library: add f_gsm_fn_{sum,sub,diff}()

Change-Id: Ie91ccef5d3f170f4a341a6cf9d3323a896b299e6
---
M library/GSM_Types.ttcn
1 file changed, 22 insertions(+), 0 deletions(-)


git pull ssh://gerrit.osmocom.org:29418/osmo-ttcn3-hacks refs/changes/15/42915/1

diff --git a/library/GSM_Types.ttcn b/library/GSM_Types.ttcn
index b3b6dc9..764b8ba 100644
--- a/library/GSM_Types.ttcn
+++ b/library/GSM_Types.ttcn
@@ -434,6 +434,28 @@
 	return (fn / 51) mod 8;
 }
 
+/* 45.002 4.3.3: TDMA frame number modular arithmetic */
+
+/* Return the sum of two given TDMA frame numbers, wrapping around the hyperframe */
+function f_gsm_fn_sum(GsmFrameNumber a, GsmFrameNumber b) return GsmFrameNumber {
+	return (a + b) mod GsmMaxFrameNumber;
+}
+
+/* Return the difference of two given TDMA frame numbers, wrapping around the hyperframe */
+function f_gsm_fn_sub(GsmFrameNumber a, GsmFrameNumber b) return GsmFrameNumber {
+	return (a - b + GsmMaxFrameNumber) mod GsmMaxFrameNumber;
+}
+
+/* Return the *minimum* distance between two given TDMA frame numbers */
+function f_gsm_fn_diff(GsmFrameNumber a, GsmFrameNumber b) return GsmFrameNumber {
+	var GsmFrameNumber d1 := f_gsm_fn_sub(a, b);
+	var GsmFrameNumber d2 := f_gsm_fn_sub(b, a);
+	if (d1 < d2) {
+		return d1;
+	}
+	return d2;
+}
+
 type hexstring GsmBcdString with { variant "HEXORDER(low)" };
 type GsmBcdString BcdMccMnc with { variant "FIELDLENGTH(6)" };
 

