Add dayofweek function

2 files changed, 26 insertions, 0 deletions

diff --git a/src/fsm/common.c b/src/fsm/common.c
index ca4c009..517eb80 100644
--- a/src/fsm/common.c
+++ b/src/fsm/common.c
@@ -26,6 +26,7 @@
 #include "FreeRTOS.h"
 #include "timers.h"
 #include <stm32f4xx.h>
+#include <time.h>
 
 static uint64_t common_timestamp = 0; // milliseconds since startup
 static TimerHandle_t common_timer;
@@ -60,6 +61,22 @@ uint64_t timestamp_now(void)
     return common_timestamp;
 }
 
+int dayofweek(uint8_t day, uint8_t month, uint16_t year)
+{
+   /* Zeller's congruence for the Gregorian calendar.
+    * With 0=Monday, ... 5=Saturday, 6=Sunday
+    */
+   if (month < 3) {
+      month += 12;
+      year--;
+   }
+   int k = year % 100;
+   int j = year / 100;
+   int h = day + 13*(month+1)/5 + k + k/4 + j/4 + 5*j;
+   return (h + 5) % 7 + 1;
+}
+
+
 // Return either 0 or 1, somewhat randomly
 int random_bool(void)
 {
diff --git a/src/fsm/common.h b/src/fsm/common.h
index b33f1f8..a2d20ab 100644
--- a/src/fsm/common.h
+++ b/src/fsm/common.h
@@ -37,6 +37,15 @@ void common_init(void);
 // wall clock time.
 uint64_t timestamp_now(void);
 
+/* Convert date to day of week.
+ * day:   1-31
+ * month: 1-12
+ * year:  1900 to 3000
+ *
+ * Returns 1=Monday, ... 6=Saturday, 7=Sunday
+ */
+int dayofweek(uint8_t day, uint8_t month, uint16_t year);
+
 // Return either 0 or 1, somewhat randomly
 int random_bool(void);