You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: fields.go
    path of file: ./repos/gitkube/vendor/k8s.io/apimachinery/third_party/forked/golang/json
    
    the code of the file until where you have to start completion: // 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.

// Package json is forked from the Go standard library to enable us to find the
// field of a struct that a given JSON key maps to.
package json

    
func foldFunc(s []byte) func(s, t []byte) bool {
	nonLetter := false
	special := false // special letter
	for _, b := range s {
		if b >= utf8.RuneSelf {
			return bytes.EqualFold
		}
		upper := b & caseMask
		if upper < 'A' || upper > 'Z' {
			nonLetter = true
		} else if upper == 'K' || upper == 'S' {
			// See above for why these letters are special.
			special = true
		}
	}
	if special {
		return equalFoldRight
	}
	if nonLetter {
		return asciiEqualFold
	}
	return simpleLetterEqualFold
}