const std = @import("std");
const debug = std.debug;
const testing = std.testing;

pub const TimeZoneTag = enum(u1) {
    utc,
    offset,
};

pub const Sign = enum(u1) {
    positive,
    negative,
};

const HoursMinutes = struct {
    hours: u4,
    minutes: ?u6,
};

pub const Offset = union(Sign) {
    positive: HoursMinutes,
    negative: HoursMinutes,

    const Self = @This();

    pub fn new(hours: i8, minutes: ?u6) ?Self {
        if (hours > 12 or hours < -12) {
            return null;
        }
        if (minutes) |m| {
            if (m > 59) return null;
            if (hours == 0 and m == 0) return null;
        } else if (hours == 0) return null;
        if (hours < 0) {
            const h = @intCast(u4, @as(i8, hours) * -1);
            return Self{ .negative = .{ .hours = h, .minutes = minutes } };
        } else {
            return Self{ .positive = .{ .hours = @intCast(u4, hours), .minutes = minutes } };
        }
    }

    pub fn asSeconds(self: Self) i64 {
        return switch (self) {
            .positive => |ofs| blk: {
                var seconds = @as(i64, ofs.hours) * 3600;
                if (ofs.minutes) |m| seconds += (@as(i64, m) * 60);
                break :blk seconds;
            },
            .negative => |ofs| blk: {
                var seconds = @as(i64, ofs.hours) * 3600;
                if (ofs.minutes) |m| seconds += (@as(i64, m) * 60);
                break :blk seconds * -1;
            },
        };
    }
};

pub const TimeZone = union(TimeZoneTag) {
    utc: void,
    offset: Offset,

    const Self = @This();

    pub fn new(hours: ?i8, minutes: ?u6) ?Self {
        return if (hours) |h| blk: {
            if (h == 0) {
                break :blk .utc;
            } else if (Offset.new(h, minutes)) |ofs| {
                break :blk Self{ .offset = ofs };
            } else {
                break :blk null;
            }
        } else if (minutes) |m| Self{ .offset = Offset.new(0, m).? } else .utc;
    }

    pub fn format(
        self: Self,
        comptime fmt: []const u8,
        options: std.fmt.FormatOptions,
        writer: anytype,
    ) !void {
        _ = fmt;
        _ = options;

        switch (self) {
            .utc => try writer.writeAll("Z"),
            .offset => |ofs| switch (ofs) {
                .positive => |p| {
                    try writer.print("+{d:0>2}", .{p.hours});
                    if (p.minutes) |m| {
                        try writer.print(":{d:0>2}", .{m});
                    }
                },
                .negative => |n| {
                    try writer.print("-{d:0>2}", .{n.hours});
                    if (n.minutes) |m| {
                        try writer.print(":{d:0>2}", .{m});
                    }
                },
            },
        }
    }
};