conv: Fix the traceback for tail biting codes

When picking the end state, looking only at the path metric
is highly suboptimal because in a tail biting code, we _know_ that
whatever treillis path is correct, it must start and end at the same
state. So we only consider path meeting that condition. We know any
path that doesn't isn't the right one. We only fallback to only
path metric if no path met that condition.

Fixes OS#4508

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Change-Id: I87e51d3880c0fe7bf3d6cd08fd46517a424a230c

include/osmocom/core/conv.h

@@ -124,6 +124,7 @@ int osmo_conv_decode_scan(struct osmo_conv_decoder *decoder,
                           const sbit_t *input, int n);
 int osmo_conv_decode_flush(struct osmo_conv_decoder *decoder,
                            const sbit_t *input);
+int osmo_conv_decode_get_best_end_state(struct osmo_conv_decoder *decoder);
 int osmo_conv_decode_get_output(struct osmo_conv_decoder *decoder,
                                 ubit_t *output, int has_flush, int end_state);
 

src/conv.c

@@ -526,38 +526,87 @@ osmo_conv_decode_flush(struct osmo_conv_decoder *decoder,
 }
 
 int
-osmo_conv_decode_get_output(struct osmo_conv_decoder *decoder,
-                            ubit_t *output, int has_flush, int end_state)
+osmo_conv_decode_get_best_end_state(struct osmo_conv_decoder *decoder)
 {
 	const struct osmo_conv_code *code = decoder->code;
 
-	int min_ae;
-	uint8_t min_state, cur_state;
-	int i, s, n;
+	int min_ae, min_state;
+	int s;
 
-	uint8_t *sh_ptr;
+	/* If flushed, we _know_ the end state */
+	if (code->term == CONV_TERM_FLUSH)
+		return 0;
 
-	/* End state ? */
-	if (end_state < 0) {
-		/* Find state with least error */
-		min_ae = MAX_AE;
-		min_state = 0xff;
+	/* Search init */
+	min_state = -1;
+	min_ae = MAX_AE;
+
+	/* If tail biting, we search for the minimum path metric that
+	 * gives a circular traceback (i.e. start_state == end_state */
+	if (code->term == CONV_TERM_TAIL_BITING) {
+		int t, n, i;
+		uint8_t *sh_ptr;
 
 		for (s=0; s<decoder->n_states; s++)
 		{
+			/* Check if that state traces back to itself */
+			n = decoder->o_idx;
+			sh_ptr = &decoder->state_history[decoder->n_states * (n-1)];
+			t = s;
+
+			for (i=n-1; i>=0; i--) {
+				t = sh_ptr[t];
+				sh_ptr -= decoder->n_states;
+			}
+
+			if (s != t)
+				continue;
+
+			/* If it does, consider it */
 			if (decoder->ae[s] < min_ae) {
 				min_ae = decoder->ae[s];
 				min_state = s;
 			}
 		}
 
-		if (min_state == 0xff)
-			return -1;
-	} else {
-		min_state = (uint8_t) end_state;
-		min_ae = decoder->ae[end_state];
+		if (min_ae < MAX_AE)
+			return min_state;
 	}
 
+	/* Finally, just the lowest path metric */
+	for (s = 0; s < decoder->n_states; s++) {
+		/* Is it smaller ? */
+		if (decoder->ae[s] < min_ae) {
+			min_ae = decoder->ae[s];
+			min_state = s;
+		}
+	}
+
+	return min_state;
+}
+
+int
+osmo_conv_decode_get_output(struct osmo_conv_decoder *decoder,
+			    ubit_t *output, int has_flush, int end_state)
+{
+	const struct osmo_conv_code *code = decoder->code;
+
+	int min_ae;
+	uint8_t min_state, cur_state;
+	int i, n;
+
+	uint8_t *sh_ptr;
+
+	/* End state ? */
+	if (end_state < 0)
+		end_state = osmo_conv_decode_get_best_end_state(decoder);
+
+	if (end_state < 0)
+		return -1;
+
+	min_state = (uint8_t) end_state;
+	min_ae = decoder->ae[end_state];
+
 	/* Traceback */
 	cur_state = min_state;
 
@@ -598,8 +647,8 @@ osmo_conv_decode_get_output(struct osmo_conv_decoder *decoder,
  *
  * This is an all-in-one function, taking care of
  * \ref osmo_conv_decode_init, \ref osmo_conv_decode_scan,
- * \ref osmo_conv_decode_flush, \ref osmo_conv_decode_get_output and
- * \ref osmo_conv_decode_deinit.
+ * \ref osmo_conv_decode_flush, \ref osmo_conv_decode_get_best_end_state,
+ * \ref osmo_conv_decode_get_output and \ref osmo_conv_decode_deinit.
  */
 int
 osmo_conv_decode(const struct osmo_conv_code *code,
@@ -626,7 +675,7 @@ osmo_conv_decode(const struct osmo_conv_code *code,
 
 	rv = osmo_conv_decode_get_output(&decoder, output,
 		code->term == CONV_TERM_FLUSH,		/* has_flush */
-		code->term == CONV_TERM_FLUSH ? 0 : -1	/* end_state */
+		-1					/* end_state */
 	);
 
 	osmo_conv_decode_deinit(&decoder);

src/conv_acc.c

@@ -483,10 +483,27 @@ static void _traceback_rec(struct vdecoder *dec,
  */
 static int traceback(struct vdecoder *dec, uint8_t *out, int term, int len)
 {
-	int i, sum, max = -1;
-	unsigned path, state = 0;
+	int i, j, sum, max = -1;
+	unsigned path, state = 0, state_scan;
 
-	if (term != CONV_TERM_FLUSH) {
+	if (term == CONV_TERM_TAIL_BITING) {
+		for (i = 0; i < dec->trellis.num_states; i++) {
+			state_scan = i;
+			for (j = len - 1; j >= 0; j--) {
+				path = dec->paths[j][state_scan] + 1;
+				state_scan = vstate_lshift(state_scan, dec->k, path);
+			}
+			if (state_scan != i)
+				continue;
+			sum = dec->trellis.sums[i];
+			if (sum > max) {
+				max = sum;
+				state = i;
+			}
+		}
+	}
+
+	if ((max < 0) && (term != CONV_TERM_FLUSH)) {
 		for (i = 0; i < dec->trellis.num_states; i++) {
 			sum = dec->trellis.sums[i];
 			if (sum > max) {