#include <ao_fec.h>
#include <stdio.h>
-/*
- * 'input' is 8-bits per symbol soft decision data
- * 'len' is output byte length
+/*
+ * 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;
};
-static const struct ao_soft_sym ao_fec_decode_table[16] = {
+#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 */
+ { { 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 */
};
-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;
-}
-
static inline uint8_t
ao_next_state(uint8_t state, uint8_t bit)
{
return ao_abs(a.a - b.a) + ao_abs(a.b - b.b);
}
-#define NUM_STATE 8
+/*
+ * '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, int len, uint8_t *out)
+ao_fec_decode(uint8_t *in, uint16_t len, uint8_t *out)
{
- uint16_t cost[2][NUM_STATE], min_cost;
- uint8_t prev[len/2 + 1][NUM_STATE];
- uint16_t prev_bits[2][NUM_STATE];
- int i, b, dump;
- uint8_t p, n;
- uint8_t state = 0, min_state;
+ 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 */
p = 0;
for (state = 0; state < NUM_STATE; state++) {
cost[0][state] = 0xffff;
- prev_bits[0][state] = 0;
+ bits[0][state] = 0;
}
cost[0][0] = 0;
- min_state = 0;
- min_cost = 0;
- dump = 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] };
+ /* 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++) {
- int zero_cost = ao_cost(s, ao_fec_decode_table[state * 2 + 0]);
- int one_cost = ao_cost(s, ao_fec_decode_table[state * 2 + 1]);
-
- uint8_t zero_state = ao_next_state(state, 0);
- uint8_t one_state = ao_next_state(state, 1);
-
- zero_cost += cost[p][state];
- one_cost += cost[p][state];
- if (zero_cost < cost[n][zero_state]) {
- prev[b+1][zero_state] = state;
- cost[n][zero_state] = zero_cost;
- prev_bits[n][zero_state] = (prev_bits[p][state] << 1) | (state & 1);
- }
-
- if (one_cost < cost[n][one_state]) {
- prev[b+1][one_state] = state;
- cost[n][one_state] = one_cost;
- prev_bits[n][one_state] = (prev_bits[p][state] << 1) | (state & 1);
+ 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], prev_bits[n][state]);
+ printf (" %5d(%04x)", cost[n][state], bits[n][state]);
}
printf ("\n");
#endif
p = n;
- b++;
- if ((b - dump) > 16 || i + 2 >= len) {
- uint8_t dist = b - (dump + 9);
- uint8_t rev;
+ /* 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++) {
min_state = state;
}
}
- for (rev = 0; rev < dist; rev++) {
- min_state = prev[b][min_state];
- b--;
+
+ /* 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, b-1, min_state, (prev_bits[p][min_state] >> dist) & 0xff);
+ 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
- out[dump/8] = prev_bits[p][min_state] >> dist;
- dump = b - 1;
+ *out++ = (bits[p][min_state] >> dist) ^ *whiten++;
+ o += 8;
}
-
}
return len/16;
}