1 files changed, 40 insertions, 7 deletions

diff --git a/src/date.rs b/src/date.rs
index 49a7642..194fd02 100644
--- a/src/date.rs
+++ b/src/date.rs
@@ -12,12 +12,10 @@ pub struct Date {
 
 impl Date {
 	/// The earliest date which can be represented
-	pub const MIN: Self =
-		unsafe { Self::from_calendar_date_unchecked(Year::MIN, Month::January, 1) };
+	pub const MIN: Self = unsafe { Self::from_ymd_unchecked(Year::MIN, Month::January, 1) };
 
 	/// The latest date which can be represented
-	pub const MAX: Self =
-		unsafe { Self::from_calendar_date_unchecked(Year::MAX, Month::December, 31) };
+	pub const MAX: Self = unsafe { Self::from_ymd_unchecked(Year::MAX, Month::December, 31) };
 
 	// TODO validated from_calendar_date
 
@@ -26,17 +24,17 @@ impl Date {
 	/// # Example
 	///
 	/// ```
-	/// use botic::Date;
+	/// use botic::{Date, Month, Year};
 	///
 	/// let y2k = unsafe {
-	///     Date::from_calendar_date_unchecked(Year::from(2000), Month::January, 1)
+	///     Date::from_ymd_unchecked(Year::from(2000), Month::January, 1)
 	/// };
 	/// ```
 	///
 	/// # Safety
 	///
 	/// This function results in undefined behavior if the given date is not a real date
-	pub const unsafe fn from_calendar_date_unchecked(year: Year, month: Month, day: u8) -> Self {
+	pub const unsafe fn from_ymd_unchecked(year: Year, month: Month, day: u8) -> Self {
 		Self { year, month, day }
 	}
 
@@ -53,6 +51,41 @@ impl Date {
 	pub const fn day(self) -> u8 {
 		self.day
 	}
+
+	pub const fn is_leap_year(self) -> bool {
+		self.year.is_leap_year()
+	}
+
+	// TODO handle BCE properly
+	pub const fn days_after_common_era(self) -> isize {
+		let year = self.year.wrapping_sub(1);
+		let leap_years = (year.as_i16() / 4 - year.as_i16() / 100 + year.as_i16() / 400) as isize;
+		let month_last_day_ordinal = self.month.last_day_ordinal(self.is_leap_year()) as isize;
+
+		year.as_i16() as isize * 365 + leap_years + month_last_day_ordinal + self.day as isize - 1
+	}
+
+	// TODO test
+	pub const fn from_days_after_common_era(days: isize) -> Self {
+		let era = days / 146097; // an era is a period of 400 year
+		let day_of_era = days - (era * 146097);
+		let year_of_era = day_of_era / 365;
+		let year = year_of_era + (era * 400);
+		let ordinal = day_of_era - (365 * year + year / 4 - year / 100);
+		// TODO look at as's
+		let year = Year::from_i16(year as i16);
+		let month = Month::from_ordinal(ordinal as u16, year.is_leap_year());
+		let day = ordinal as u16 - month.previous().last_day_ordinal(year.is_leap_year());
+		let day = day as u8;
+
+		unsafe { Self::from_ymd_unchecked(year, month, day) }
+	}
+
+	#[must_use]
+	pub const fn add_days(self, days: isize) -> Self {
+		let total_days_since_ce = self.days_after_common_era() + days;
+		Self::from_days_after_common_era(total_days_since_ce)
+	}
 }
 
 impl PartialOrd for Date {