mirror of
https://github.com/f4exb/sdrangel.git
synced 2024-11-23 08:28:36 -05:00
270 lines
8.7 KiB
C++
270 lines
8.7 KiB
C++
// This file is part of LeanSDR Copyright (C) 2016-2018 <pabr@pabr.org>.
|
|
// See the toplevel README for more information.
|
|
//
|
|
// This program is free software: you can redistribute it and/or modify
|
|
// it under the terms of the GNU General Public License as published by
|
|
// the Free Software Foundation, either version 3 of the License, or
|
|
// (at your option) any later version.
|
|
//
|
|
// This program is distributed in the hope that it will be useful,
|
|
// but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
|
|
// GNU General Public License for more details.
|
|
//
|
|
// You should have received a copy of the GNU General Public License
|
|
// along with this program. If not, see <http://www.gnu.org/licenses/>.
|
|
|
|
#ifndef LEANSDR_BCH_H
|
|
#define LEANSDR_BCH_H
|
|
|
|
#include "leansdr/discrmath.h"
|
|
|
|
namespace leansdr
|
|
{
|
|
|
|
// Interface to hide the template parameters
|
|
|
|
struct bch_interface
|
|
{
|
|
virtual void encode(const uint8_t *msg, size_t msgbytes, uint8_t *out) = 0;
|
|
virtual int decode(uint8_t *cw, size_t cwbytes) = 0;
|
|
}; // bch_interface
|
|
|
|
// BCH error correction.
|
|
// T: Unsigned type for packing binary polynomials.
|
|
// N: Number of parity bits.
|
|
// NP: Width of the polynomials supplied.
|
|
// DP: Actual degree of the minimum polynomials (all must be same).
|
|
// TGF: Unsigned type for syndromes (must be wider than DP).
|
|
// GFTRUNCGEN: Generator polynomial for GF(2^DP), with X^DP omitted.
|
|
|
|
template <typename T, int N, int NP, int DP, typename TGF, int GFTRUNCGEN>
|
|
struct bch_engine : bch_interface
|
|
{
|
|
bch_engine(const bitvect<T, NP> *polys, int _npolys)
|
|
: npolys(_npolys)
|
|
{
|
|
// Build the generator polynomial (product of polys[]).
|
|
g = 1;
|
|
for (int i = 0; i < npolys; ++i)
|
|
g = g * polys[i];
|
|
// Convert the polynomials to truncated representation
|
|
// (with X^DP omitted) for use with divmod().
|
|
truncpolys = new bitvect<T, DP>[npolys];
|
|
for (int i = 0; i < npolys; ++i)
|
|
truncpolys[i].copy(polys[i]);
|
|
// Check which polynomial contains each root.
|
|
// Note: The DVB-S2 polynomials are numbered so that
|
|
// syndpoly[2*i]==i, but we don't use that property.
|
|
syndpolys = new int[2 * npolys];
|
|
for (int i = 0; i < 2 * npolys; ++i)
|
|
{
|
|
int j;
|
|
for (j = 0; j < npolys; ++j)
|
|
if (!eval_poly(truncpolys[j], true, 1 + i))
|
|
break;
|
|
if (j == npolys)
|
|
fail("Bad polynomials/root");
|
|
syndpolys[i] = j;
|
|
}
|
|
}
|
|
|
|
// Generate BCH parity bits.
|
|
|
|
void encode(const uint8_t *msg, size_t msgbytes, uint8_t *out)
|
|
{
|
|
bitvect<T, N> parity = shiftdivmod(msg, msgbytes, g);
|
|
// Output as bytes, coefficient of highest degree first
|
|
for (int i = N / 8; i--; ++out)
|
|
*out = parity.v[i / sizeof(T)] >> ((i & (sizeof(T) - 1)) * 8);
|
|
}
|
|
|
|
// Decode BCH.
|
|
// Return number of bits corrected, or -1 on failure.
|
|
|
|
int decode(uint8_t *cw, size_t cwbytes)
|
|
{
|
|
//again:
|
|
bool corrupted = false;
|
|
// Divide by individual polynomials.
|
|
// TBD Maybe do in parallel, scanning cw only once.
|
|
bitvect<T, DP> *rem = new bitvect<T, DP>[npolys]; // npolys is not static hence 'bitvect<T, DP> rem[npolys]' does not compile in all compilers
|
|
for (int j = 0; j < npolys; ++j)
|
|
{
|
|
rem[j] = divmod(cw, cwbytes, truncpolys[j]);
|
|
}
|
|
// Compute syndromes.
|
|
TGF *S = new TGF[2 * npolys]; // npolys is not static hence 'TGF S[2 * npolys]' does not compile in all compilers
|
|
for (int i = 0; i < 2 * npolys; ++i)
|
|
{
|
|
// Compute R(alpha^(1+i)), exploiting the fact that
|
|
// R(x)=Q(x)g_j(X)+rem_j(X) and g_j(alpha^(1+i))=0
|
|
// for some j that we already determined.
|
|
// TBD Compute even exponents using conjugates.
|
|
S[i] = eval_poly(rem[syndpolys[i]], false, 1 + i);
|
|
if (S[i])
|
|
corrupted = true;
|
|
}
|
|
if (!corrupted)
|
|
{
|
|
delete[] S;
|
|
delete[] rem;
|
|
return 0;
|
|
}
|
|
#if 0
|
|
fprintf(stderr, "synd:");
|
|
for ( int i=0; i<2*npolys; ++i ) fprintf(stderr, " %04x", S[i]);
|
|
fprintf(stderr, "\n");
|
|
#endif
|
|
|
|
// S_j = R(alpha_j) = 0+E(alpha_j) = sum(l=1..L)((alpha^j)^i_l)
|
|
// where i_1 .. i_L are the degrees of the non-zero coefficients of E.
|
|
// S_j = sum(l=1..L)((alpha^i_l)^j) = sum(l=1..L)(X_l^j)
|
|
|
|
// Berlekamp - Massey
|
|
// http://en.wikipedia.org/wiki/Berlekamp%E2%80%93Massey_algorithm
|
|
// TBD More efficient to work with logs of syndromes ?
|
|
|
|
int NN = 2 * npolys;
|
|
TGF *C = new TGF[NN];
|
|
C[0] = 1;
|
|
TGF *B = new TGF[NN];
|
|
B[0] = 1;
|
|
// TGF C[NN] = { crap code
|
|
// 1,
|
|
// },
|
|
// B[NN] = {
|
|
// 1,
|
|
// };
|
|
int L = 0, m = 1;
|
|
TGF b = 1;
|
|
for (int n = 0; n < NN; ++n)
|
|
{
|
|
TGF d = S[n];
|
|
for (int i = 1; i <= L; ++i)
|
|
d = GF.add(d, GF.mul(C[i], S[n - i]));
|
|
if (d == 0)
|
|
++m;
|
|
else
|
|
{
|
|
TGF d_div_b = GF.mul(d, GF.inv(b));
|
|
if (2 * L <= n)
|
|
{
|
|
TGF *tmp = new TGF[NN]; // replaced crap code
|
|
std::copy(C, C+NN, tmp); //memcpy(tmp, C, sizeof(tmp));
|
|
for (int i = 0; i < NN - m; ++i)
|
|
C[m + i] = GF.sub(C[m + i], GF.mul(d_div_b, B[i]));
|
|
L = n + 1 - L;
|
|
std::copy(tmp, tmp+NN, B); //memcpy(B, tmp, sizeof(B));
|
|
b = d;
|
|
m = 1;
|
|
delete[] tmp;
|
|
}
|
|
else
|
|
{
|
|
for (int i = 0; i < NN - m; ++i)
|
|
C[m + i] = GF.sub(C[m + i], GF.mul(d_div_b, B[i]));
|
|
++m;
|
|
}
|
|
}
|
|
}
|
|
// L is the number of errors.
|
|
// C of degree L is the error locator polynomial (Lambda).
|
|
// C(X) = sum(l=1..L)(1-X_l*X).
|
|
#if 0
|
|
fprintf(stderr, "C[%d]=", L);
|
|
for ( int i=0; i<NN; ++i ) fprintf(stderr, " %04x", C[i]);
|
|
fprintf(stderr, "\n");
|
|
#endif
|
|
|
|
// Forney
|
|
// http://en.wikipedia.org/wiki/Forney_algorithm
|
|
// Simplified because coefficients are in GF(2).
|
|
|
|
// Find zeroes of C by exhaustive search.
|
|
// TODO Chien method
|
|
int roots_found = 0;
|
|
for (int i = 0; i < (1 << DP) - 1; ++i)
|
|
{
|
|
// Candidate root ALPHA^i
|
|
TGF v = eval_poly(C, L, i);
|
|
if (!v)
|
|
{
|
|
// ALPHA^i is a root of C, i.e. the inverse of an X_l.
|
|
int loc = (i ? (1 << DP) - 1 - i : 0); // exponent of inverse
|
|
// Reverse because cw[0..cwbytes-1] is stored MSB first
|
|
int rloc = cwbytes * 8 - 1 - loc;
|
|
if (rloc < 0)
|
|
{
|
|
// This may happen if the code is used truncated.
|
|
delete[] C;
|
|
delete[] B;
|
|
delete[] S;
|
|
delete[] rem;
|
|
return -1;
|
|
}
|
|
cw[rloc / 8] ^= 128 >> (rloc & 7);
|
|
++roots_found;
|
|
if (roots_found == L)
|
|
break;
|
|
}
|
|
}
|
|
|
|
delete[] C;
|
|
delete[] B;
|
|
delete[] S;
|
|
delete[] rem;
|
|
|
|
if (roots_found != L)
|
|
return -1;
|
|
return L;
|
|
}
|
|
|
|
private:
|
|
// Eval a GF(2)[X] polynomial at a power of ALPHA.
|
|
|
|
TGF eval_poly(const bitvect<T, DP> &poly, bool is_trunc, int rootexp)
|
|
{
|
|
TGF acc = 0;
|
|
int re = 0;
|
|
for (int i = 0; i < DP; ++i)
|
|
{
|
|
if (poly[i])
|
|
acc = GF.add(acc, GF.exp(re));
|
|
re += rootexp;
|
|
if (re >= (1 << DP) - 1)
|
|
re -= (1 << DP) - 1; // mod 2^DP-1 incrementally
|
|
}
|
|
if (is_trunc)
|
|
acc = GF.add(acc, GF.exp(re));
|
|
return acc;
|
|
}
|
|
|
|
// Eval a GF(2^16)[X] polynomial at a power of ALPHA.
|
|
|
|
TGF eval_poly(const TGF *poly, int deg, int rootexp)
|
|
{
|
|
TGF acc = 0;
|
|
int re = 0;
|
|
for (int i = 0; i <= deg; ++i)
|
|
{
|
|
acc = GF.add(acc, GF.mul(poly[i], GF.exp(re)));
|
|
re += rootexp;
|
|
if (re >= (1 << DP) - 1)
|
|
re -= (1 << DP) - 1; // mod 2^DP-1 incrementally
|
|
}
|
|
return acc;
|
|
}
|
|
|
|
bitvect<T, DP> *truncpolys;
|
|
int npolys;
|
|
int *syndpolys;
|
|
bitvect<T, N> g;
|
|
// Finite group for syndrome calculations
|
|
gf2n<TGF, DP, 2, GFTRUNCGEN> GF;
|
|
}; // bch_engine
|
|
|
|
} // namespace leansdr
|
|
|
|
#endif // LEANSDR_BCH_H
|