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quick and dirty packed bits for the BCH encoder

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Ahmet Inan 2018-09-22 14:50:25 +02:00
commit 1f354379d3
2 changed files with 101 additions and 5 deletions
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66
bose_chaudhuri_hocquenghem_encoder.hh
View file

@ -11,16 +11,78 @@ Copyright 2018 Ahmet Inan <inan@aicodix.de>
namespace CODE {
template <int NR, int FCR, int MSG, typename GF>
template <int LEN, int MSG>
class BoseChaudhuriHocquenghemEncoder
{
public:
static const int N = LEN, K = MSG, NP = N - K;
static const int G = ((NP+1)+7)/8;
uint8_t generator[G];
BoseChaudhuriHocquenghemEncoder(std::initializer_list<int> minimal_polynomials)
{
// $generator(x) = \prod_i(minpoly_i(x))$
int generator_degree = 1;
generator[0] = 1;
for (int i = 1; i < G; ++i)
generator[i] = 0;
for (auto m: minimal_polynomials) {
int m_degree = 0;
while (m>>m_degree)
++m_degree;
--m_degree;
assert(generator_degree + m_degree <= NP + 1);
for (int i = generator_degree; i >= 0; --i) {
if (!((generator[i/8]>>(i%8))&1))
continue;
generator[i/8] &= ~(1<<(i%8));
generator[i/8] |= (m&1)<<(i%8);
for (int j = 1; j <= m_degree; ++j)
generator[(i+j)/8] ^= ((m>>j)&1)<<((i+j)%8);
}
generator_degree += m_degree;
}
assert(generator_degree == NP + 1);
if (0) {
std::cerr << "generator =";
for (int i = 0; i <= NP; ++i)
std::cerr << " " << int((generator[i/8] >> (i%8))&1);
std::cerr << std::endl;
}
}
void operator()(uint8_t *code)
{
// $code = data * x^{NP} + (data * x^{NP}) \mod{generator}$
for (int i = 0; i < NP; ++i)
code[(K+i)/8] &= ~(128>>((K+i)%8));
for (int i = 0; i < K; ++i) {
if (((code[i/8]>>(7-i%8))&1) != ((code[K/8]>>(7-K%8))&1)) {
for (int j = 1; j < NP; ++j) {
code[(K+j-1)/8] &= ~(128>>((K+j-1)%8));
code[(K+j-1)/8] |= (((generator[(NP-j)/8]>>((NP-j)%8))^(code[(K+j)/8]>>(7-(K+j)%8)))&1)<<(7-(K+j-1)%8);
}
code[(N-1)/8] &= ~(128>>((N-1)%8));
code[(N-1)/8] |= (generator[0]&1)<<(7-(N-1)%8);
} else {
for (int j = 1; j < NP; ++j) {
code[(K+j-1)/8] &= ~(128>>((K+j-1)%8));
code[(K+j-1)/8] |= ((code[(K+j)/8]>>(7-(K+j)%8))&1)<<(7-(K+j-1)%8);
}
code[(N-1)/8] &= ~(128>>((N-1)%8));
}
}
}
};
template <int NR, int FCR, int MSG, typename GF>
class BoseChaudhuriHocquenghemEncoderReference
{
public:
typedef typename GF::value_type value_type;
typedef typename GF::ValueType ValueType;
typedef typename GF::IndexType IndexType;
static const int N = GF::N, K = MSG, NP = N - K;
ValueType generator[NP+1];
BoseChaudhuriHocquenghemEncoder(std::initializer_list<int> minimal_polynomials)
BoseChaudhuriHocquenghemEncoderReference(std::initializer_list<int> minimal_polynomials)
{
// $generator(x) = \prod_i(minpoly_i(x))$
int generator_degree = 1;

40
tests/bch_encoder_test.cc
View file

@ -12,9 +12,43 @@ Copyright 2018 Ahmet Inan <inan@aicodix.de>
int main()
{
if (1) {
// NASA INTRO BCH(15, 5) T=3
typedef CODE::BoseChaudhuriHocquenghemEncoder<15, 5> BCH;
const int L = (BCH::N + 7) / 8;
BCH encode({0b10011, 0b11111, 0b00111});
uint8_t code[L] = { 0b11001000 };
uint8_t target[L] = { 0b11001000, 0b11110100 };
encode(code);
for (int i = 0; i < L; ++i)
assert(code[i] == target[i]);
}
if (1) {
// DVB-S2 FULL BCH(65535, 65343) T=12
typedef CODE::BoseChaudhuriHocquenghemEncoder<65535, 65343> BCH;
const int L = (BCH::N + 7) / 8;
BCH *encode = new BCH({0b10000000000101101, 0b10000000101110011, 0b10000111110111101, 0b10101101001010101, 0b10001111100101111, 0b11111011110110101, 0b11010111101100101, 0b10111001101100111, 0b10000111010100001, 0b10111010110100111, 0b10011101000101101, 0b10001101011100011});
uint8_t *target = new uint8_t[L];
for (int i = 0; i < L; ++i)
target[i] = 0;
for (int i = 0, s = 0; i < BCH::K; ++i, s=(s*(s*s*51767+71287)+35149)&0xffffff)
target[i/8] |= ((s^=s>>7)&1) << (7-i%8);
uint8_t *code = new uint8_t[L];
for (int i = 0; i < L; ++i)
code[i] = target[i];
bool parity[BCH::NP] = { 0, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 0, 1, 1, 0, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 1, 1, 0, 1, 1, 0, 1, 1, 0, 0, 0, 1, 0, 0, 1, 0, 0, 1, 0, 0, 0, 1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 0, 1, 0, 1, 0, 1, 0, 0, 1, 1, 1, 0, 0, 0, 1, 0, 1, 0, 0, 1, 1, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0, 0, 0, 0, 1, 1, 0, 1, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 1, 1, 0, 1, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 1, 1, 0, 0, 1, 0, 1, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 0, 1, 0, 1, 0, 1, 1, 0, 0 };
for (int i = 0; i < BCH::NP; ++i)
target[(BCH::K+i)/8] |= parity[i] << (7-(BCH::K+i)%8);
(*encode)(code);
for (int i = 0; i < L; ++i)
assert(code[i] == target[i]);
delete[] target;
delete[] code;
delete encode;
}
if (0) {
// NASA INTRO BCH(15, 5) T=3
typedef CODE::GaloisField<4, 0b10011, uint8_t> GF;
typedef CODE::BoseChaudhuriHocquenghemEncoder<6, 1, 5, GF> BCH;
typedef CODE::BoseChaudhuriHocquenghemEncoderReference<6, 1, 5, GF> BCH;
GF instance;
BCH encode({0b10011, 0b11111, 0b00111});
uint8_t code[BCH::N] = { 1, 1, 0, 0, 1 };
@ -23,10 +57,10 @@ int main()
for (int i = 0; i < BCH::N; ++i)
assert(code[i] == target[i]);
}
if (1) {
if (0) {
// DVB-S2 FULL BCH(65535, 65343) T=12
typedef CODE::GaloisField<16, 0b10000000000101101, uint16_t> GF;
typedef CODE::BoseChaudhuriHocquenghemEncoder<24, 1, 65343, GF> BCH;
typedef CODE::BoseChaudhuriHocquenghemEncoderReference<24, 1, 65343, GF> BCH;
GF *instance = new GF();
BCH *encode = new BCH({0b10000000000101101, 0b10000000101110011, 0b10000111110111101, 0b10101101001010101, 0b10001111100101111, 0b11111011110110101, 0b11010111101100101, 0b10111001101100111, 0b10000111010100001, 0b10111010110100111, 0b10011101000101101, 0b10001101011100011});
uint16_t *code = new uint16_t[BCH::N];