splitflap: use timezone

This is based on time_get_timezone_offset from pycardium, which really
should be available as part of the epicardium API to l0dables. For now
we copy-paste this.

l0dables/splitflap/main.c

@@ -5,6 +5,8 @@
 #include "tmr_utils.h"
 
 #include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
 
 #include "render.h"
 #include "splitflap.h"
@@ -26,6 +28,36 @@ void SysTick_Handler() {
     tick++;
 }
 
+static uint32_t get_timezone_offset(void) {
+	static uint32_t offset = INT32_MAX;
+
+	/* Only read the timezone setting once after starting
+	 * the interpeter to not impact performance */
+	if (offset == INT32_MAX) {
+		offset = 3600;
+
+		char buf[128];
+		int ret = epic_config_get_string("timezone", buf, sizeof(buf));
+
+		/* Understands formats like +0100, 0100, 100, -0800 */
+		if (ret == 0 && strlen(buf) > 0) {
+			errno             = 0;
+			long int timezone = strtol(buf, NULL, 10);
+			int sign          = 1;
+			if (timezone < 0) {
+				sign     = -1;
+				timezone = -timezone;
+			}
+			if (errno == 0) {
+				offset = (timezone / 100) * 3600;
+				offset += (timezone % 100) * 60;
+				offset *= sign;
+			}
+		}
+	}
+	return offset;
+}
+
 static void sample_rtc(uint32_t *ticks, uint32_t *rtc) {
     *rtc = epic_rtc_get_seconds();
     *ticks = tick;
@@ -68,7 +100,7 @@ int main(void) {
         // operate in UTC+2. Compensate and show UTC+2 time, even if the user is
         // not in this timezone.
         // TODO(q3k): file a bug
-        seconds += 7200;
+        seconds += get_timezone_offset();
 
         SplitFlap_Draw(theta(), seconds);