    //! Epoch reference times in terms of their difference from
    //! UTC 1970-01-01 in seconds.
    const std = @import("../std.zig");
    const testing = std.testing;
    const math = std.math;

    /// Jan 01, 1970 AD
    pub const posix = 0;
    /// Jan 01, 1980 AD
    pub const dos = 315532800;
    /// Jan 01, 2001 AD
    pub const ios = 978307200;
    /// Nov 17, 1858 AD
    pub const openvms = -3506716800;
    /// Jan 01, 1900 AD
    pub const zos = -2208988800;
    /// Jan 01, 1601 AD
    pub const windows = -11644473600;
    /// Jan 01, 1978 AD
    pub const amiga = 252460800;
    /// Dec 31, 1967 AD
    pub const pickos = -63244800;
    /// Jan 06, 1980 AD
    pub const gps = 315964800;
    /// Jan 01, 0001 AD
    pub const clr = -62135769600;

    pub const unix = posix;
    pub const android = posix;
    pub const os2 = dos;
    pub const bios = dos;
    pub const vfat = dos;
    pub const ntfs = windows;
    pub const ntp = zos;
    pub const jbase = pickos;
    pub const aros = amiga;
    pub const morphos = amiga;
    pub const brew = gps;
    pub const atsc = gps;
    pub const go = clr;

    /// The type that holds the current year, i.e. 2016
    pub const Year = u16;

    pub const epoch_year = 1970;
    pub const secs_per_day: u17 = 24 * 60 * 60;

    pub fn isLeapYear(year: Year) bool {
        if (@mod(year, 4) != 0)
            return false;
        if (@mod(year, 100) != 0)
            return true;
        return (0 == @mod(year, 400));
    }

    test "isLeapYear" {
        try testing.expectEqual(false, isLeapYear(2095));
        try testing.expectEqual(true, isLeapYear(2096));
        try testing.expectEqual(false, isLeapYear(2100));
        try testing.expectEqual(true, isLeapYear(2400));
    }

    pub fn getDaysInYear(year: Year) u9 {
        return if (isLeapYear(year)) 366 else 365;
    }

    pub const YearLeapKind = enum(u1) { not_leap, leap };

    pub const Month = enum(u4) {
        jan = 1,
        feb,
        mar,
        apr,
        may,
        jun,
        jul,
        aug,
        sep,
        oct,
        nov,
        dec,

        /// return the numeric calendar value for the given month
        /// i.e. jan=1, feb=2, etc
        pub fn numeric(self: Month) u4 {
            return @intFromEnum(self);
        }
    };

    /// Get the number of days in the given month
    pub fn getDaysInMonth(leap_year: YearLeapKind, month: Month) u5 {
        return switch (month) {
            .jan => 31,
            .feb => @as(u5, switch (leap_year) {
                .leap => 29,
                .not_leap => 28,
            }),
            .mar => 31,
            .apr => 30,
            .may => 31,
            .jun => 30,
            .jul => 31,
            .aug => 31,
            .sep => 30,
            .oct => 31,
            .nov => 30,
            .dec => 31,
        };
    }

    pub const YearAndDay = struct {
        year: Year,
        /// The number of days into the year (0 to 365)
        day: u9,

        pub fn calculateMonthDay(self: YearAndDay) MonthAndDay {
            var month: Month = .jan;
            var days_left = self.day;
            const leap_kind: YearLeapKind = if (isLeapYear(self.year)) .leap else .not_leap;
            while (true) {
                const days_in_month = getDaysInMonth(leap_kind, month);
                if (days_left < days_in_month)
                    break;
                days_left -= days_in_month;
                month = @as(Month, @enumFromInt(@intFromEnum(month) + 1));
            }
            return .{ .month = month, .day_index = @as(u5, @intCast(days_left)) };
        }
    };

    pub const MonthAndDay = struct {
        month: Month,
        day_index: u5, // days into the month (0 to 30)

    };

    // days since epoch Oct 1, 1970

    pub const EpochDay = struct {
        day: u47, // u47 = u64 - u17 (because day = sec(u64) / secs_per_day(u17)

        pub fn calculateYearDay(self: EpochDay) YearAndDay {
            var year_day = self.day;
            var year: Year = epoch_year;
            while (true) {
                const year_size = getDaysInYear(year);
                if (year_day < year_size)
                    break;
                year_day -= year_size;
                year += 1;
            }
            return .{ .year = year, .day = @as(u9, @intCast(year_day)) };
        }
    };

    /// seconds since start of day
    pub const DaySeconds = struct {
        secs: u17, // max is 24*60*60 = 86400

        /// the number of hours past the start of the day (0 to 23)
        pub fn getHoursIntoDay(self: DaySeconds) u5 {
            return @as(u5, @intCast(@divTrunc(self.secs, 3600)));
        }
        /// the number of minutes past the hour (0 to 59)
        pub fn getMinutesIntoHour(self: DaySeconds) u6 {
            return @as(u6, @intCast(@divTrunc(@mod(self.secs, 3600), 60)));
        }
        /// the number of seconds past the start of the minute (0 to 59)
        pub fn getSecondsIntoMinute(self: DaySeconds) u6 {
            return math.comptimeMod(self.secs, 60);
        }
    };

    /// seconds since epoch Oct 1, 1970 at 12:00 AM
    pub const EpochSeconds = struct {
        secs: u64,

        /// Returns the number of days since the epoch as an EpochDay.
        /// Use EpochDay to get information about the day of this time.
        pub fn getEpochDay(self: EpochSeconds) EpochDay {
            return EpochDay{ .day = @as(u47, @intCast(@divTrunc(self.secs, secs_per_day))) };
        }

        /// Returns the number of seconds into the day as DaySeconds.
        /// Use DaySeconds to get information about the time.
        pub fn getDaySeconds(self: EpochSeconds) DaySeconds {
            return DaySeconds{ .secs = math.comptimeMod(self.secs, secs_per_day) };
        }
    };

    fn testEpoch(secs: u64, expected_year_day: YearAndDay, expected_month_day: MonthAndDay, expected_day_seconds: struct {
        /// 0 to 23
        hours_into_day: u5,
        /// 0 to 59
        minutes_into_hour: u6,
        /// 0 to 59
        seconds_into_minute: u6,
    }) !void {
        const epoch_seconds = EpochSeconds{ .secs = secs };
        const epoch_day = epoch_seconds.getEpochDay();
        const day_seconds = epoch_seconds.getDaySeconds();
        const year_day = epoch_day.calculateYearDay();
        try testing.expectEqual(expected_year_day, year_day);
        try testing.expectEqual(expected_month_day, year_day.calculateMonthDay());
        try testing.expectEqual(expected_day_seconds.hours_into_day, day_seconds.getHoursIntoDay());
        try testing.expectEqual(expected_day_seconds.minutes_into_hour, day_seconds.getMinutesIntoHour());
        try testing.expectEqual(expected_day_seconds.seconds_into_minute, day_seconds.getSecondsIntoMinute());
    }

    test "epoch decoding" {
        try testEpoch(0, .{ .year = 1970, .day = 0 }, .{
            .month = .jan,
            .day_index = 0,
        }, .{ .hours_into_day = 0, .minutes_into_hour = 0, .seconds_into_minute = 0 });

        try testEpoch(31535999, .{ .year = 1970, .day = 364 }, .{
            .month = .dec,
            .day_index = 30,
        }, .{ .hours_into_day = 23, .minutes_into_hour = 59, .seconds_into_minute = 59 });

        try testEpoch(1622924906, .{ .year = 2021, .day = 31 + 28 + 31 + 30 + 31 + 4 }, .{
            .month = .jun,
            .day_index = 4,
        }, .{ .hours_into_day = 20, .minutes_into_hour = 28, .seconds_into_minute = 26 });

        try testEpoch(1625159473, .{ .year = 2021, .day = 31 + 28 + 31 + 30 + 31 + 30 }, .{
            .month = .jul,
            .day_index = 0,
        }, .{ .hours_into_day = 17, .minutes_into_hour = 11, .seconds_into_minute = 13 });
    }