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gsm: add gsm0502_fn_is_greater() 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, 49 insertions(+), 0 deletions(-)

git pull ssh://gerrit.osmocom.org:29418/libosmocore refs/changes/20/34520/1

diff --git a/include/osmocom/gsm/gsm0502.h b/include/osmocom/gsm/gsm0502.h
index ffd6e20..0d70aaf 100644
--- a/include/osmocom/gsm/gsm0502.h
+++ b/include/osmocom/gsm/gsm0502.h
@@ -6,6 +6,7 @@
 #pragma once
 
 #include <stdint.h>
+#include <stdbool.h>
 
 #include <osmocom/gsm/protocol/gsm_04_08.h>
 #include <osmocom/gsm/protocol/gsm_08_58.h>
@@ -64,6 +65,20 @@
 #define GSM_NBITS_AB_GMSK_TAIL		GSM_NBITS_NB_GMSK_TAIL
 #define GSM_NBITS_AB_GMSK_BURST		GSM_NBITS_NB_GMSK_BURST
 
+/* Check if the given TDMA Fn 'fn1' is greater than TDMA Fn 'fn2',
+ * taking the wrapping into account (i.e. 0 is greater than 2715647).
+ * Based on fn_cmp() implementation from osmo-pcu.git, simplified. */
+static inline bool gsm0502_fn_is_greater(uint32_t fn1, uint32_t fn2)
+{
+	const uint32_t thresh = GSM_TDMA_HYPERFRAME / 2;
+
+	if ((fn1 < fn2 && (fn2 - fn1) < thresh) ||
+	    (fn1 > fn2 && (fn1 - fn2) > thresh))
+		return false;
+
+	return true;
+}
+
 /* 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..6390d8b 100644
--- a/tests/gsm0502/gsm0502_test.c
+++ b/tests/gsm0502/gsm0502_test.c
@@ -148,8 +148,27 @@
 	printf("\n");
 }
 
+static void test_gsm0502_fn_is_greater(void)
+{
+	OSMO_ASSERT(gsm0502_fn_is_greater(42, 1337)		== false);
+	OSMO_ASSERT(gsm0502_fn_is_greater(1337, 42)		== true);
+	OSMO_ASSERT(gsm0502_fn_is_greater(42, 0)		== true);
+
+	/* 2715642 is very close to the Fn period (GSM_TDMA_HYPERFRAME) */
+	OSMO_ASSERT(gsm0502_fn_is_greater(2715642, 42)		== false);
+	OSMO_ASSERT(gsm0502_fn_is_greater(42, 2715642)		== true);
+	OSMO_ASSERT(gsm0502_fn_is_greater(0, 2715642)		== true);
+
+	/* 1357824 is half of the Fn period (GSM_TDMA_HYPERFRAME) */
+	OSMO_ASSERT(gsm0502_fn_is_greater(1357820, 1357824)	== false);
+	OSMO_ASSERT(gsm0502_fn_is_greater(1357820, 1357825)	== false);
+	OSMO_ASSERT(gsm0502_fn_is_greater(1357824, 1357820)	== true);
+	OSMO_ASSERT(gsm0502_fn_is_greater(1357825, 1357820)	== true);
+}
+
 int main(int argc, char **argv)
 {
 	test_gsm0502_fn_remap();
+	test_gsm0502_fn_is_greater();
 	return EXIT_SUCCESS;
 }

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