calendrical_calculations/
julian.rs

1// This file is part of ICU4X.
2//
3// The contents of this file implement algorithms from Calendrical Calculations
4// by Reingold & Dershowitz, Cambridge University Press, 4th edition (2018),
5// which have been released as Lisp code at <https://github.com/EdReingold/calendar-code2/>
6// under the Apache-2.0 license. Accordingly, this file is released under
7// the Apache License, Version 2.0 which can be found at the calendrical_calculations
8// package root or at http://www.apache.org/licenses/LICENSE-2.0.
9
10use crate::helpers::{i64_to_i32, k_day_after, I32CastError};
11use crate::rata_die::RataDie;
12
13// Julian epoch is equivalent to fixed_from_iso of December 30th of 0 year
14// 1st Jan of 1st year Julian is equivalent to December 30th of 0th year of ISO year
15const JULIAN_EPOCH: RataDie = RataDie::new(-1);
16
17/// Lisp code reference: <https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1684-L1687>
18#[inline(always)]
19pub const fn is_leap_year(year: i32) -> bool {
20    year % 4 == 0
21}
22
23/// Lisp code reference: <https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1689-L1709>
24pub const fn fixed_from_julian(year: i32, month: u8, day: u8) -> RataDie {
25    let mut fixed =
26        JULIAN_EPOCH.to_i64_date() - 1 + 365 * (year as i64 - 1) + (year as i64 - 1).div_euclid(4);
27    debug_assert!(month > 0 && month < 13, "Month should be in range 1..=12.");
28    fixed += match month {
29        1 => 0,
30        2 => 31,
31        3 => 59,
32        4 => 90,
33        5 => 120,
34        6 => 151,
35        7 => 181,
36        8 => 212,
37        9 => 243,
38        10 => 273,
39        11 => 304,
40        12 => 334,
41        _ => -1,
42    };
43    // Only add one if the month is after February (month > 2), since leap days are added to the end of February
44    if month > 2 && is_leap_year(year) {
45        fixed += 1;
46    }
47    RataDie::new(fixed + (day as i64))
48}
49
50/// Lisp code reference: <https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1711-L1738>
51pub fn julian_from_fixed(date: RataDie) -> Result<(i32, u8, u8), I32CastError> {
52    let approx = (4 * date.to_i64_date() + 1464).div_euclid(1461);
53    let year = i64_to_i32(approx)?;
54    let prior_days = date
55        - fixed_from_julian(year, 1, 1)
56        - if is_leap_year(year) && date > fixed_from_julian(year, 2, 28) {
57            1
58        } else {
59            0
60        };
61    let adjusted_year = if prior_days >= 365 {
62        year.saturating_add(1)
63    } else {
64        year
65    };
66    let adjusted_prior_days = prior_days.rem_euclid(365);
67    debug_assert!((0..365).contains(&adjusted_prior_days));
68    let month = if adjusted_prior_days < 31 {
69        1
70    } else if adjusted_prior_days < 59 {
71        2
72    } else if adjusted_prior_days < 90 {
73        3
74    } else if adjusted_prior_days < 120 {
75        4
76    } else if adjusted_prior_days < 151 {
77        5
78    } else if adjusted_prior_days < 181 {
79        6
80    } else if adjusted_prior_days < 212 {
81        7
82    } else if adjusted_prior_days < 243 {
83        8
84    } else if adjusted_prior_days < 273 {
85        9
86    } else if adjusted_prior_days < 304 {
87        10
88    } else if adjusted_prior_days < 334 {
89        11
90    } else {
91        12
92    };
93    let day = (date - fixed_from_julian(adjusted_year, month, 1) + 1) as u8; // as days_in_month is < u8::MAX
94    debug_assert!(day <= 31, "Day assertion failed; date: {date:?}, adjusted_year: {adjusted_year}, prior_days: {prior_days}, month: {month}, day: {day}");
95
96    Ok((adjusted_year, month, day))
97}
98
99/// Get a fixed date from the ymd of a Julian date.
100///
101/// Years are counted as in _Calendrical Calculations_ by Reingold & Dershowitz,
102/// meaning there is no year 0. For instance, near the epoch date, years are counted: -3, -2, -1, 1, 2, 3 instead of -2, -1, 0, 1, 2, 3.
103///
104/// Primarily useful for use with code constructing epochs specified in the bookg
105pub const fn fixed_from_julian_book_version(book_year: i32, month: u8, day: u8) -> RataDie {
106    debug_assert!(book_year != 0);
107    // TODO: Should we check the bounds here?
108    fixed_from_julian(
109        if book_year < 0 {
110            book_year + 1
111        } else {
112            book_year
113        },
114        month,
115        day,
116    )
117}
118
119/// Calculates the date of Easter in the given year
120pub fn easter(year: i32) -> RataDie {
121    let shifted_epact = (14 + 11 * year.rem_euclid(19)) % 30;
122    let paschal_moon = fixed_from_julian(year, 4, 19) - shifted_epact as i64;
123    k_day_after(0, paschal_moon)
124}
125
126#[test]
127fn test_easter() {
128    // https://en.wikipedia.org/wiki/List_of_dates_for_Easter, dates in Gregorian
129    for (y, m, d) in [
130        (2021, 5, 2),
131        (2022, 4, 24),
132        (2023, 4, 16),
133        (2024, 5, 5),
134        (2025, 4, 20),
135        (2026, 4, 12),
136        (2027, 5, 2),
137        (2028, 4, 16),
138        (2029, 4, 8),
139    ] {
140        assert_eq!(easter(y), crate::iso::fixed_from_iso(y, m, d));
141    }
142}
calendrical_calculations/julian.rs

calendrical_calculations/
julian.rs
