gsm: add gsm0502_fn_compare() for comparing TDMA FNs

We need this function in:

* osmocom-bb.git for trxcon and l1gprs,
* osmo-pcu.git replacing fn_cmp().

Change-Id: I9590f2e836fc48650decf1564b6ab46306c4fe2d
Related: OS#5500
---
M include/osmocom/gsm/gsm0502.h
M tests/gsm0502/gsm0502_test.c
2 files changed, 54 insertions(+), 0 deletions(-)

diff --git a/include/osmocom/gsm/gsm0502.h b/include/osmocom/gsm/gsm0502.h
index ffd6e20..6624e70 100644
--- a/include/osmocom/gsm/gsm0502.h
+++ b/include/osmocom/gsm/gsm0502.h
@@ -64,6 +64,25 @@
 #define GSM_NBITS_AB_GMSK_TAIL		GSM_NBITS_NB_GMSK_TAIL
 #define GSM_NBITS_AB_GMSK_BURST		GSM_NBITS_NB_GMSK_BURST
 
+/*! Compare the given TDMA FNs, taking the wrapping into account.
+ * \param[in] fn1 First TDMA Fn value to compare.
+ * \param[in] fn2 Second TDMA Fn value to compare.
+ * \returns similarly to memcmp(), -1 if fn1 goes before fn2;
+ *                                  0 if fn1 equals fn2;
+ *                                  1 if fn1 goes after fn2. */
+static inline int gsm0502_fn_compare(uint32_t fn1, uint32_t fn2)
+{
+	const uint32_t thresh = GSM_TDMA_HYPERFRAME / 2;
+
+	if (fn1 == fn2)
+		return 0;
+	if ((fn1 < fn2 && (fn2 - fn1) < thresh) ||
+	    (fn1 > fn2 && (fn1 - fn2) > thresh))
+		return -1;
+
+	return 1;
+}
+
 /* Table 5 Clause 7 TS 05.02 */
 static inline unsigned int
 gsm0502_get_n_pag_blocks(const struct gsm48_control_channel_descr *chan_desc)
diff --git a/tests/gsm0502/gsm0502_test.c b/tests/gsm0502/gsm0502_test.c
index e9deaa9..d78a94d 100644
--- a/tests/gsm0502/gsm0502_test.c
+++ b/tests/gsm0502/gsm0502_test.c
@@ -148,8 +148,28 @@
 	printf("\n");
 }
 
+static void test_gsm0502_fn_compare(void)
+{
+	OSMO_ASSERT(gsm0502_fn_compare(1337, 1337)		==  0);
+	OSMO_ASSERT(gsm0502_fn_compare(42, 1337)		== -1);
+	OSMO_ASSERT(gsm0502_fn_compare(1337, 42)		==  1);
+	OSMO_ASSERT(gsm0502_fn_compare(42, 0)			==  1);
+
+	/* 2715642 is very close to the Fn period (GSM_TDMA_HYPERFRAME) */
+	OSMO_ASSERT(gsm0502_fn_compare(2715642, 42)		== -1);
+	OSMO_ASSERT(gsm0502_fn_compare(42, 2715642)		==  1);
+	OSMO_ASSERT(gsm0502_fn_compare(0, 2715642)		==  1);
+
+	/* 1357824 is half of the Fn period (GSM_TDMA_HYPERFRAME) */
+	OSMO_ASSERT(gsm0502_fn_compare(1357820, 1357824)	== -1);
+	OSMO_ASSERT(gsm0502_fn_compare(1357820, 1357825)	== -1);
+	OSMO_ASSERT(gsm0502_fn_compare(1357824, 1357820)	==  1);
+	OSMO_ASSERT(gsm0502_fn_compare(1357825, 1357820)	==  1);
+}
+
 int main(int argc, char **argv)
 {
 	test_gsm0502_fn_remap();
+	test_gsm0502_fn_compare();
 	return EXIT_SUCCESS;
 }

