--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libgsmfrp/prng.c	Mon Nov 28 04:00:18 2022 +0000
@@ -0,0 +1,53 @@
+/*
+ * We use a pseudorandom sequence generator function in two places:
+ * when generating comfort noise (GSM 06.12 section 6.1), and when
+ * randomizing grid position parameters as part of speech muting
+ * during error handling (GSM 06.11 section 6).
+ *
+ * Our PRNG is a copy of the pseudonoise() function from ETSI EFR code,
+ * where it is used for similar purposes.
+ */
+
+#include <stdint.h>
+#include "gsm_fr_preproc.h"
+#include "internal.h"
+
+uint16_t gsmfr_preproc_prng(struct gsmfr_preproc_state *st, uint16_t no_bits)
+{
+    uint16_t noise_bits, Sn, i;
+
+    noise_bits = 0;
+    for (i = 0; i < no_bits; i++)
+    {
+        /* State n == 31 */
+        if ((st->cn_random_lfsr & 0x00000001L) != 0)
+        {
+            Sn = 1;
+        }
+        else
+        {
+            Sn = 0;
+        }
+
+        /* State n == 3 */
+        if ((st->cn_random_lfsr & 0x10000000L) != 0)
+        {
+            Sn = Sn ^ 1;
+        }
+        else
+        {
+            Sn = Sn ^ 0;
+        }
+
+        noise_bits = noise_bits << 1;
+        noise_bits = noise_bits | st->cn_random_lfsr & 1;
+
+        st->cn_random_lfsr >>= 1;
+        if (Sn & 1)
+        {
+            st->cn_random_lfsr |= 0x40000000L;
+        }
+    }
+
+    return noise_bits;
+}