Speed up gogf127.Mul()

Cache results of the shift. Also add test for checking if
implementation can work when result is one of the arguments.
This commit is contained in:
Evgenii Stratonikov 2019-10-11 11:50:32 +03:00
parent b27c17ce19
commit c5fb08aece
2 changed files with 20 additions and 16 deletions

View file

@ -38,6 +38,21 @@ func TestMul(t *testing.T) {
}
}
func TestMulInPlace(t *testing.T) {
for _, tc := range testCasesMul {
a := *tc[0]
b := *tc[1]
Mul(&a, &b, &b)
require.Equal(t, a, *tc[0])
require.Equal(t, b, *tc[2])
b = *tc[1]
Mul(&a, &b, &a)
require.Equal(t, b, *tc[1])
require.Equal(t, a, *tc[2])
}
}
var testCasesMul10 = [][2]*GF127{
{&GF127{123, 0}, &GF127{246, 0}},
{&GF127{maxUint64, 2}, &GF127{maxUint64 - 1, 5}},

View file

@ -160,35 +160,24 @@ func Add(a, b, c *GF127) {
}
// Mul sets c to a*b.
// TODO make it work in-place without allocations
// TODO optimization: no need to perform shift by i every time, cache results
func Mul(a, b, c *GF127) {
r := new(GF127)
d := new(GF127)
d := *a
for i := uint(0); i < 64; i++ {
if b[0]&(1<<i) != 0 {
shl(i, a, d)
Add(r, d, r)
Add(r, &d, r)
}
Mul10(&d, &d)
}
for i := uint(0); i < 63; i++ {
if b[1]&(1<<i) != 0 {
shl(i+64, a, d)
Add(r, d, r)
Add(r, &d, r)
}
Mul10(&d, &d)
}
*c = *r
}
// shl performs left shift by consecutive multiplications by 2.
func shl(count uint, a, b *GF127) {
b[0] = a[0]
b[1] = a[1]
for i := uint(0); i < count; i++ {
Mul10(b, b)
}
}
// Mul10 sets b to a*x.
func Mul10(a, b *GF127) {
c := (a[0] & msb64) >> 63