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: s2k.go
    path of file: ./repos/bytom/lib/golang.org/x/crypto/openpgp/s2k
    
    the code of the file until where you have to start completion: // Copyright 2011 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 s2k implements the various OpenPGP string-to-key transforms as
// specified in RFC 4800 section 3.7.1.
package s2k // import "golang.org/x/crypto/openpgp/s2k"

import (
	"crypto"
	"hash"
	"io"
	"strconv"

	"golang.org/x/crypto/openpgp/errors"
)

// Config collects configuration parameters for s2k key-stretching
// transformatioms. A nil *Config is valid and results in all default
// values. Currently, Config is u
    
func Parse(r io.Reader) (f func(out, in []byte), err error) {
	var buf [9]byte

	_, err = io.ReadFull(r, buf[:2])
	if err != nil {
		return
	}

	hash, ok := HashIdToHash(buf[1])
	if !ok {
		return nil, errors.UnsupportedError("hash for S2K function: " + strconv.Itoa(int(buf[1])))
	}
	if !hash.Available() {
		return nil, errors.UnsupportedError("hash not available: " + strconv.Itoa(int(hash)))
	}
	h := hash.New()

	switch buf[0] {
	case 0:
		f := func(out, in []byte) {
			Simple(out, h, in)
		}
		return f, nil
	case 1:
		_, err = io.ReadFull(r, buf[:8])
		if err != nil {
			return
		}
		f := func(out, in []byte) {
			Salted(out, h, in, buf[:8])
		}
		return f, nil
	case 3:
		_, err = io.ReadFull(r, buf[:9])
		if err != nil {
			return
		}
		count := decodeCount(buf[8])
		f := func(out, in []byte) {
			Iterated(out, h, in, buf[:8], count)
		}
		return f, nil
	}

	return nil, errors.UnsupportedError("S2K function")
}