mirror of
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647 lines
18 KiB
C++
647 lines
18 KiB
C++
// This file is part of LeanSDR Copyright (C) 2016-2018 <pabr@pabr.org>.
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// See the toplevel README for more information.
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//
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// This program is free software: you can redistribute it and/or modify
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// it under the terms of the GNU General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program. If not, see <http://www.gnu.org/licenses/>.
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#ifndef LEANSDR_LDPC_H
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#define LEANSDR_LDPC_H
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#define lfprintf(...) \
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{ \
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}
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namespace leansdr
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{
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// LDPC sparse matrix specified like in the DVB-S2 standard
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// Taddr must be wide enough to index message bits and address bits.
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template <typename Taddr>
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struct ldpc_table
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{
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// TBD Save space
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static const int MAX_ROWS = 162; // 64800 * (9/10) / 360
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static const int MAX_COLS = 13;
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int q;
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int nrows;
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struct row
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{
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int ncols;
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Taddr cols[MAX_COLS];
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} rows[MAX_ROWS];
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};
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// LDPC ENGINE
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// SOFTBITs can be hard (e.g. bool) or soft (e.g. llr_t).
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// They are stored as SOFTWORDs containing SWSIZE SOFTBITs.
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// See interface in softword.h.
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template <typename SOFTBIT, typename SOFTWORD, int SWSIZE, typename Taddr>
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struct ldpc_engine
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{
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// vnodes: Value/variable nodes (message bits)
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// cnodes: Check nodes (parity bits)
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int k; // Message size in bits
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int n; // Codeword size in bits
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struct node
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{
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Taddr *edges;
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int nedges;
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static const int CHUNK = 4; // Grow edges[] in steps of CHUNK.
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void append(Taddr a)
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{
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if (nedges % CHUNK == 0)
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{ // Full ?
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edges = (Taddr *)realloc(edges, (nedges + CHUNK) * sizeof(Taddr));
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if (!edges) {
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fatal("realloc");
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}
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}
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edges[nedges++] = a;
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}
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};
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node *vnodes; // [k]
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node *cnodes; // [n-k]
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ldpc_engine() :
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vnodes(nullptr),
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cnodes(nullptr)
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{
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}
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// Initialize from a S2-style table.
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ldpc_engine(
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const ldpc_table<Taddr> *table,
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int _k,
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int _n
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) :
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k(_k),
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n(_n)
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{
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// Sanity checks
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if (360 % SWSIZE) {
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fatal("Bad LDPC word size");
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}
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if (k % SWSIZE) {
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fatal("Bad LDPC k");
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}
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if (n % SWSIZE) {
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fatal("Bad LDPC n");
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}
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if (k != table->nrows * 360) {
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fatal("Bad table");
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}
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int n_k = n - k;
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if (table->q * 360 != n_k) {
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fatal("Bad q");
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}
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vnodes = new node[k];
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memset(vnodes, 0, sizeof(node) * k);
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cnodes = new node[n_k];
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memset(cnodes, 0, sizeof(node) * n_k);
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// Expand the graph.
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int m = 0;
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// Iterate over rows
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for (const typename ldpc_table<Taddr>::row *prow = table->rows;
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prow < table->rows + table->nrows;
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++prow)
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{
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// Process 360 bits per row.
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int q = table->q;
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int qoffs = 0;
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for (int mw = 360; mw--; ++m, qoffs += q)
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{
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const Taddr *pa = prow->cols;
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for (int nc = prow->ncols; nc--; ++pa)
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{
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int a = (int)*pa + qoffs;
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if (a >= n_k) {
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a -= n_k; // Modulo n-k. Note qoffs<360*q.
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}
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if (a >= n_k) {
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fail("Invalid LDPC table");
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}
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vnodes[m].append(a);
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cnodes[a].append(m);
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}
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}
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}
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}
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~ldpc_engine()
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{
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if (vnodes) {
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delete[] vnodes;
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}
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if (cnodes) {
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delete[] cnodes;
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}
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}
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void print_node_stats()
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{
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int nedges = count_edges(vnodes, k);
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fprintf(stderr, "LDPC(%5d,%5d)(%.2f)"
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" %5.2f edges/vnode, %5.2f edges/cnode\n",
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k, n - k, (float)k / n, (float)nedges / k, (float)nedges / (n - k));
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}
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int count_edges(node *nodes, int nnodes)
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{
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int c = 0;
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for (int i = 0; i < nnodes; ++i) {
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c += nodes[i].nedges;
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}
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return c;
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}
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// k: Message size in bits
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// n: Codeword size in bits
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// integrate: Optional S2-style post-processing
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#if 0
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void encode_hard(const ldpc_table<Taddr> *table, const uint8_t *msg,
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int k, int n, uint8_t *parity, bool integrate=true) {
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// Sanity checks
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if ( 360 % SWSIZE ) fatal("Bad LDPC word size");
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if ( k % SWSIZE ) fatal("Bad LDPC k");
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if ( n % SWSIZE ) fatal("Bad LDPC n");
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if ( k != table->nrows*360 ) fatal("Bad table");
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int n_k = n - k;
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if ( table->q*360 != n_k ) fatal("Bad q");
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for ( int i=0; i<n_k/SWSIZE; ++i ) softword_zero(&parity[i]);
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// Iterate over rows
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for ( const typename ldpc_table<Taddr>::row *prow = table->rows; // quirk
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prow < table->rows+table->nrows;
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++prow ) {
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// Process 360 bits per row, in words of SWSIZE bits
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int q = table->q;
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int qoffs = 0;
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for ( int mw=360/SWSIZE; mw--; ++msg ) {
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SOFTWORD msgword = *msg;
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for ( int wbit=0; wbit<SWSIZE; ++wbit,qoffs+=q ) {
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SOFTBIT msgbit = softword_get(msgword, wbit);
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if ( ! msgbit ) continue; // TBD Generic soft version
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const Taddr *pa = prow->cols;
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for ( int nc=prow->ncols; nc--; ++pa ) {
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// Don't wrap modulo range of Taddr
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int a = (int)*pa + qoffs;
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// Note: qoffs < 360*q=n-k
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if ( a >= n_k ) a -= n_k; // TBD not predictable
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softwords_flip(parity, a);
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}
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}
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}
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}
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if ( integrate )
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integrate_bits(parity, parity, n_k/SWSIZE);
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}
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#endif
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void encode(
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const ldpc_table<Taddr> *table,
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const SOFTWORD *msg,
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int k,
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int n,
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SOFTWORD *parity,
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int integrate = true
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)
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{
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// Sanity checks
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if (360 % SWSIZE) {
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fatal("Bad LDPC word size");
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}
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if (k % SWSIZE) {
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fatal("Bad LDPC k");
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}
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if (n % SWSIZE) {
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fatal("Bad LDPC n");
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}
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if (k != table->nrows * 360) {
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fatal("Bad table");
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}
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int n_k = n - k;
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if (table->q * 360 != n_k) {
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fatal("Bad q");
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}
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for (int i = 0; i < n_k / SWSIZE; ++i) {
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softword_zero(&parity[i]);
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}
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// Iterate over rows
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for (const typename ldpc_table<Taddr>::row *prow = table->rows; // quirk
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prow < table->rows + table->nrows;
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++prow)
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{
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// Process 360 bits per row, in words of SWSIZE bits
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int q = table->q;
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int qoffs = 0;
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for (int mw = 360 / SWSIZE; mw--; ++msg)
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{
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SOFTWORD msgword = *msg;
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for (int wbit = 0; wbit < SWSIZE; ++wbit, qoffs += q)
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{
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SOFTBIT msgbit = softword_get(msgword, wbit);
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if (!softbit_harden(msgbit)) {
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continue;
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}
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const Taddr *pa = prow->cols;
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for (int nc = prow->ncols; nc--; ++pa)
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{
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int a = (int)*pa + qoffs;
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// Note: qoffs < 360*q=n-k
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if (a >= n_k) {
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a -= n_k; // TBD not predictable
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}
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softwords_flip(parity, a);
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}
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}
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}
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}
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if (integrate) {
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integrate_bits(parity, parity, n_k / SWSIZE);
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}
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}
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// Flip bits connected to parity errors, one at a time,
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// as long as things improve and max_bitflips is not exceeded.
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// cw: codeword (k value bits followed by n-k check bits)
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static const int PPCM = 39;
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typedef int64_t score_t;
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score_t compute_scores(
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SOFTWORD *m,
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SOFTWORD *p,
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SOFTWORD *q,
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int nc,
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score_t *score,
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int k)
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{
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int total = 0;
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memset(score, 0, k * sizeof(*score));
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for (int c = 0; c < nc; ++c)
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{
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SOFTBIT err = softwords_xor(p, q, c);
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if (softbit_harden(err))
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{
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Taddr *pe = cnodes[c].edges;
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for (int e = cnodes[c].nedges; e--; ++pe)
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{
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int v = *pe;
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int s = err * softwords_weight<SOFTBIT, SOFTWORD>(m, v) * PPCM / vnodes[v].nedges;
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//fprintf(stderr, "c[%d] bad => v[%d] += %d (%d*%d)\n",
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///c, v, s, err, softwords_weight<SOFTBIT,SOFTWORD>(m,*pe));
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score[v] += s;
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total += s;
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}
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}
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}
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return total;
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}
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int decode_bitflip(
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const ldpc_table<Taddr> *table,
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SOFTWORD *cw,
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int k,
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int n,
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int max_bitflips)
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{
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if (!vnodes) {
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fail("LDPC graph not initialized");
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}
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int n_k = n - k;
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// Compute the expected check bits (without the final mixing)
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SOFTWORD *expected = new SOFTWORD[n_k / SWSIZE]; // Forbidden to statically allocate with non constant size
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encode(table, cw, k, n, expected, false);
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// Reverse the integrator mixing from the received check bits
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SOFTWORD *received = new SOFTWORD[n_k / SWSIZE]; // Forbidden to statically allocate with non constant size
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diff_bits(cw + k / SWSIZE, received, n_k / SWSIZE);
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// Compute initial scores
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score_t *score = new score_t[k]; // Forbidden to statically allocate with non constant size
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score_t tots = compute_scores(cw, expected, received, n_k, score, k);
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lfprintf(stderr, "Initial score %d\n", (int)tots);
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int nflipped = 0;
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score_t score_threshold;
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{
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SOFTBIT one;
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softbit_set(&one, true);
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score_threshold = (int)one * 2;
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}
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bool progress = true;
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while (progress && nflipped < max_bitflips)
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{
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progress = false;
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// Try to flip parity bits.
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// Due to differential decoding, they appear as consecutive errors.
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SOFTBIT prev_err = softwords_xor(expected, received, 0);
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for (int b = 0; b < n - k - 1; ++b)
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{
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prev_err = softwords_xor(expected, received, b); //TBD
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SOFTBIT err = softwords_xor(expected, received, b + 1);
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if (softbit_harden(prev_err) && softbit_harden(err))
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{
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lfprintf(stderr, "flip parity %d\n", b);
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softwords_flip(received, b);
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softwords_flip(received, b + 1);
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++nflipped; // Counts as one flip before differential decoding.
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progress = true;
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int dtot = 0;
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// Depenalize adjacent message bits.
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{
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Taddr *pe = cnodes[b].edges;
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for (int e = cnodes[b].nedges; e--; ++pe)
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{
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int d = prev_err * softwords_weight<SOFTBIT, SOFTWORD>(cw, *pe) * PPCM / vnodes[*pe].nedges;
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score[*pe] -= d;
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dtot -= d;
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}
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}
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{
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Taddr *pe = cnodes[b + 1].edges;
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for (int e = cnodes[b + 1].nedges; e--; ++pe)
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{
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int d = err * softwords_weight<SOFTBIT, SOFTWORD>(cw, *pe) * PPCM / vnodes[*pe].nedges;
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score[*pe] -= d;
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dtot -= d;
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}
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}
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tots += dtot;
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#if 1
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// Also update the codeword in-place.
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// TBD Useful for debugging only.
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softwords_flip(cw, k + b);
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#endif
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// TBD version soft. err = ! err;
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}
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prev_err = err;
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} // c nodes
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score_t maxs = -(1 << 30);
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for (int v = 0; v < k; ++v)
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{
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if (score[v] > maxs) {
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maxs = score[v];
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}
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}
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if (!maxs) {
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break;
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}
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lfprintf(stderr, "maxs %d\n", (int)maxs);
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// Try to flip each message bits with maximal score
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for (int v = 0; v < k; ++v)
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{
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if (score[v] < score_threshold) {
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continue;
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}
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// if ( score[v] < maxs*9/10 ) continue;
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if (score[v] < maxs - 4) {
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continue;
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}
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lfprintf(stderr, " flip %d score=%d\n", (int)v, (int)score[v]);
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// Update expected parities and scores that depend on them.
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score_t dtot = 0;
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for (int commit = 0; commit <= 1; ++commit)
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{
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Taddr *pe = vnodes[v].edges;
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for (int e = vnodes[v].nedges; e--; ++pe)
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{
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Taddr c = *pe;
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SOFTBIT was_bad = softwords_xor(expected, received, c);
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if (softbit_harden(was_bad))
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{
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Taddr *pe = cnodes[c].edges;
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for (int e = cnodes[c].nedges; e--; ++pe)
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{
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int d = was_bad * softwords_weight<SOFTBIT, SOFTWORD>(cw, *pe) * PPCM / vnodes[*pe].nedges;
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if (commit) {
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score[*pe] -= d;
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} else {
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dtot -= d;
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}
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}
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}
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softwords_flip(expected, c);
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SOFTBIT is_bad = softwords_xor(expected, received, c);
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if (softbit_harden(is_bad))
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{
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Taddr *pe = cnodes[c].edges;
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for (int e = cnodes[c].nedges; e--; ++pe)
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{
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int d = is_bad * softwords_weight<SOFTBIT, SOFTWORD>(cw, *pe) * PPCM / vnodes[*pe].nedges;
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if (commit) {
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score[*pe] += d;
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} else {
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dtot += d;
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}
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}
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}
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if (!commit) {
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softwords_flip(expected, c);
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}
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}
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if (!commit)
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{
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if (dtot >= 0)
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{
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lfprintf(stderr, " abort %d\n", v);
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break; // Next v
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}
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}
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else
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{
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softwords_flip(cw, v);
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++nflipped;
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tots += dtot;
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progress = true;
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v = k - 1; // Force exit to update maxs ?
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}
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} // commit
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} // v
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lfprintf(stderr, "progress %d\n", progress);
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#if 0
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fprintf(stderr, "CHECKING TOTS INCREMENT (slow) %d\n", tots);
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score_t tots2 = compute_scores(cw, expected, received, n_k, score, k);
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if ( tots2 != tots ) fail("bad tots update");
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#endif
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}
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delete[] score;
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delete[] received;
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delete[] expected;
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return nflipped;
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}
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// EN 302 307-1 5.3.2.1 post-processing of parity bits.
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// In-place allowed.
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#if 1
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static void integrate_bits(const SOFTWORD *in, SOFTWORD *out, int nwords)
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{
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SOFTBIT sum;
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softbit_clear(&sum);
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for (int i = 0; i < nwords; ++i)
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{
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SOFTWORD w = in[i];
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for (int b = 0; b < SWSIZE; ++b)
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{
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sum = softbit_xor(sum, softword_get(w, b));
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softword_write(w, b, sum);
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}
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out[i] = w;
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}
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}
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#else
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// Optimized for hard_sb
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static void integrate_bits(const uint8_t *in, uint8_t *out, int nwords)
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{
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// TBD Optimize
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uint8_t prev = 0;
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for (int i = 0; i < nwords; ++i)
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{
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uint8_t c = in[i];
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for (int j = SWSIZE; j--;)
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{
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c ^= prev << j;
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prev = (c >> j) & 1;
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}
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out[i] = c;
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}
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}
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#endif
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// Undo EN 302 307-1 5.3.2.1, post-processing of parity bits.
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// In-place allowed.
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|
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#if 1
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static void diff_bits(const SOFTWORD *in, SOFTWORD *out, int nwords)
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{
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SOFTBIT prev;
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softbit_clear(&prev);
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|
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for (int i = 0; i < nwords; ++i, ++in, ++out)
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{
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SOFTWORD w = *in;
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|
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for (int b = 0; b < SWSIZE; ++b)
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{
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SOFTBIT n = softword_get(w, b);
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softword_write(w, b, softbit_xor(prev, n));
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prev = n;
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}
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|
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*out = w;
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}
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}
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#else
|
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// Optimized for hard_sb
|
|
static void diff_bits(const uint8_t *in, uint8_t *out, int nwords)
|
|
{
|
|
uint8_t prev = 0;
|
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for (int i = 0; i < nwords; ++i)
|
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{
|
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uint8_t c = in[i];
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out[i] = c ^ (prev | (c >> 1));
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prev = (c & 1) << (SWSIZE - 1);
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}
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}
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#endif
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}; // ldpc_engine
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} // namespace leansdr
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#endif // LEANSDR_LDPC_H
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