X-Git-Url: https://git.gag.com/?a=blobdiff_plain;f=src%2Fcore%2Fao_fec_rx.c;h=c4f5559a11f8ea79b07d069e99ee60a2208fe58f;hb=d96fd33aa8a220d547512eb43c88fc8f5651e39e;hp=d4c98475833d246ca1a66f380cbb160894703c27;hpb=84f9a525c64491afa9b7a565e3c10a4cee106e14;p=fw%2Faltos

diff --git a/src/core/ao_fec_rx.c b/src/core/ao_fec_rx.c
index d4c98475..c4f5559a 100644
--- a/src/core/ao_fec_rx.c
+++ b/src/core/ao_fec_rx.c
@@ -18,7 +18,7 @@
 #include <ao_fec.h>
 #include <stdio.h>
 
-#ifdef MEGAMETRUM
+#ifdef TELEMEGA
 #include <ao.h>
 #endif
 
@@ -62,7 +62,9 @@ static inline uint16_t ao_interleave_index(uint16_t i) {
 }
 
 #define NUM_STATE	8
-#define NUM_HIST	8
+#define NUM_HIST	24
+
+typedef uint32_t	bits_t;
 
 #define V_0		0xff
 #define V_1		0x00
@@ -94,10 +96,10 @@ ao_next_state(uint8_t state, uint8_t bit)
  */
 
 uint8_t
-ao_fec_decode(const uint8_t *in, uint16_t len, uint8_t *out, uint8_t out_len, uint16_t (*callback)())
+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 uint16_t	bits[2][NUM_STATE];		/* save bits to quickly output them */
+	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) */
@@ -154,42 +156,93 @@ ao_fec_decode(const uint8_t *in, uint16_t len, uint8_t *out, uint8_t out_len, ui
 
 		avail -= 2;
 
-		/* Reset next costs to 'impossibly high' values so that
-		 * the first path through this state is cheaper than this
+		/* 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
 		 */
-		for (state = 0; state < NUM_STATE; state++)
-			cost[n][state] = 0x7fffffff;
 
-		/* Compute path costs and accumulate output bit path
-		 * for each state and encoded bit value
+		/* 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.
 		 */
-		for (state = 0; state < NUM_STATE; state++) {
-			uint32_t	bitcost = ((uint32_t) (s0 ^ ao_fec_decode_table[(state<<1)]) +
-						   (uint32_t) (s1 ^ ao_fec_decode_table[(state<<1)+1]));
-			{
-				uint32_t	cost0 = cost[p][state] + bitcost;
-				uint8_t		state0 = ao_next_state(state, 0);
-
-				if (cost0 < cost[n][state0]) {
-					cost[n][state0] = cost0;
-					bits[n][state0] = (bits[p][state] << 1) | (state & 1);
-				}
-			}
-			{
-				uint32_t	cost1 = cost[p][state] + 510 - bitcost;
-				uint8_t		state1 = ao_next_state(state, 1);
 
-				if (cost1 < cost[n][state1]) {
-					cost[n][state1] = cost1;
-					bits[n][state1] = (bits[p][state] << 1) | (state & 1);
-				}
-			}
+#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 (" %5d(%04x)", cost[n][state], bits[n][state]);
+			printf (" %8u(%08x)", cost[n][state], bits[n][state]);
 		}
 		printf ("\n");
 #endif