--- /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