gsm_utils: improve gsm_gsmtime2fn()
The function gsm_gsmtime2fn() uses a hack to account for the truncated
modulo implementation of C/C++. libosmocore offers proven modulo
functions, so lets use OSMO_MOD_FLR() instead. Also arrange the formula
so that it looks more like the one in the spec.
Also add better spec references and a final modulo GSM_MAX_FN to
prevent frame number results that exceed the valid range.
Change-Id: Ibf94bca8223f1f7858a6dd67bf27de0ab6feab20
diff --git a/src/gsm/gsm_utils.c b/src/gsm/gsm_utils.c
index 3b0ec6a..bbc7dd6 100644
--- a/src/gsm/gsm_utils.c
+++ b/src/gsm/gsm_utils.c
@@ -887,8 +887,19 @@
* \returns GSM Frame Number */
uint32_t gsm_gsmtime2fn(struct gsm_time *time)
{
- /* TS 05.02 Chapter 4.3.3 TDMA frame number */
- return (51 * ((time->t3 - time->t2 + 26) % 26) + time->t3 + (26 * 51 * time->t1));
+ uint32_t fn;
+
+ /* See also:
+ * 3GPP TS 44.018, section 10.5.2.38, 3GPP TS 45.002 section 4.3.3, and
+ * 3GPP TS 48.058, section 9.3.8 */
+ fn = 51 * OSMO_MOD_FLR((time->t3-time->t2), 26) + time->t3 + 51 * 26 * time->t1;
+
+ /* Note: Corrupted input values may cause a resulting frame number
+ * larger then the maximum permitted value of GSM_MAX_FN. Even though
+ * the caller is expected to check the input values beforehand we must
+ * make sure that the result cannot exceed the value range of a valid
+ * GSM frame number. */
+ return fn % GSM_MAX_FN;
}
char *osmo_dump_gsmtime_buf(char *buf, size_t buf_len, const struct gsm_time *tm)