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// This file is part of ICU4X.
//
// The contents of this file implement algorithms from Calendrical Calculations
// by Reingold & Dershowitz, Cambridge University Press, 4th edition (2018),
// under the Apache-2.0 license. Accordingly, this file is released under
// the Apache License, Version 2.0 which can be found at the calendrical_calculations
use crate::helpers::{i64_to_i32, I32CastError};
use crate::rata_die::RataDie;
// The Gregorian epoch is equivalent to first day in fixed day measurement
const EPOCH: RataDie = RataDie::new(1);
/// Whether or not `year` is a leap year
pub fn is_leap_year(year: i32) -> bool {
year % 4 == 0 && (year % 400 == 0 || year % 100 != 0)
}
// Fixed is day count representation of calendars starting from Jan 1st of year 1.
// The fixed calculations algorithms are from the Calendrical Calculations book.
//
/// Lisp code reference: <https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1167-L1189>
pub fn fixed_from_iso(year: i32, month: u8, day: u8) -> RataDie {
let prev_year = (year as i64) - 1;
// Calculate days per year
let mut fixed: i64 = (EPOCH.to_i64_date() - 1) + 365 * prev_year;
// Calculate leap year offset
let offset = prev_year.div_euclid(4) - prev_year.div_euclid(100) + prev_year.div_euclid(400);
// Adjust for leap year logic
fixed += offset;
// Days of current year
fixed += (367 * (month as i64) - 362).div_euclid(12);
// Leap year adjustment for the current year
fixed += if month <= 2 {
0
} else if is_leap_year(year) {
-1
} else {
-2
};
// Days passed in current month
fixed += day as i64;
RataDie::new(fixed)
}
/// Lisp code reference: <https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1191-L1217>
pub(crate) fn iso_year_from_fixed(date: RataDie) -> i64 {
// Shouldn't overflow because it's not possbile to construct extreme values of RataDie
let date = date - EPOCH;
// 400 year cycles have 146097 days
let (n_400, date) = (date.div_euclid(146097), date.rem_euclid(146097));
// 100 year cycles have 36524 days
let (n_100, date) = (date.div_euclid(36524), date.rem_euclid(36524));
// 4 year cycles have 1461 days
let (n_4, date) = (date.div_euclid(1461), date.rem_euclid(1461));
let n_1 = date.div_euclid(365);
let year = 400 * n_400 + 100 * n_100 + 4 * n_4 + n_1;
if n_100 == 4 || n_1 == 4 {
year
} else {
year + 1
}
}
fn iso_new_year(year: i32) -> RataDie {
fixed_from_iso(year, 1, 1)
}
/// Lisp code reference: <https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1525-L1540>
pub fn iso_from_fixed(date: RataDie) -> Result<(i32, u8, u8), I32CastError> {
let year = iso_year_from_fixed(date);
let year = i64_to_i32(year)?;
// Calculates the prior days of the adjusted year, then applies a correction based on leap year conditions for the correct ISO date conversion.
let prior_days = date - iso_new_year(year);
let correction = if date < fixed_from_iso(year, 3, 1) {
0
} else if is_leap_year(year) {
1
} else {
2
};
let month = (12 * (prior_days + correction) + 373).div_euclid(367) as u8; // in 1..12 < u8::MAX
let day = (date - fixed_from_iso(year, month, 1) + 1) as u8; // <= days_in_month < u8::MAX
Ok((year, month, day))
}