X-Git-Url: https://git.gag.com/?p=fw%2Faltos;a=blobdiff_plain;f=src%2Fcore%2Fao_fec_rx.c;fp=src%2Fcore%2Fao_fec_rx.c;h=69e9c1f5b7da393f9757e9b88b6448a1cf95b31d;hp=0000000000000000000000000000000000000000;hb=eab18714ed9eabbcef0ff81b07427da042a58ccc;hpb=09761fe0f6ed40ff74317fbb47d6a74068fb4ce4 diff --git a/src/core/ao_fec_rx.c b/src/core/ao_fec_rx.c new file mode 100644 index 00000000..69e9c1f5 --- /dev/null +++ b/src/core/ao_fec_rx.c @@ -0,0 +1,226 @@ +/* + * Copyright © 2012 Keith Packard + * + * 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 +#include + +/* + * 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 inline uint16_t ao_interleave_index(uint16_t i) { + uint8_t l = i & 0x1e; + uint16_t h = i & ~0x1e; + uint8_t o = 0x1e ^ (((l >> 2) & 0x6) | ((l << 2) & 0x18)); + return h | o; +} + +struct ao_soft_sym { + uint8_t a, b; +}; + +#define NUM_STATE 8 +#define NUM_HIST 8 +#define MOD_HIST(b) ((b) & 7) + +#define V_0 0xc0 +#define V_1 0x40 + +static const struct ao_soft_sym ao_fec_decode_table[NUM_STATE][2] = { +/* next 0 1 state */ + { { V_0, V_0 }, { V_1, V_1 } } , /* 000 */ + { { V_0, V_1 }, { V_1, V_0 } }, /* 001 */ + { { V_1, V_1 }, { V_0, V_0 } }, /* 010 */ + { { V_1, V_0 }, { V_0, V_1 } }, /* 011 */ + { { V_1, V_1 }, { V_0, V_0 } }, /* 100 */ + { { V_1, V_0 }, { V_0, V_1 } }, /* 101 */ + { { V_0, V_0 }, { V_1, V_1 } }, /* 110 */ + { { V_0, V_1 }, { V_1, V_0 } } /* 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, uint8_t out_len, uint16_t (*callback)()) +{ + 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 */ + const uint8_t *whiten = ao_fec_whiten_table; + uint16_t interleave; /* input byte array index */ + struct ao_soft_sym s; /* input symbol pair */ + uint16_t avail; + + p = 0; + for (state = 0; state < NUM_STATE; state++) { + cost[0][state] = 0xffff; + bits[0][state] = 0; + } + cost[0][0] = 0; + + if (callback) + avail = 0; + else + avail = len; + + o = 0; + for (i = 0; i < len; i += 2) { + b = i/2; + n = p ^ 1; + + if (!avail) { + avail = callback(); + if (!avail) + break; + } + + /* Fetch one pair of input bytes, de-interleaving + * the input. + */ + interleave = ao_interleave_index(i); + s.a = in[interleave]; + s.b = in[interleave+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 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 */ + 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 whiten %0x\n", + i/2, min_cost, o + 8, min_state, (bits[p][min_state] >> dist) & 0xff, *whiten); +#endif + if (out_len) { + *out++ = (bits[p][min_state] >> dist) ^ *whiten++; + --out_len; + } + o += 8; + } + } + return len/16; +}