const std = @import("std");

const expect = std.testing.expect;
const expectEqualStrings = std.testing.expectEqualStrings;
const expectEqual = std.testing.expectEqual;
const Allocator = std.mem.Allocator;

pub const TimeError = error{ParsingError};

pub const Time = struct {
    /// expecting a date in the format yyyy-MM-dd'T'HH:mm:ss
    /// returning a unix timestamp
    /// does not handle leap seconds
    /// only handles UTC
    pub fn parseIso8601(data: []const u8) !i64 {
        const format = "nnnn-nn-nnTnn:nn:nn";
        //              0123456789012345678

        // validate the format
        if (data.len != 19) {
            return TimeError.ParsingError;
        }

        for (format, 0..) |formatChar, index| {
            if (index >= data.len) {
                return TimeError.ParsingError;
            }

            const c = data[index];
            switch (formatChar) {
                'n' => {
                    if (!isDigit(c)) return TimeError.ParsingError;
                },
                else => {
                    if (formatChar != c) return TimeError.ParsingError;
                },
            }
        }

        const year = try std.fmt.parseInt(i64, data[0..4], 10);
        const month = try std.fmt.parseInt(i64, data[5..7], 10);
        const day = try std.fmt.parseInt(i64, data[8..10], 10);
        const hour = try std.fmt.parseInt(i64, data[11..13], 10);
        const minute = try std.fmt.parseInt(i64, data[14..16], 10);
        const second = try std.fmt.parseInt(i64, data[17..19], 10);

        const unixTime: i64 = (year - 1970) * 365 * 24 * 3600 //
        + leapDays(year, month) * 24 * 3600 //
        + dayInYear(month, day) * 24 * 3600 //
        + hour * 3600 //
        + minute * 60 //
        + second;

        return unixTime;
    }

    fn isDigit(byte: u8) bool {
        return byte >= '0' and byte <= '9';
    }

    fn dayInYear(month: i64, day: i64) i64 {
        const daysPerMonth = [_]u8{ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
        var result: i64 = 0;
        var i: u8 = 0;
        while (i < (month - 1)) : (i += 1) {
            result += daysPerMonth[i];
        }
        result += day - 1;
        return result;
    }
    fn leapDays(year: i64, month: i64) i64 {
        var i: u64 = 1972;
        var result: i64 = 0;
        while (i < year) : (i += 1) {
            if (@mod(i, 4) == 0 and (i % 100 != 0 or @mod(i, 400) == 0)) {
                result += 1;
            }
        }
        if (month > 2 and @mod(year, 4) == 0 and (@mod(year, 100) != 0 or @mod(year, 400) == 0)) {
            result += 1;
        }
        return result;
    }
};

test "parsing error 'x025-01-01T00:00:00'" {
    try std.testing.expectError(TimeError.ParsingError, Time.parseIso8601("x025-01-01T00:00:00"));
}

test "parsing error '2025-01-01'" {
    try std.testing.expectError(TimeError.ParsingError, Time.parseIso8601("2025-01-01T"));
}

//test "parsing validation successfull" {
//    const y = try Time.parseIso8601("2025-12-13T01:02:03");
//    try expectEqual(12, y);
//}

test "parse 2025-02-18T18:37:45, expecting 1739903865" {
    const unixTime = try Time.parseIso8601("2025-02-18T18:37:45");
    try expectEqual(1739903865, unixTime);
}