From e2a1fac9d016fb835446135ba1d6f439b6dc0fff Mon Sep 17 00:00:00 2001
From: KatolaZ <katolaz@freaknet.org>
Date: Fri, 7 Oct 2022 12:31:03 +0100
Subject: add getgrid.c and amend README.md accordingly

---
 Makefile  | 13 +++++++++----
 README.md | 12 ++++++++++++
 getgrid.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 82 insertions(+), 4 deletions(-)
 create mode 100644 getgrid.c

diff --git a/Makefile b/Makefile
index fa58aa4..f9d958d 100644
--- a/Makefile
+++ b/Makefile
@@ -1,10 +1,11 @@
 # gridcal - compute great-circle distance between two grid locators 
 .POSIX:
 
-SRC = gridcal.c 
+SRC = gridcal.c getgrid.c
 OBJ = ${SRC:.c=.o}
+PROG= gridcal getgrid
 
-all: options gridcal 
+all: options gridcal getgrid
 
 options:
 	@echo "-+- build options -+-"
@@ -15,11 +16,15 @@ options:
 	@echo "-+-+-+-+-+-+-+-+-+-+-" 
 
 gridcal: ${OBJ}
-	$(CC) $(LDFLAGS) -o $@ $(OBJ) -lm
+	$(CC) $(LDFLAGS) -o $@ gridcal.o -lm
+
+getgrid: ${OBJ}
+	$(CC) $(LDFLAGS) -o $@ getgrid.o -lm
+
 
 debug: clean
 	${MAKE} CFLAGS="${DEBUG}" all
 
 clean:
-	rm -f gridcal ${OBJ}
+	rm -f ${PROG} ${OBJ}
 
diff --git a/README.md b/README.md
index f9de941..86a4292 100644
--- a/README.md
+++ b/README.md
@@ -36,6 +36,18 @@ The latter can be useful to check if a QSO with another ham radio
 station qualifies for one of the thousand miles per Watt awards.
 
 
+`getgrid` -- compute the grid locator associated to lat-long coordinates
+========================================================================
+
+`getgrid` is a simple tool to compute the grid locator associated to a
+latitude/longitude pair. The coordinates must be in degrees. 
+
+Usage: getgrid <lat> <lon>
+
+Example: 
+
+    getgrid 
+
 Dependencies
 ============
 
diff --git a/getgrid.c b/getgrid.c
new file mode 100644
index 0000000..84ac26d
--- /dev/null
+++ b/getgrid.c
@@ -0,0 +1,61 @@
+/**
+*
+* Compute the grid locator associated to a pair of latitude/longitude
+* coordinates, given in degrees
+*
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+
+void latlon_to_grid(double lat, double lon, char *loc){
+	
+	double q;
+
+	q = (int)((lon + 180)/20);
+	lon = (q==0? lon: fmod(lon+180, 20));
+	loc[0] = 'A' + q;
+	q = (int)((lat + 90)/10);
+	lat = (q==0? lat: fmod(lat+90, 10));
+	loc[1] = 'A' + q;
+	q = (int)(lon/2);
+	lon = (q==0? lon: fmod(lon, 2));
+	loc[2] = '0' + q;
+	q = (int)(lat);
+	lat = (q==0? lat: lat - q);
+	loc[3] = '0' + q;
+	q = (int)(lon*12);
+	loc[4] = 'A' + q;
+	q = (int)(lat*24);
+	loc[5] = 'A' + q;
+	loc[6] = '\0';
+}
+
+void usage(char s[]){
+	printf("Usage: get_grid <lat> <lon>\n");
+	exit(1);
+}
+
+int main (int argc, char *argv[]){
+	
+	double lat, lon;
+	char loc[7];
+
+	if (argc < 3){
+		usage(argv[0]);
+	}
+
+	lat = atof(argv[1]);
+	lon = atof(argv[2]);
+	while (lat < -90) lat +=180;
+	while (lat > 90) lat -=180;
+	while (lon < -180) lon += 360;
+	while (lon > 180) lon -= 360;
+	//fprintf(stderr, "lat: %g lon: %g\n", lat, lon);
+	latlon_to_grid(lat, lon, loc);
+	printf("%s\n", loc);
+
+}
+
+
-- 
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