotted property within trie somehow.
// A Caser transforms given input to a certain case. It implements
// transform.Transformer.
// A Caser may be stateful and should therefore not be shared between
// goroutines.
type Caser struct {
t transform.SpanningTransformer
// Bytes returns a new byte slice with the result of converting b to the case
// form implemented by c.
func (c Caser) Bytes(b []byte) []byte {
b, _, _ = transform.Bytes(c.t, b)
return b
// String returns a string with the result of transforming s to the case form
// implemented by c.
func (c Caser) String(s string) string {
s, _, _ = transform.String(c.t, s)
return s
// Reset resets the Caser to be reused for new input after a previous call to
// Transform.
func (c Caser) Reset() { c.t.Reset() }
// Transform implements the transform.Transformer interface and transforms the
// given input to the case form implemented by c.
func (c Caser) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
return c.t.Transform(dst, src, atEOF)
// Span implements the transform.SpanningTransformer interface.
func (c Caser) Span(src []byte, atEOF bool) (n int, err error) {
return c.t.Span(src, atEOF)
// Upper returns a Caser for language-specific uppercasing.
func Upper(t language.Tag, opts ...Option) Caser {
return Caser{makeUpper(t, getOpts(opts...))}
// Lower returns a Caser for language-specific lowercasing.
func Lower(t language.Tag, opts ...Option) Caser {
return Caser{makeLower(t, getOpts(opts...))}
// Title returns a Caser for language-specific title casing. It uses an
// approximation of the default Unicode Word Break algorithm.
func Title(t language.Tag, opts ...Option) Caser {
return Caser{makeTitle(t, getOpts(opts...))}
// Fold returns a Caser that implements Unicode case folding. The returned Caser
// is stateless and safe to use concurrently by multiple goroutines.
// Case folding does not normalize the input and may not preserve a normal form.
// Use the collate or search package for more convenient and linguistically
// sound comparisons. Use for string comparisons
// where security aspects are a concern.
func Fold(opts ...Option) Caser {
return Caser{makeFold(getOpts(opts...))}
// An Option is used to modify the behavior of a Caser.
type Option func(o options) options
// TODO: consider these options to take a boolean as well, like FinalSigma.
// The advantage of using this approach is that other providers of a lower-case
// algorithm could set different defaults by prefixing a user-provided slice
// of options with their own. This is handy, for instance, for the precis
// package which would override the default to not handle the Greek final sigma.
var (
// NoLower disables the lowercasing of non-leading letters for a title
// caser.
NoLower Option = noLower
// Compact omits mappings in case folding for characters that would grow the
// input. (Unimplemented.)
Compact Option = compact
// TODO: option to preserve a normal form, if applicable?
type options struct {
noLower bool
simple bool
// TODO: segmenter, max ignorable, alternative versions, etc.
ignoreFinalSigma bool
func getOpts(o ...Option) (res options) {
for _, f := range o {
res = f(res)
func noLower(o options) options {
o.noLower = true
return o
func compact(o options) options {
o.simple = true
return o
// HandleFinalSigma specifies whether the special handling of Greek final sigma
// should be enabled. Unicode prescribes handling the Greek final sigma for all
// locales, but standards like IDNA and PRECIS override this default.
func HandleFinalSigma(enable bool) Option {
if enable {
return handleFinalSigma
return ignoreFinalSigma
func ignoreFinalSigma(o options) options {
o.ignoreFinalSigma = true
return o
func handleFinalSigma(o options) options {
o.ignoreFinalSigma = false
return o