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Draft: Implement DST #39

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Draft: Implement DST #39

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136 changes: 113 additions & 23 deletions src/datetime.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -64,6 +64,15 @@ pub const Month = enum(u4) {
}
};

pub const Occurance = enum(u3) {
First = 0,
Second = 1,
Third = 2,
Fourth = 3,
Fifth = 4,
Last = 5,
};

test "month-parse-abbr" {
try testing.expectEqual(try Month.parseAbbr("Jan"), .January);
try testing.expectEqual(try Month.parseAbbr("Oct"), .October);
Expand Down Expand Up @@ -216,6 +225,26 @@ pub const Date = struct {
};
}

// Create the date for a given year, month, and n'th weekday
pub fn fromNthWeekday(year: u32, month: u32, nth: Occurance, wd: Weekday) !Date {
if (year < MIN_YEAR or year > MAX_YEAR) return error.InvalidDate;
if (month < 1 or month > 12) return error.InvalidDate;
// Since we just validated the ranges we can now savely cast
const start = ymd2ord(@intCast(year), @intCast(month), 1);
const dow = start % 7;
const exp_dow: u32 = @intFromEnum(wd);
const n: u32 = @intFromEnum(nth);
var day = if (exp_dow >= dow) (exp_dow - dow) else (7 + exp_dow - dow);
day += 1 + n * 7;
if (nth == .Last) {
const max = daysInMonth(year, month);
while (day > max) {
day -= 7;
}
}
return Date.create(@intCast(year), @intCast(month), @intCast(day));
}

// Return a copy of the date
pub fn copy(self: Date) !Date {
return Date.create(self.year, self.month, self.day);
Expand Down Expand Up @@ -468,10 +497,15 @@ pub const Date = struct {
return @tagName(self.dayOfWeek());
}

// Return the month as an enum
pub fn monthOfYear(self: Date) Month {
assert(self.month >= 1 and self.month <= 12);
return @enumFromInt(self.month);
}

// Return the name of the day of the month, eg "January"
pub fn monthName(self: Date) []const u8 {
assert(self.month >= 1 and self.month <= 12);
return @tagName(@as(Month, @enumFromInt(self.month)));
return @tagName(self.monthOfYear());
}

// ------------------------------------------------------------------------
Expand Down Expand Up @@ -602,6 +636,34 @@ test "date-from-seconds" {
// try testing.expectEqual(date.toSeconds(), tmax);
}

test "date-from-nth-weekday" {
var date = Date.fromNthWeekday(2025, 4, .First, .Monday);
try testing.expectEqual(try Date.create(2025, 4, 7), date); // First Mon
date = Date.fromNthWeekday(2025, 4, .First, .Tuesday);
try testing.expectEqual(try Date.create(2025, 4, 1), date); // First Tues
date = Date.fromNthWeekday(2025, 4, .First, .Wednesday);
try testing.expectEqual(try Date.create(2025, 4, 2), date); // First Wed
date = Date.fromNthWeekday(2025, 4, .First, .Thursday);
try testing.expectEqual(try Date.create(2025, 4, 3), date); // First Thu
date = Date.fromNthWeekday(2025, 4, .First, .Friday);
try testing.expectEqual(try Date.create(2025, 4, 4), date); // First Fri
date = Date.fromNthWeekday(2025, 4, .First, .Saturday);
try testing.expectEqual(try Date.create(2025, 4, 5), date); // First Sat
date = Date.fromNthWeekday(2025, 4, .First, .Sunday);
try testing.expectEqual(try Date.create(2025, 4, 6), date); // First Sun

date = Date.fromNthWeekday(2025, 4, .Second, .Monday);
try testing.expectEqual(try Date.create(2025, 4, 14), date);
date = Date.fromNthWeekday(2025, 4, .Second, .Tuesday);
try testing.expectEqual(try Date.create(2025, 4, 8), date);

date = Date.fromNthWeekday(2025, 4, .Last, .Tuesday);
try testing.expectEqual(try Date.create(2025, 4, 29), date);

date = Date.fromNthWeekday(2024, 11, .Last, .Sunday);
try testing.expectEqual(try Date.create(2024, 11, 24), date); // Last sunday in november 2024
}

test "date-day-of-year" {
var date = try Date.create(1970, 1, 1);
try testing.expect(date.dayOfYear() == 1);
Expand Down Expand Up @@ -794,17 +856,33 @@ test "date-isocalendar" {
}

pub const Timezone = struct {
// Function to compute DST offset
pub const DaylightSavings = *const fn (date: Date, time: Time) i32;

offset: i16, // In minutes
name: []const u8,
dst: ?DaylightSavings,

// Auto register timezones
pub fn create(name: []const u8, offset: i16) Timezone {
const self = Timezone{ .offset = offset, .name = name };
return self;
return Timezone{ .offset = offset, .name = name, .dst = null };
}

pub fn offsetSeconds(self: Timezone) i32 {
return @as(i32, self.offset) * time.s_per_min;
pub fn createDst(name: []const u8, offset: i16, dst: DaylightSavings) Timezone {
return Timezone{ .offset = offset, .name = name, .dst = dst };
}

// Check if two timezones are the same or differ only by their name
pub fn isSame(self: *const Timezone, other: *const Timezone) bool {
return self.offset == other.offset and self.dst == other.dst;
}

// Calculate the timezone offset in minutes for the given date and time factoring in daylight savings if relevant
pub fn dstOffset(self: Timezone, d: Date, t: Time) i32 {
const mins = @as(i32, self.offset);
if (self.dst) |f| {
return mins + f(d, t);
}
return mins;
}
};

Expand Down Expand Up @@ -1196,7 +1274,7 @@ pub const Datetime = struct {
pub fn toTimestamp(self: Datetime) i128 {
const ds = self.date.toTimestamp();
const ts = self.time.toTimestamp();
const zs = self.zone.offsetSeconds() * time.ms_per_s;
const zs = self.zone.dstOffset(self.date, self.time) * time.s_per_min * time.ms_per_s;
return ds + ts - zs;
}

Expand All @@ -1208,20 +1286,18 @@ pub const Datetime = struct {
}

pub fn cmpSameTimezone(self: Datetime, other: Datetime) Order {
assert(self.zone.offset == other.zone.offset);
assert(self.zone.isSame(other.zone));
const r = self.date.cmp(other.date);
if (r != .eq) return r;
return self.time.cmp(other.time);
}

pub fn cmp(self: Datetime, other: Datetime) Order {
if (self.zone.offset == other.zone.offset) {
if (self.zone.isSame(other.zone)) {
return self.cmpSameTimezone(other);
}
// Shift both to utc
const a = self.shiftTimezone(&timezones.UTC);
const b = other.shiftTimezone(&timezones.UTC);
return a.cmpSameTimezone(b);
const shifted = other.shiftTimezone(self.zone);
return self.cmpSameTimezone(shifted);
}

pub fn gt(self: Datetime, other: Datetime) bool {
Expand Down Expand Up @@ -1249,7 +1325,7 @@ pub const Datetime = struct {
// Return a Datetime.Delta relative to this date
pub fn sub(self: Datetime, other: Datetime) Delta {
var days = @as(i32, @intCast(self.date.toOrdinal())) - @as(i32, @intCast(other.date.toOrdinal()));
const offset = (self.zone.offset - other.zone.offset) * time.s_per_min;
const offset = (self.zone.dstOffset(self.date, self.time) - other.zone.dstOffset(other.date, other.time)) * time.s_per_min;
var seconds = (self.time.totalSeconds() - other.time.totalSeconds()) - offset;
var ns = @as(i32, @intCast(self.time.nanosecond)) - @as(i32, @intCast(other.time.nanosecond));
while (seconds > 0 and ns < 0) {
Expand Down Expand Up @@ -1286,10 +1362,14 @@ pub const Datetime = struct {
// Convert to the given timeszone
pub fn shiftTimezone(self: Datetime, zone: *const Timezone) Datetime {
var dt =
if (self.zone.offset == zone.offset)
(self.copy() catch unreachable)
else
self.shiftMinutes(zone.offset - self.zone.offset);
if (self.zone.isSame(zone))
(self.copy() catch unreachable)
else if (self.zone.dst == zone.dst)
// Any DST effects whill be the same so just compare the offset directly
self.shiftMinutes(zone.offset - self.zone.offset)
else
// Shift adjusting for any DST effects
self.shiftMinutes(zone.dstOffset(self.date, self.time) - self.zone.dstOffset(self.date, self.time));
dt.zone = zone;
return dt;
}
Expand Down Expand Up @@ -1393,10 +1473,11 @@ pub const Datetime = struct {
/// e.g. "2023-06-10T14:06:40.015006+08:00"
pub fn formatISO8601(self: Datetime, allocator: Allocator, with_micro: bool) ![]const u8 {
var sign: u8 = '+';
if (self.zone.offset < 0) {
const dst_offset = self.zone.dstOffset(self.date, self.time);
if (dst_offset < 0) {
sign = '-';
}
const offset = @abs(self.zone.offset);
const offset = @abs(dst_offset);

var micro_part_len: u3 = 0;
var micro_part: [7]u8 = undefined;
Expand Down Expand Up @@ -1425,10 +1506,11 @@ pub const Datetime = struct {

pub fn formatISO8601Buf(self: Datetime, buf: []u8, with_micro: bool) ![]const u8 {
var sign: u8 = '+';
if (self.zone.offset < 0) {
const dst_offset = self.zone.dstOffset(self.date, self.time);
if (dst_offset < 0) {
sign = '-';
}
const offset = @abs(self.zone.offset);
const offset = @abs(dst_offset);

var micro_part_len: usize = 0;
var micro_part: [7]u8 = undefined;
Expand Down Expand Up @@ -1673,6 +1755,14 @@ test "readme-example" {

}

test "datetime-now-in-ny" {
const allocator = std.testing.allocator;
const now = Datetime.now().shiftTimezone(&timezones.America.New_York);
const now_str = try now.formatHttp(allocator);
defer allocator.free(now_str);
std.log.warn("New york time is: {s}\n", .{now_str});
}

test "datetime-format-ISO8601" {
const allocator = std.testing.allocator;

Expand Down
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