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Frame Sync: Replace per-symbol full sort with single-pass top-5 selection

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The frame sync hunt loop sorted the entire symbol amplitude window
(lbuf2, t_max entries) with qsort on every iteration only to read
three ranked values off each end. Replace the copy + qsort with a
single O(n) pass maintaining the five smallest and five largest
values in two fixed 5-slot arrays, reading the exact same ranks.

Rank equivalence: lmn[] ascending = sorted[0..], lmx[] descending =
sorted[..t_max-1], so both the short (t_max < 13) and long rank
formulas map 1:1. The now-unused lbuf2 scratch buffer is removed.

Decode output verified byte-identical (timestamps stripped) against
DMR, dPMR, D-Star and YSF sample captures, including runs with a
10 second no-carrier lead-in.
This commit is contained in:
xdab
2026-09-21 20:48:33 +02:00
parent 15515158b5
commit 9232d66fcb
1 changed files with 22 additions and 8 deletions
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30
src/dsd_frame_sync.c
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@@ -16,6 +16,7 @@
*/
#include "dsd.h"
#include <limits.h>
#include <locale.h>
void
@@ -197,11 +198,10 @@ getFrameSync (dsd_opts * opts, dsd_state * state)
}
else t_max = 24; //24 for everything else
int lbuf[48], lbuf2[48]; //if we use t_max in these arrays, and t >= t_max in condition below, then it can overflow those checks in there if t exceeds t_max
int lbuf[48]; //if we use t_max in these arrays, and t >= t_max in condition below, then it can overflow those checks in there if t exceeds t_max
int lsum;
//init the lbuf
memset (lbuf, 0, sizeof(lbuf));
memset (lbuf2, 0, sizeof(lbuf2));
// detect frame sync
t = 0;
@@ -374,23 +374,37 @@ getFrameSync (dsd_opts * opts, dsd_state * state)
*synctest_p = dibit;
if (t >= t_max) //works excelent now with short sync patterns, and no issues with large ones!
{
int lmn[5] = {INT_MAX, INT_MAX, INT_MAX, INT_MAX, INT_MAX};
int lmx[5] = {INT_MIN, INT_MIN, INT_MIN, INT_MIN, INT_MIN};
int j, x;
for (i = 0; i < t_max; i++) //24
{
lbuf2[i] = lbuf[i];
x = lbuf[i];
if (x < lmn[4])
{
for (j = 4; j > 0 && x < lmn[j - 1]; j--)
lmn[j] = lmn[j - 1];
lmn[j] = x;
}
if (x > lmx[4])
{
for (j = 4; j > 0 && x > lmx[j - 1]; j--)
lmx[j] = lmx[j - 1];
lmx[j] = x;
}
}
qsort (lbuf2, t_max, sizeof (int), comp);
//on shorter sync patterns, we need to look at the edges since we don't have many to pull from
if (t_max < 13)
{
lmin = (lbuf2[0] + lbuf2[1] + lbuf2[2]) / 3;
lmax = (lbuf2[t_max - 3] + lbuf2[t_max - 2] + lbuf2[t_max - 1]) / 3;
lmin = (lmn[0] + lmn[1] + lmn[2]) / 3;
lmax = (lmx[2] + lmx[1] + lmx[0]) / 3;
}
//on longer sync patterns, we can get more towards the middle and eliminate outlier values
else
{
lmin = (lbuf2[2] + lbuf2[3] + lbuf2[4]) / 3;
lmax = (lbuf2[t_max - 5] + lbuf2[t_max - 4] + lbuf2[t_max - 3]) / 3;
lmin = (lmn[2] + lmn[3] + lmn[4]) / 3;
lmax = (lmx[4] + lmx[3] + lmx[2]) / 3;
}
if (state->rf_mod == 1)