conversion from DateTime structure to timestamp (partial)

This only handles dates after 1970. The code
is getting ugly and I think it is the wrong approach.
Will clean up in the next commit

src/date.zig

@@ -6,12 +6,13 @@ const std = @import("std");
 
 pub const DateTime = struct { day: u8, month: u8, year: u16, hour: u8, minute: u8, second: u8 };
 
+const SECONDS_PER_DAY = 86400; //*  24* 60 * 60 */
+const DAYS_PER_YEAR = 365; //* Normal year (no leap year) */
+
 pub fn timestampToDateTime(timestamp: i64) DateTime {
 
     // aus https://de.wikipedia.org/wiki/Unixzeit
     const unixtime = @intCast(u64, timestamp);
-    const SECONDS_PER_DAY = 86400; //*  24* 60 * 60 */
-    const DAYS_PER_YEAR = 365; //* Normal year (no leap year) */
     const DAYS_IN_4_YEARS = 1461; //*   4*365 +   1 */
     const DAYS_IN_100_YEARS = 36524; //* 100*365 +  25 - 1 */
     const DAYS_IN_400_YEARS = 146097; //* 400*365 + 100 - 4 + 1 */
@@ -54,6 +55,127 @@ pub fn timestampToDateTime(timestamp: i64) DateTime {
     return DateTime{ .day = day, .month = month, .year = year, .hour = hours, .minute = minutes, .second = seconds };
 }
 
+/// Converts a string to a timestamp value. May not handle dates before the
+/// epoch
+pub fn parseIso8601Timestamp(data: []const u8) !i64 {
+    _ = data;
+    return error.NotImplemented;
+
+    // TODO: Use a parsing for loop with a state machine implementation
+    // to tell us where we are in the string
+    // if (data.len < 4) return error.NotEnoughData;
+    // var year = try std.fmt.parseInt(u8, data[0..4], 10);
+    //
+    // var month:u4 = 0;
+    // if (data.len > 5) {
+    //     if (data[5] != '-') return error.InvalidCharacter;
+    //     var next_dash = std.mem.indexOf(u8, data[6..], "-");
+    //     if (next_dash == null)
+    //         next_dash = data.len - 6;
+    //     month = std.fmt.parseInt(u8, data[6..next_dash + 6], 10);
+    // }
+    // var day:u5 = 0;
+    // var hours: u5 = 0;
+    // var minutes: u6 = 0;
+    // var seconds: u6 = 0;
+    // var milliseconds: u9 = 0;
+    // ISO 8601 is complicated. We're going
+}
+
+fn dateTimeToTimestamp(datetime: DateTime) !i64 {
+    if (datetime.month > 12 or
+        datetime.day > 31 or
+        datetime.hour >= 24 or
+        datetime.minute >= 60 or
+        datetime.second >= 60) return error.DateTimeOutOfRange;
+    const epoch_year = 1970;
+    if (datetime.year < epoch_year) return error.DatesBeforeEpochNotImplemented;
+    const leap_years_between = leapYearsBetween(epoch_year, datetime.year);
+    var add_days: u1 = 0;
+    const years_diff = std.math.absCast(@as(i17, datetime.year) - @as(i17, epoch_year));
+    std.log.debug("Years from epoch: {d}, Leap years: {d}", .{ years_diff, leap_years_between });
+    var days_diff: i32 = (years_diff * DAYS_PER_YEAR) + leap_years_between + add_days;
+    std.log.debug("Days with leap year, without month: {d}", .{days_diff});
+
+    const seconds_into_year = secondsFromBeginningOfYear(
+        datetime.year,
+        datetime.month,
+        datetime.day,
+        datetime.hour,
+        datetime.minute,
+        datetime.second,
+    );
+    return (days_diff * SECONDS_PER_DAY) + @as(i64, seconds_into_year);
+}
+
+fn secondsFromBeginningOfYear(year: u16, month: u8, day: u8, hour: u8, minute: u8, second: u8) u32 {
+    const current_year_is_leap_year = isLeapYear(year);
+    const leap_year_days_per_month: [12]u5 = .{ 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
+    const normal_days_per_month: [12]u5 = .{ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
+    const days_per_month = if (current_year_is_leap_year) leap_year_days_per_month else normal_days_per_month;
+    var current_month: usize = 1;
+    var end_month = month;
+    var days_diff: u32 = 0;
+    while (current_month != end_month) {
+        days_diff += days_per_month[current_month - 1]; // months are 1-based vs array is 0-based
+        current_month += 1;
+    }
+    std.log.debug("Days with month, without day: {d}. Day of month {d}, will add {d} days", .{
+        days_diff,
+        day,
+        day - 1,
+    });
+    // We need -1 because we're not actually including the ending day (that's up to hour/minute)
+    // In other words, days in the month are 1-based, while hours/minutes are zero based
+    days_diff += day - 1;
+    std.log.debug("Total days diff: {d}", .{days_diff});
+    var seconds_diff: u32 = days_diff * SECONDS_PER_DAY;
+
+    // From here out, we want to get everything into seconds
+    seconds_diff += @as(u32, hour) * 60 * 60;
+    seconds_diff += @as(u32, minute) * 60;
+    seconds_diff += @as(u32, second);
+
+    return seconds_diff;
+}
+fn isLeapYear(year: u16) bool {
+    if (year % 4 != 0) return false;
+    if (year % 400 == 0) return true;
+    if (year % 100 == 0) return false;
+    return true;
+}
+
+fn leapYearsBetween(start_year_inclusive: u16, end_year_exclusive: u16) u16 {
+    const start = std.math.min(start_year_inclusive, end_year_exclusive);
+    const end = std.math.max(start_year_inclusive, end_year_exclusive);
+    var current = start;
+    std.log.debug("Leap years starting from {d}, ending at {d}", .{ start, end });
+    while (current % 4 != 0 and current < end) {
+        current += 1;
+    }
+    if (current == end) return 0; // No leap years here. E.g. 1971-1973
+    // We're on a potential leap year, and now we can step by 4
+    var rc: u16 = 0;
+    while (current < end) {
+        if (current % 4 == 0) {
+            if (current % 100 != 0) {
+                std.log.debug("Year {d} is leap year", .{current});
+                rc += 1;
+                current += 4;
+                continue;
+            }
+            // We're on a century, which is normally not a leap year, unless
+            // it's divisible by 400
+            if (current % 400 == 0) {
+                std.log.debug("Year {d} is leap year", .{current});
+                rc += 1;
+            }
+        }
+        current += 4;
+    }
+    return rc;
+}
+
 fn printDateTime(dt: DateTime) void {
     std.log.debug("{:0>4}-{:0>2}-{:0>2}T{:0>2}:{:0>2}:{:0<2}Z", .{
         dt.year,
@@ -69,9 +191,7 @@ pub fn printNowUtc() void {
     printDateTime(timestampToDateTime(std.time.timestamp()));
 }
 
-test "GMT and localtime" {
+test "Convert timestamp to datetime" {
-    std.testing.log_level = .debug;
-    std.log.debug("\n", .{});
     printDateTime(timestampToDateTime(std.time.timestamp()));
     try std.testing.expectEqual(DateTime{ .year = 2020, .month = 8, .day = 28, .hour = 9, .minute = 32, .second = 27 }, timestampToDateTime(1598607147));
 
@@ -79,3 +199,31 @@ test "GMT and localtime" {
     // Get time for date: https://wtools.io/convert-date-time-to-unix-time
     try std.testing.expectEqual(DateTime{ .year = 2015, .month = 08, .day = 30, .hour = 12, .minute = 36, .second = 00 }, timestampToDateTime(1440938160));
 }
+
+test "Convert datetime to timestamp" {
+    std.testing.log_level = .debug;
+    std.log.debug("\n", .{});
+    try std.testing.expectEqual(@as(i64, 1598607147), try dateTimeToTimestamp(DateTime{ .year = 2020, .month = 8, .day = 28, .hour = 9, .minute = 32, .second = 27 }));
+    try std.testing.expectEqual(@as(i64, 1604207167), try dateTimeToTimestamp(DateTime{ .year = 2020, .month = 11, .day = 1, .hour = 5, .minute = 6, .second = 7 }));
+    try std.testing.expectEqual(@as(i64, 1440938160), try dateTimeToTimestamp(DateTime{ .year = 2015, .month = 08, .day = 30, .hour = 12, .minute = 36, .second = 00 }));
+}
+
+test "Convert ISO8601 string to timestamp" {
+    try std.testing.expectEqual(@as(i64, 1598607147), try dateTimeToTimestamp(DateTime{ .year = 2020, .month = 8, .day = 28, .hour = 9, .minute = 32, .second = 27 }));
+    try std.testing.expectEqual(@as(i64, 1604207167), try dateTimeToTimestamp(DateTime{ .year = 2020, .month = 11, .day = 1, .hour = 5, .minute = 6, .second = 7 }));
+    try std.testing.expectEqual(@as(i64, 1440938160), try dateTimeToTimestamp(DateTime{ .year = 2015, .month = 08, .day = 30, .hour = 12, .minute = 36, .second = 00 }));
+}
+// TODO: I think before epoch, the best approach is to flip the epoch and the
+//       input date, calculate the answer, then flip signs. However, this requires
+//       re-designing the algorithm to start from something other than midnight
+//       January 1st. This will require an overhaul, so for now, we'll leave
+//       this unimplemented.
+//
+// test "Convert datetime to timestamp before 1970" {
+//     std.testing.log_level = .debug;
+//     std.log.debug("\n", .{});
+//     try std.testing.expectEqual(@as(i64, -449392815000), try dateTimeToTimestamp(DateTime{ .year = 1955, .month = 10, .day = 05, .hour = 16, .minute = 39, .second = 45 }));
+//
+//
+//   1955.1 - .1 + x = 1970
+// }