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tnt gerrit-no-reply at lists.osmocom.orgtnt has uploaded this change for review. ( https://gerrit.osmocom.org/c/libosmocore/+/26657 )
Change subject: conv: Fix the traceback for tail biting codes
......................................................................
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 #4508
Signed-off-by: Sylvain Munaut <tnt at 246tNt.com>
Change-Id: I87e51d3880c0fe7bf3d6cd08fd46517a424a230c
---
M include/osmocom/core/conv.h
M src/conv.c
M src/conv_acc.c
3 files changed, 93 insertions(+), 24 deletions(-)
git pull ssh://gerrit.osmocom.org:29418/libosmocore refs/changes/57/26657/1
diff --git a/include/osmocom/core/conv.h b/include/osmocom/core/conv.h
index 1c14c3b..84ef0f8 100644
--- a/include/osmocom/core/conv.h
+++ b/include/osmocom/core/conv.h
@@ -124,6 +124,7 @@
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);
diff --git a/src/conv.c b/src/conv.c
index 0e07e1f..453e942 100644
--- a/src/conv.c
+++ b/src/conv.c
@@ -526,6 +526,68 @@
}
int
+osmo_conv_decode_get_best_end_state(struct osmo_conv_decoder *decoder)
+{
+ const struct osmo_conv_code *code = decoder->code;
+
+ int min_ae, min_state;
+ int s;
+
+ /* If flushed, we _know_ the end state */
+ if (code->term == CONV_TERM_FLUSH)
+ return 0;
+
+ /* 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_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)
{
@@ -533,30 +595,19 @@
int min_ae;
uint8_t min_state, cur_state;
- int i, s, n;
+ int i, n;
uint8_t *sh_ptr;
/* End state ? */
- if (end_state < 0) {
- /* Find state with least error */
- min_ae = MAX_AE;
- min_state = 0xff;
+ if (end_state < 0)
+ end_state = osmo_conv_decode_get_best_end_state(decoder);
- for (s=0; s<decoder->n_states; s++)
- {
- if (decoder->ae[s] < min_ae) {
- min_ae = decoder->ae[s];
- min_state = s;
- }
- }
+ if (end_state < 0)
+ return -1;
- if (min_state == 0xff)
- return -1;
- } else {
- min_state = (uint8_t) end_state;
- min_ae = decoder->ae[end_state];
- }
+ min_state = (uint8_t) end_state;
+ min_ae = decoder->ae[end_state];
/* Traceback */
cur_state = min_state;
@@ -598,8 +649,8 @@
*
* 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 +677,7 @@
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);
diff --git a/src/conv_acc.c b/src/conv_acc.c
index a26ed3b..4bd3b07 100644
--- a/src/conv_acc.c
+++ b/src/conv_acc.c
@@ -483,10 +483,27 @@
*/
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) {
--
To view, visit https://gerrit.osmocom.org/c/libosmocore/+/26657
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Gerrit-Project: libosmocore
Gerrit-Branch: master
Gerrit-Change-Id: I87e51d3880c0fe7bf3d6cd08fd46517a424a230c
Gerrit-Change-Number: 26657
Gerrit-PatchSet: 1
Gerrit-Owner: tnt <tnt at 246tNt.com>
Gerrit-MessageType: newchange
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