#include <ao_fec.h>
#include <stdio.h>
-/*
- * 'input' is 8-bits per symbol soft decision data
- * 'len' is output byte length
- */
-
-static const uint8_t ao_fec_encode_table[16] = {
-/* next 0 1 state */
- 0, 3, /* 000 */
- 1, 2, /* 001 */
- 3, 0, /* 010 */
- 2, 1, /* 011 */
- 3, 0, /* 100 */
- 2, 1, /* 101 */
- 0, 3, /* 110 */
- 1, 2 /* 111 */
-};
-
struct ao_soft_sym {
uint8_t a, b;
};
-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;
-}
+#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 */
+};
-uint8_t
+static inline uint8_t
ao_next_state(uint8_t state, uint8_t bit)
{
return ((state << 1) | bit) & 0x7;
}
-static inline abs(int x) { return x < 0 ? -x : x; }
+static inline uint16_t ao_abs(int16_t x) { return x < 0 ? -x : x; }
-int
+static inline uint16_t
ao_cost(struct ao_soft_sym a, struct ao_soft_sym b)
{
- return abs(a.a - b.a) + abs(a.b - b.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, int len, uint8_t *out)
+ao_fec_decode(uint8_t *in, uint16_t len, uint8_t *out)
{
- int cost[len/2 + 1][8];
- uint8_t prev[len/2 + 1][8];
- int c;
- int i, b;
- uint8_t state = 0, min_state;
- uint8_t bits[len/2];
-
- for (c = 0; c < 8; c++)
- cost[0][c] = 10000;
+ 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] };
- for (state = 0; state < 8; state++)
- cost[b+1][state] = 10000;
-
- for (state = 0; state < 8; state++) {
- struct ao_soft_sym zero = ao_soft_sym(ao_fec_encode_table[state * 2 + 0]);
- struct ao_soft_sym one = ao_soft_sym(ao_fec_encode_table[state * 2 + 1]);
- uint8_t zero_state = ao_next_state(state, 0);
- uint8_t one_state = ao_next_state(state, 1);
- int zero_cost = ao_cost(s, zero);
- int one_cost = ao_cost(s, one);
-
-#if 0
- printf ("saw %02x %02x expected %02x %02x (%d) or %02x %02x (%d)\n",
- s.a, s.b, zero.a, zero.b, zero_cost, one.a, one.b, one_cost);
-#endif
- zero_cost += cost[b][state];
- one_cost += cost[b][state];
- if (zero_cost < cost[b+1][zero_state]) {
- prev[b+1][zero_state] = state;
- cost[b+1][zero_state] = zero_cost;
- }
-
- if (one_cost < cost[b+1][one_state]) {
- prev[b+1][one_state] = state;
- cost[b+1][one_state] = one_cost;
+ /* 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 < 8; state++) {
- printf (" %5d", cost[b+1][state]);
+ for (state = 0; state < NUM_STATE; state++) {
+ printf (" %5d(%04x)", cost[n][state], bits[n][state]);
}
printf ("\n");
- }
-
- b = len / 2;
- c = cost[b][0];
- min_state = 0;
- for (state = 1; state < 8; state++) {
- if (cost[b][state] < c) {
- c = cost[b][state];
- min_state = state;
- }
- }
-
- for (b = len/2; b > 0; b--) {
- bits[b-1] = min_state & 1;
- min_state = prev[b][min_state];
- }
+#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;
+ }
+ }
- for (i = 0; i < len/2; i += 8) {
- uint8_t byte;
+ /* 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;
+ }
- byte = 0;
- for (b = 0; b < 8; b++)
- byte = (byte << 1) | bits[i + b];
- out[i/8] = byte;
+#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;
}
projects / fw / altos / blobdiff