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: curve25519_compat.go
    path of file: ./repos/gotosocial/vendor/golang.org/x/crypto/curve25519
    
    the code of the file until where you have to start completion: // Copyright 2019 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 !go1.20

package curve25519

import (
	"crypto/subtle"
	"errors"
	"strconv"

	"golang.org/x/crypto/curve25519/internal/field"
)

func scalarMult(dst, scalar, point *[32]byte) {
	var e [32]byte

	copy(e[:], scalar[:])
	e[0] &= 248
	e[31] &= 127
	e[31] |= 64

	var x1, x2, z2, x3, z3, tmp0, tmp1 field.Element
	x1.SetBytes(point[:])
	x2.One()
	x3.Set(&x1)
	z3.One()

	swap := 0
    
func x25519(dst *[32]byte, scalar, point []byte) ([]byte, error) {
	var in [32]byte
	if l := len(scalar); l != 32 {
		return nil, errors.New("bad scalar length: " + strconv.Itoa(l) + ", expected 32")
	}
	if l := len(point); l != 32 {
		return nil, errors.New("bad point length: " + strconv.Itoa(l) + ", expected 32")
	}
	copy(in[:], scalar)
	if &point[0] == &Basepoint[0] {
		scalarBaseMult(dst, &in)
	} else {
		var base, zero [32]byte
		copy(base[:], point)
		scalarMult(dst, &in, &base)
		if subtle.ConstantTimeCompare(dst[:], zero[:]) == 1 {
			return nil, errors.New("bad input point: low order point")
		}
	}
	return dst[:], nil
}