Initial bch functions

1 files changed, 76 insertions, 0 deletions

diff --git a/dumpocsag.c b/dumpocsag.c
new file mode 100644
index 0000000..47a292b
--- /dev/null
+++ b/dumpocsag.c
@@ -0,0 +1,76 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <stdint.h>
+#include <errno.h>
+#include <unistd.h>
+#include <math.h>
+#include <sys/time.h>
+#include <signal.h>
+#include <fcntl.h>
+#include <ctype.h>
+#include "rtl-sdr.h"
+
+/* POCSAG parameters */
+#define POCSAG_ID           "POCSAG"
+#define POCSAG_SYM_RATE     1200
+#define POCSAG_FM_DEV       4500
+#define POCSAG_SPS          8
+
+#define POCSAG_BCH_POLY     0x769
+#define POCSAG_BCH_N        31
+#define POCSAG_BCH_K        21
+
+int hammingWeight(int n) {
+    unsigned int c;
+    for (c = 0; n; c++)
+        n &= n - 1;
+    return c;
+}
+
+int evenParity(int n) {
+    return hammingWeight(n) & 1;
+}
+
+int bchSyndrome(int data, int poly, int n, int k) {
+    int mask = 1 << (n - 1);
+    int coeff = poly << (k - 1);
+    n = k;
+
+    int s = data >> 1;
+    while (n > 0) {
+        if (s & mask)
+            s ^= coeff;
+        n -= 1;
+        mask >>= 1;
+        coeff >>= 1;
+    }
+
+    if (evenParity(data))
+        s |= 1 << (n - k);
+
+    return s;
+}
+
+int bchFix(int data, int poly, int n, int k) {
+    int i, j;
+    for (i=0; i<32; i++) {
+        int t = data ^ (1 << i);
+        if (!bchSyndrome(t, poly, n, k))
+            return t;
+    }
+    for (i=0; i<32; i++) {
+        for (j=i+1; j<32; j++) {
+            int t = data ^ ((1 << i) | (1 << j));
+            if (!bchSyndrome(t, poly, n, k))
+                return t;
+        }
+    }
+    return data;
+}
+
+int main(int argc, char **argv) {
+    printf("Dumpocsag v0.1\n");
+    exit(0);
+}