non-table driven interleaver b

1 files changed, 15 insertions, 28 deletions

diff --git a/src/gp_interleaver.c b/src/gp_interleaver.c
index 6710b0c..70b8ad0 100644
--- a/src/gp_interleaver.c
+++ b/src/gp_interleaver.c
@@ -31,45 +31,32 @@
 #include "gp_interleaver.h"
 
 #include <assert.h>
+#include <math.h>
 #include <stdio.h>
 
 /*
   Choose b for Golden Prime Interleaver.  b is chosen to be the
   closest integer, which is relatively prime to N, to the Golden
   section of N.
-
-  Implemented with a LUT in C for convenience, Octave version
-  has a more complete implementation.  If you find you need some more
-  numbers head back to the Octave choose_interleaver_b() function.
 */
 
-static const int b_table[] = {
-    48,   31,  /* datac14: HRA_56_56, 40 data bits used          */
-    56,   37,  /* 700E:   HRA_56_56                              */
-    106,  67,  /* 2020B:  (112,56) partial protection            */
-    112,  71,  /* 700D:   HRA_112_112                            */
-    128,  83,  /* datac0: H_128_256_5                            */
-    192,  127, /* datac13: H_256_512_4, 128 data bits used       */
-    210,  131, /* 2020:   HRAb_396_504 with 312 data bits used   */
-    736,  457, /* datac4: H_1024_2048_4f, 448 data bits used     */
-    1024, 641, /* datac3: H_1024_2048_4f                         */
-    1290, 797, /* datac2: H2064_516_sparse                       */
-    4096, 2531 /* datac1: H_4096_8192_3d                         */
-};
-
-int choose_interleaver_b(int Nbits) {
-  int i;
-  for (i = 0; i < sizeof(b_table) / sizeof(int); i += 2) {
-    if (b_table[i] == Nbits) {
-      return b_table[i + 1];
-    }
+int is_prime(int x) {
+  for (int i = 2; i < x; i++) {
+    if ((x % i) == 0) return 0;
   }
+  return 1;
+}
 
-  /* if we get to here it means a Nbits we don't have in our table so choke */
+int next_prime(int x) {
+  x++;
+  while (is_prime(x) == 0) x++;
+  return x;
+}
 
-  fprintf(stderr, "gp_interleaver: Nbits: %d, b not found!\n", Nbits);
-  assert(0);
-  return -1;
+int choose_interleaver_b(int Nbits) {
+  int b = floor(Nbits / 1.62);
+  b = next_prime(b);
+  return b;
 }
 
 void gp_interleave_comp(COMP interleaved_frame[], COMP frame[], int Nbits) {