Added gmtime() and a placeholder implementation of localtime()

photon/libc/time/gmtime.c

@@ -0,0 +1,75 @@
+#include <time.h>
+#include <stdbool.h>
+
+static struct tm g_time_struct = {};
+
+/* to the suprise of nobody, this is from stack overflow.
+ *      https://stackoverflow.com/questions/21593692/convert-unix-timestamp-to-date-without-system-libs
+ */
+struct tm *gmtime(const time_t *timer)
+{
+    unsigned int seconds, minutes, hours, days, year, month;
+    unsigned int day_of_week;
+    seconds = *timer;
+
+    /* calculate minutes */
+    minutes  = seconds / 60;
+    seconds -= minutes * 60;
+    /* calculate hours */
+    hours    = minutes / 60;
+    minutes -= hours   * 60;
+    /* calculate days */
+    days     = hours   / 24;
+    hours   -= days    * 24;
+
+    /* Unix time starts in 1970 on a Thursday */
+    year      = 1970;
+    day_of_week = 4;
+
+    while(1) {
+        bool     leap_year   = (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0));
+        uint16_t days_in_year = leap_year ? 366 : 365;
+        if (days >= days_in_year) {
+            day_of_week += leap_year ? 2 : 1;
+            days      -= days_in_year;
+            if (day_of_week >= 7) {
+                day_of_week -= 7;
+            }
+
+            ++year;
+        } else {
+            g_time_struct.tm_yday = days;
+            day_of_week  += days;
+            day_of_week  %= 7;
+
+            /* calculate the month and day */
+            static const uint8_t days_in_month[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
+            for (month = 0; month < 12; ++month) {
+                uint8_t dim = days_in_month[month];
+
+                /* add a day to feburary if this is a leap year */
+                if (month == 1 && leap_year) {
+                    ++dim;
+                }
+
+                if (days >= dim) {
+                    days -= dim;
+                } else {
+                    break;
+                }
+            }
+
+            break;
+        }
+    }
+
+    g_time_struct.tm_sec  = seconds;
+    g_time_struct.tm_min  = minutes;
+    g_time_struct.tm_hour = hours;
+    g_time_struct.tm_mday = days + 1;
+    g_time_struct.tm_mon  = month;
+    g_time_struct.tm_year = year;
+    g_time_struct.tm_wday = day_of_week;
+
+    return &g_time_struct;
+}

photon/libc/time/localtime.c

@@ -0,0 +1,7 @@
+#include <time.h>
+
+struct tm *localtime(const time_t *timer)
+{
+    /* TODO timezone support */
+    return gmtime(timer);
+}