+++ /dev/null
-/*
- * 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>
-
-#ifdef TELEMEGA
-#include <ao.h>
-#endif
-
-#if AO_PROFILE
-#include <ao_profile.h>
-
-uint32_t ao_fec_decode_start, ao_fec_decode_end;
-#endif
-
-/*
- * byte order repeats through 3 2 1 0
- *
- * bit-pair order repeats through
- *
- * 1/0 3/2 5/4 7/6
- *
- * So, the over all order is:
- *
- * 3,1/0 2,1/0 1,1/0 0,1/0
- * 3,3/2 2,3/2 1,3/2 0,3/2
- * 3,5/4 2,5/4 1,5/4 0,5/4
- * 3,7/6 2,7/6 1,7/6 0,7/6
- *
- * The raw bit order is thus
- *
- * 1e/1f 16/17 0e/0f 06/07
- * 1c/1d 14/15 0c/0d 04/05
- * 1a/1b 12/13 0a/0b 02/03
- * 18/19 10/11 08/09 00/01
- */
-
-static const uint8_t ao_interleave_order[] = {
- 0x1e, 0x16, 0x0e, 0x06,
- 0x1c, 0x14, 0x0c, 0x04,
- 0x1a, 0x12, 0x0a, 0x02,
- 0x18, 0x10, 0x08, 0x00
-};
-
-static inline uint16_t ao_interleave_index(uint16_t i) {
- return (i & ~0x1e) | ao_interleave_order[(i & 0x1e) >> 1];
-}
-
-#define NUM_STATE 8
-#define NUM_HIST 24
-
-typedef uint32_t bits_t;
-
-#define V_0 0xff
-#define V_1 0x00
-
-/*
- * These are just the 'zero' states; the 'one' states mirror them
- */
-static const uint8_t ao_fec_decode_table[NUM_STATE*2] = {
- V_0, V_0, /* 000 */
- V_0, V_1, /* 001 */
- V_1, V_1, /* 010 */
- V_1, V_0, /* 011 */
- V_1, V_1, /* 100 */
- V_1, V_0, /* 101 */
- V_0, V_0, /* 110 */
- V_0, V_1 /* 111 */
-};
-
-static inline uint8_t
-ao_next_state(uint8_t state, uint8_t bit)
-{
- return ((state << 1) | bit) & 0x7;
-}
-
-/*
- * '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(const uint8_t *in, uint16_t len, uint8_t *out, uint8_t out_len, uint16_t (*callback)(void))
-{
- static uint32_t cost[2][NUM_STATE]; /* path cost */
- static bits_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 */
- const uint8_t *whiten = ao_fec_whiten_table;
- uint16_t interleave; /* input byte array index */
- uint8_t s0, s1;
- uint16_t avail;
- uint16_t crc = AO_FEC_CRC_INIT;
-#if AO_PROFILE
- uint32_t start_tick;
-#endif
-
- p = 0;
- for (state = 0; state < NUM_STATE; state++) {
- cost[0][state] = 0x7fffffff;
- bits[0][state] = 0;
- }
- cost[0][0] = 0;
-
- if (callback)
- avail = 0;
- else
- avail = len;
-
-#if AO_PROFILE
- if (!avail) {
- avail = callback();
- if (!avail)
- return 0;
- }
- start_tick = ao_profile_tick();
-#endif
- o = 0;
- for (i = 0; i < len; i += 2) {
- b = i/2;
- n = p ^ 1;
-
- if (!avail) {
- avail = callback();
- if (!avail)
- return 0;
- }
-
- /* Fetch one pair of input bytes, de-interleaving
- * the input.
- */
- interleave = ao_interleave_index(i);
- s0 = in[interleave];
- s1 = in[interleave+1];
-
- avail -= 2;
-
- /* Compute path costs and accumulate output bit path
- * for each state and encoded bit value. Unrolling
- * this loop is worth about > 30% performance boost.
- * Decoding 76-byte remote access packets is reduced
- * from 14.700ms to 9.3ms. Redoing the loop to
- * directly compare the two pasts for each future state
- * reduces this down to 5.7ms
- */
-
- /* Ok, of course this is tricky, it's optimized.
- *
- * First, it's important to realize that we have 8
- * states representing the combinations of the three
- * most recent bits from the encoder. Flipping any
- * of these three bits flips both output bits.
- *
- * 'state<<1' represents the target state for a new
- * bit value of 0. '(state<<1)+1' represents the
- * target state for a new bit value of 1.
- *
- * 'state' is the previous state with an oldest bit
- * value of 0. 'state + 4' is the previous state with
- * an oldest bit value of 1. These two states will
- * either lead to 'state<<1' or '(state<<1)+1', depending
- * on whether the next encoded bit was a zero or a one.
- *
- * m0 and m1 are the cost of coming to 'state<<1' from
- * one of the two possible previous states 'state' and
- * 'state + 4'.
- *
- * Because we know the expected values of each
- * received bit are flipped between these two previous
- * states:
- *
- * bitcost(state+4) = 510 - bitcost(state)
- *
- * With those two total costs in hand, we then pick
- * the lower as the cost of the 'state<<1', and compute
- * the path of bits leading to that state.
- *
- * Then, do the same for '(state<<1) + 1'. This time,
- * instead of computing the m0 and m1 values from
- * scratch, because the only difference is that we're
- * expecting a one bit instead of a zero bit, we just
- * flip the bitcost values around to match the
- * expected transmitted bits with some tricky
- * arithmetic which is equivalent to:
- *
- * m0 = cost[p][state] + (510 - bitcost);
- * m1 = cost[p][state+4] + bitcost
- *
- * Then, the lowest cost and bit trace of the new state
- * is saved.
- */
-
-#define DO_STATE(state) { \
- uint32_t bitcost; \
- \
- uint32_t m0; \
- uint32_t m1; \
- uint32_t bit; \
- \
- bitcost = ((uint32_t) (s0 ^ ao_fec_decode_table[(state<<1)]) + \
- (uint32_t) (s1 ^ ao_fec_decode_table[(state<<1)|1])); \
- \
- m0 = cost[p][state] + bitcost; \
- m1 = cost[p][state+4] + (510 - bitcost); \
- bit = m0 > m1; \
- cost[n][state<<1] = bit ? m1 : m0; \
- bits[n][state<<1] = (bits[p][state + (bit<<2)] << 1) | (state&1); \
- \
- m0 -= (bitcost+bitcost-510); \
- m1 += (bitcost+bitcost-510); \
- bit = m0 > m1; \
- cost[n][(state<<1)+1] = bit ? m1 : m0; \
- bits[n][(state<<1)+1] = (bits[p][state + (bit<<2)] << 1) | (state&1); \
- }
-
- DO_STATE(0);
- DO_STATE(1);
- DO_STATE(2);
- DO_STATE(3);
-
-#if 0
- printf ("bit %3d symbol %2x %2x:", i/2, s0, s1);
- for (state = 0; state < NUM_STATE; state++) {
- printf (" %8u(%08x)", 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 any 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 */
- uint32_t min_cost; /* lowest cost */
- uint8_t min_state; /* lowest cost state */
- uint8_t byte;
-
- /* 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 whiten %0x\n",
- i/2, min_cost, o + 8, min_state, (bits[p][min_state] >> dist) & 0xff, *whiten);
-#endif
- byte = (bits[p][min_state] >> dist) ^ *whiten++;
- *out++ = byte;
- if (out_len > 2)
- crc = ao_fec_crc_byte(byte, crc);
-
- if (!--out_len) {
- if ((out[-2] == (uint8_t) (crc >> 8)) &&
- out[-1] == (uint8_t) crc)
- out[-1] = AO_FEC_DECODE_CRC_OK;
- else
- out[-1] = 0;
- out[-2] = 0;
- goto done;
- }
- o += 8;
- }
- }
-done:
-#if AO_PROFILE
- ao_fec_decode_start = start_tick;
- ao_fec_decode_end = ao_profile_tick();
-#endif
- return 1;
-}
projects / fw / altos / blobdiff