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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>
struct ao_soft_sym {
uint8_t a, b;
};
#define NUM_STATE 8
#define NUM_HIST 8
#define MOD_HIST(b) ((b) & 7)
static const struct ao_soft_sym ao_fec_decode_table[NUM_STATE][2] = {
/* next 0 1 state */
{ { 0x00, 0x00 }, { 0xff, 0xff } } , /* 000 */
{ { 0x00, 0xff }, { 0xff, 0x00 } }, /* 001 */
{ { 0xff, 0xff }, { 0x00, 0x00 } }, /* 010 */
{ { 0xff, 0x00 }, { 0x00, 0xff } }, /* 011 */
{ { 0xff, 0xff }, { 0x00, 0x00 } }, /* 100 */
{ { 0xff, 0x00 }, { 0x00, 0xff } }, /* 101 */
{ { 0x00, 0x00 }, { 0xff, 0xff } }, /* 110 */
{ { 0x00, 0xff }, { 0xff, 0x00 } } /* 111 */
};
static inline uint8_t
ao_next_state(uint8_t state, uint8_t bit)
{
return ((state << 1) | bit) & 0x7;
}
static inline uint16_t ao_abs(int16_t x) { return x < 0 ? -x : x; }
static inline uint16_t
ao_cost(struct ao_soft_sym a, struct ao_soft_sym b)
{
return ao_abs(a.a - b.a) + ao_abs(a.b - b.b);
}
/*
* 'in' is 8-bits per symbol soft decision data
* 'len' is input byte length. 'out' must be
* 'len'/16 bytes long
*/
uint8_t
ao_fec_decode(uint8_t *in, uint16_t len, uint8_t *out)
{
static uint16_t cost[2][NUM_STATE]; /* path cost */
static uint16_t bits[2][NUM_STATE]; /* save bits to quickly output them */
uint16_t i; /* input byte index */
uint16_t b; /* encoded symbol index (bytes/2) */
uint16_t o; /* output bit index */
uint8_t p; /* previous cost/bits index */
uint8_t n; /* next cost/bits index */
uint8_t state; /* state index */
uint8_t bit; /* original encoded bit index */
p = 0;
for (state = 0; state < NUM_STATE; state++) {
cost[0][state] = 0xffff;
bits[0][state] = 0;
}
cost[0][0] = 0;
o = 0;
for (i = 0; i < len; i += 2) {
b = i/2;
n = p ^ 1;
struct ao_soft_sym s = { .a = in[i], .b = in[i+1] };
/* Reset next costs to 'impossibly high' values so that
* the first path through this state is cheaper than this
*/
for (state = 0; state < NUM_STATE; state++)
cost[n][state] = 0xffff;
/* Compute path costs and accumulate output bit path
* for each state and encoded bit value
*/
for (state = 0; state < NUM_STATE; state++) {
for (bit = 0; bit < 2; bit++) {
int bit_cost = cost[p][state] + ao_cost(s, ao_fec_decode_table[state][bit]);
uint8_t bit_state = ao_next_state(state, bit);
/* Only track the minimal cost to reach
* this state; the best path can never
* go through the higher cost paths as
* total path cost is cumulative
*/
if (bit_cost < cost[n][bit_state]) {
cost[n][bit_state] = bit_cost;
bits[n][bit_state] = (bits[p][state] << 1) | (state & 1);
}
}
}
#if 0
printf ("bit %3d symbol %2x %2x:", i/2, s.a, s.b);
for (state = 0; state < NUM_STATE; state++) {
printf (" %5d(%04x)", cost[n][state], bits[n][state]);
}
printf ("\n");
#endif
p = n;
/* A loop is needed to handle the last output byte. It
* won't have a full NUM_HIST bits of future data to
* perform full error correction, but we might as well
* give the best possible answer anyways.
*/
while ((b - o) >= (8 + NUM_HIST) || (i + 2 >= len && b > o)) {
/* Compute number of bits to the end of the
* last full byte of data. This is generally
* NUM_HIST, unless we've reached
* the end of the input, in which case
* it will be seven.
*/
int8_t dist = b - (o + 8); /* distance to last ready-for-writing bit */
uint16_t min_cost; /* lowest cost */
uint8_t min_state; /* lowest cost state */
/* Find the best fit at the current point
* of the decode.
*/
min_cost = cost[p][0];
min_state = 0;
for (state = 1; state < NUM_STATE; state++) {
if (cost[p][state] < min_cost) {
min_cost = cost[p][state];
min_state = state;
}
}
/* The very last byte of data has the very last bit
* of data left in the state value; just smash the
* bits value in place and reset the 'dist' from
* -1 to 0 so that the full byte is read out
*/
if (dist < 0) {
bits[p][min_state] = (bits[p][min_state] << 1) | (min_state & 1);
dist = 0;
}
#if 0
printf ("\tbit %3d min_cost %5d old bit %3d old_state %x bits %02x\n",
i/2, min_cost, o + 8, min_state, (bits[p][min_state] >> dist) & 0xff);
#endif
out[o >> 3] = bits[p][min_state] >> dist;
o += 8;
}
}
return len/16;
}
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