1 files changed, 130 insertions, 0 deletions
diff --git a/unixtime.h b/unixtime.h
new file mode 100644
index 0000000..7dc4824
--- /dev/null
+++ b/unixtime.h
@@ -0,0 +1,130 @@
+#ifndef UNIXTIME_H
+#define UNIXTIME_H
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#include "dlog.h"
+
+#define UNIX_EPOCH_YEAR 1970
+#define DOS_EPOCH_YEAR  1980
+
+static bool is_leap_year(unsigned year)
+{
+	return ((year % 4) == 0 && (year % 100) != 0 || (year % 400) == 0);
+}
+
+static int days_per_year(unsigned year)
+{
+	return is_leap_year(year) ? 366 : 365;
+}
+
+static int days_per_month(unsigned month, bool leapyear)
+{
+	switch (month) {
+	case 1:
+	case 3:
+	case 5:
+	case 7:
+	case 8:
+	case 10:
+	case 12:
+		return 31;
+	case 4:
+	case 6:
+	case 9:
+	case 11:
+		return 30;
+	case 2:
+		return leapyear ? 29 : 28;
+	}
+
+	return 0;
+}
+
+static void timestampns_to_dos_time(uint16_t __far *dos_time, uint16_t __far *dos_date, int64_t timestampns, int32_t tzoffset)
+{
+	// To maximize range of 32-bit ints, we'll use DOS's "2 seconds" as a unit instead of 1 second
+	long days_since_epoch = 0;
+	unsigned hours = 0, minutes = 0, seconds2 = 0;
+	unsigned year, month, day;
+	int per_year, per_month;
+	bool is_leap;
+
+	__asm {
+		push eax                     /* Preserve 32-bit regs */
+		push ecx
+		push edx
+		mov eax, dword ptr [timestampns]
+		mov edx, dword ptr [timestampns + 4]
+		mov ecx, 2 * 1000000000      /* nanoseconds in 2 seconds, should still fit in a dword */
+
+		idiv ecx                     /*  64-bit signed divison edx:eax / ecx, returns quotient in eax, remainder in edx */
+		                             /* TODO: Handle overflow, which will trigger an interrupt */
+
+		/* eax now contains seconds_since_epoch / 2 */
+		xor edx, edx                 /* Discard the remainder (less than 2 seconds) */
+
+		add eax, [tzoffset]          /* Add tzoffset now (which is in seconds / 2 units) */
+
+		mov ecx, (24 * 60 * 60) / 2  /* seconds in one day / 2 */
+
+		idiv ecx
+		/* eax now contains days_since_epoch */
+		/* edx contains seconds (since start of day) /2 , this now fits in a 16-bit word */
+
+		mov [days_since_epoch], eax
+
+		mov eax, edx                /* eax = seconds / 2 */
+		xor edx, edx
+		mov ecx, 60 / 2             /* seconds in a minute / 2; also clears upper part of ecx, we'll start using 16-bit registers from now on */
+
+		div cx
+		/* ax contains minutes, dx contains remainder seconds/2 -- should be < 30 */
+		mov [seconds2], dx
+
+		xor dx, dx
+		mov cx, 60                  /* minutes per hour */
+		div cx
+		/* ax contains hours (< 24), dx contains remainder minutes (< 60) */
+		mov [minutes], dx
+		mov [hours], ax
+
+		pop edx
+		pop ecx
+		pop eax
+	}
+
+	year = UNIX_EPOCH_YEAR;
+	if (days_since_epoch > 0) {
+		while (days_since_epoch >= (per_year = days_per_year(year))) {
+			days_since_epoch -= per_year;
+			year++;
+		}
+	} else {
+		while (days_since_epoch < 0) {
+			days_since_epoch += days_per_year(year);
+			year--;
+		}
+	}
+
+	month = 1;
+	is_leap = is_leap_year(year);
+	while (days_since_epoch >= (per_month = days_per_month(month, is_leap))) {
+		days_since_epoch -= per_month;
+		month++;
+	}
+	day = 1 + days_since_epoch; // (day is 1-based)
+
+	if (year < DOS_EPOCH_YEAR) {
+		dlog_puts("Year is too old, will show as 0");
+		year = 0;
+	} else {
+		year -= DOS_EPOCH_YEAR;
+	}
+
+	*dos_time = ((hours << 11) & 0xF800) | ((minutes << 5) & 0x7E0) | (seconds2 & 0x1F);
+	*dos_date = ((year << 9) & 0xFE00)   | ((month << 5) & 0x1E0)   | (day & 0x1F);
+}
+
+#endif // UNIXTIME_H