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/*
* Copyright © 2012 Keith Packard <keithp@keithp.com>
*
* 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; version 2 of the License.
*
* 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, write to the Free Software Foundation, Inc.,
* 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
*/
#include <ao_fec.h>
#include <stdio.h>
/*
* 'input' is 8-bits per symbol soft decision data
* 'len' is output byte length
*/
static const uint8_t ao_fec_encode_table[16] = {
/* next 0 1 state */
0, 3, /* 000 */
1, 2, /* 001 */
3, 0, /* 010 */
2, 1, /* 011 */
3, 0, /* 100 */
2, 1, /* 101 */
0, 3, /* 110 */
1, 2 /* 111 */
};
struct ao_soft_sym {
uint8_t a, b;
};
struct ao_soft_sym
ao_soft_sym(uint8_t bits)
{
struct ao_soft_sym s;
s.a = ((bits & 2) >> 1) * 0xff;
s.b = (bits & 1) * 0xff;
return s;
}
uint8_t
ao_next_state(uint8_t state, uint8_t bit)
{
return ((state << 1) | bit) & 0x7;
}
static inline abs(int x) { return x < 0 ? -x : x; }
int
ao_cost(struct ao_soft_sym a, struct ao_soft_sym b)
{
return abs(a.a - b.a) + abs(a.b - b.b);
}
uint8_t
ao_fec_decode(uint8_t *in, int len, uint8_t *out)
{
int cost[len/2 + 1][8];
uint8_t prev[len/2 + 1][8];
int c;
int i, b;
uint8_t state = 0, min_state;
uint8_t bits[len/2];
for (c = 0; c < 8; c++)
cost[0][c] = 10000;
cost[0][0] = 0;
for (i = 0; i < len; i += 2) {
b = i/2;
struct ao_soft_sym s = { .a = in[i], .b = in[i+1] };
for (state = 0; state < 8; state++)
cost[b+1][state] = 10000;
for (state = 0; state < 8; state++) {
struct ao_soft_sym zero = ao_soft_sym(ao_fec_encode_table[state * 2 + 0]);
struct ao_soft_sym one = ao_soft_sym(ao_fec_encode_table[state * 2 + 1]);
uint8_t zero_state = ao_next_state(state, 0);
uint8_t one_state = ao_next_state(state, 1);
int zero_cost = ao_cost(s, zero);
int one_cost = ao_cost(s, one);
#if 0
printf ("saw %02x %02x expected %02x %02x (%d) or %02x %02x (%d)\n",
s.a, s.b, zero.a, zero.b, zero_cost, one.a, one.b, one_cost);
#endif
zero_cost += cost[b][state];
one_cost += cost[b][state];
if (zero_cost < cost[b+1][zero_state]) {
prev[b+1][zero_state] = state;
cost[b+1][zero_state] = zero_cost;
}
if (one_cost < cost[b+1][one_state]) {
prev[b+1][one_state] = state;
cost[b+1][one_state] = one_cost;
}
}
printf ("bit %3d symbol %2x %2x:", i/2, s.a, s.b);
for (state = 0; state < 8; state++) {
printf (" %5d", cost[b+1][state]);
}
printf ("\n");
}
b = len / 2;
c = cost[b][0];
min_state = 0;
for (state = 1; state < 8; state++) {
if (cost[b][state] < c) {
c = cost[b][state];
min_state = state;
}
}
for (b = len/2; b > 0; b--) {
bits[b-1] = min_state & 1;
min_state = prev[b][min_state];
}
for (i = 0; i < len/2; i += 8) {
uint8_t byte;
byte = 0;
for (b = 0; b < 8; b++)
byte = (byte << 1) | bits[i + b];
out[i/8] = byte;
}
return len/16;
}
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