src/encoding/json/fold.go

// Copyright 2013 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:build !goexperiment.jsonv2

package json

import (
	"unicode"
	"unicode/utf8"
)

// foldName returns a folded string such that foldName(x) == foldName(y)
// is identical to bytes.EqualFold(x, y).
func foldName(in []byte) []byte {
	// This is inlinable to take advantage of "function outlining".
	var arr [32]byte // large enough for most JSON names
	return appendFoldedName(arr[:0], in)
}

func appendFoldedName(out, in []byte) []byte {
	for i := 0; i < len(in); {
		// Handle single-byte ASCII.
		if c := in[i]; c < utf8.RuneSelf {
			if 'a' <= c && c <= 'z' {
				c -= 'a' - 'A'
			}
			out = append(out, c)
			i++
			continue
		}
		// Handle multi-byte Unicode.
		r, n := utf8.DecodeRune(in[i:])
		out = utf8.AppendRune(out, foldRune(r))
		i += n
	}
	return out
}

// foldRune is returns the smallest rune for all runes in the same fold set.
func foldRune(r rune) rune {
	for {
		r2 := unicode.SimpleFold(r)
		if r2 <= r {
			return r2
		}
		r = r2
	}
}