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// Copyright 2012 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.
// This file contains a straightforward implementation of
// Reed-Solomon encoding, along with a benchmark.
// It goes with http://research.swtch.com/field.
//
// For an optimized implementation, see gf256.go.
package gf256
import (
"bytes"
"fmt"
"testing"
)
// BlogECC writes to check the error correcting code bytes
// for data using the given Reed-Solomon parameters.
func BlogECC(rs *RSEncoder, m []byte, check []byte) {
if len(check) < rs.c {
panic("gf256: invalid check byte length")
}
if rs.c == 0 {
return
}
// The check bytes are the remainder after dividing
// data padded with c zeros by the generator polynomial.
// p = data padded with c zeros.
var p []byte
n := len(m) + rs.c
if len(rs.p) >= n {
p = rs.p
} else {
p = make([]byte, n)
}
copy(p, m)
for i := len(m); i < len(p); i++ {
p[i] = 0
}
gen := rs.gen
// Divide p by gen, leaving the remainder in p[len(data):].
// p[0] is the most significant term in p, and
// gen[0] is the most significant term in the generator.
for i := 0; i < len(m); i++ {
k := f.Mul(p[i], f.Inv(gen[0])) // k = pi / g0
// p -= k·g
for j, g := range gen {
p[i+j] = f.Add(p[i+j], f.Mul(k, g))
}
}
copy(check, p[len(m):])
rs.p = p
}
func BenchmarkBlogECC(b *testing.B) {
data := []byte{0x10, 0x20, 0x0c, 0x56, 0x61, 0x80, 0xec, 0x11, 0xec, 0x11, 0xec, 0x11, 0xec, 0x11, 0xec, 0x11, 0x10, 0x20, 0x0c, 0x56, 0x61, 0x80, 0xec, 0x11, 0xec, 0x11, 0xec, 0x11, 0xec, 0x11, 0xec, 0x11}
check := []byte{0x29, 0x41, 0xb3, 0x93, 0x8, 0xe8, 0xa3, 0xe7, 0x63, 0x8f}
out := make([]byte, len(check))
rs := NewRSEncoder(f, len(check))
for i := 0; i < b.N; i++ {
BlogECC(rs, data, out)
}
b.SetBytes(int64(len(data)))
if !bytes.Equal(out, check) {
fmt.Printf("have %#v want %#v\n", out, check)
}
}
func TestBlogECC(t *testing.T) {
data := []byte{0x10, 0x20, 0x0c, 0x56, 0x61, 0x80, 0xec, 0x11, 0xec, 0x11, 0xec, 0x11, 0xec, 0x11, 0xec, 0x11}
check := []byte{0xa5, 0x24, 0xd4, 0xc1, 0xed, 0x36, 0xc7, 0x87, 0x2c, 0x55}
out := make([]byte, len(check))
rs := NewRSEncoder(f, len(check))
BlogECC(rs, data, out)
if !bytes.Equal(out, check) {
t.Errorf("have %x want %x", out, check)
}
}
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