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// 1st Jan of 1st year proleptic Julian is equivalent to December 30th of 0th year proleptic Gregorian
14const JULIAN_EPOCH: RataDie = crate::gregorian::fixed_from_gregorian(0, 12, 30);
15
16/// Lisp code reference: <https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1684-L1687>
17#[inline(always)]
18pub const fn is_leap_year(year: i32) -> bool {
19    year % 4 == 0
20}
21
22/// Lisp code reference: <https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1689-L1709>
23pub const fn fixed_from_julian(year: i32, month: u8, day: u8) -> RataDie {
24    let mut fixed =
25        JULIAN_EPOCH.to_i64_date() - 1 + 365 * (year as i64 - 1) + (year as i64 - 1).div_euclid(4);
26    debug_assert!(month > 0 && month < 13, "Month should be in range 1..=12.");
27    fixed += match month {
28        1 => 0,
29        2 => 31,
30        3 => 59,
31        4 => 90,
32        5 => 120,
33        6 => 151,
34        7 => 181,
35        8 => 212,
36        9 => 243,
37        10 => 273,
38        11 => 304,
39        12 => 334,
40        _ => -1,
41    };
42    // Only add one if the month is after February (month > 2), since leap days are added to the end of February
43    if month > 2 && is_leap_year(year) {
44        fixed += 1;
45    }
46    RataDie::new(fixed + (day as i64))
47}
48
49/// Lisp code reference: <https://github.com/EdReingold/calendar-code2/blob/1ee51ecfaae6f856b0d7de3e36e9042100b4f424/calendar.l#L1711-L1738>
50pub fn julian_from_fixed(date: RataDie) -> Result<(i32, u8, u8), I32CastError> {
51    let approx = (4 * date.to_i64_date() + 1464).div_euclid(1461);
52    let year = i64_to_i32(approx)?;
53    let prior_days = date
54        - fixed_from_julian(year, 1, 1)
55        - if is_leap_year(year) && date > fixed_from_julian(year, 2, 28) {
56            1
57        } else {
58            0
59        };
60    let adjusted_year = if prior_days >= 365 {
61        year.saturating_add(1)
62    } else {
63        year
64    };
65    let adjusted_prior_days = prior_days.rem_euclid(365);
66    debug_assert!((0..365).contains(&adjusted_prior_days));
67    let month = if adjusted_prior_days < 31 {
68        1
69    } else if adjusted_prior_days < 59 {
70        2
71    } else if adjusted_prior_days < 90 {
72        3
73    } else if adjusted_prior_days < 120 {
74        4
75    } else if adjusted_prior_days < 151 {
76        5
77    } else if adjusted_prior_days < 181 {
78        6
79    } else if adjusted_prior_days < 212 {
80        7
81    } else if adjusted_prior_days < 243 {
82        8
83    } else if adjusted_prior_days < 273 {
84        9
85    } else if adjusted_prior_days < 304 {
86        10
87    } else if adjusted_prior_days < 334 {
88        11
89    } else {
90        12
91    };
92    let day = (date - fixed_from_julian(adjusted_year, month, 1) + 1) as u8; // as days_in_month is < u8::MAX
93    debug_assert!(day <= 31, "Day assertion failed; date: {date:?}, adjusted_year: {adjusted_year}, prior_days: {prior_days}, month: {month}, day: {day}");
94
95    Ok((adjusted_year, month, day))
96}
97
98/// Get a fixed date from the ymd of a Julian date.
99///
100/// Years are counted as in _Calendrical Calculations_ by Reingold & Dershowitz,
101/// 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.
102///
103/// Primarily useful for use with code constructing epochs specified in the bookg
104pub const fn fixed_from_julian_book_version(book_year: i32, month: u8, day: u8) -> RataDie {
105    debug_assert!(book_year != 0);
106    // TODO: Should we check the bounds here?
107    fixed_from_julian(
108        if book_year < 0 {
109            book_year + 1
110        } else {
111            book_year
112        },
113        month,
114        day,
115    )
116}
117
118/// Calculates the date of Easter in the given year
119pub fn easter(year: i32) -> RataDie {
120    let shifted_epact = (14 + 11 * year.rem_euclid(19)) % 30;
121    let paschal_moon = fixed_from_julian(year, 4, 19) - shifted_epact as i64;
122    k_day_after(0, paschal_moon)
123}
124
125#[test]
126fn test_easter() {
127    // https://en.wikipedia.org/wiki/List_of_dates_for_Easter, dates in Gregorian
128    for (y, m, d) in [
129        (2021, 5, 2),
130        (2022, 4, 24),
131        (2023, 4, 16),
132        (2024, 5, 5),
133        (2025, 4, 20),
134        (2026, 4, 12),
135        (2027, 5, 2),
136        (2028, 4, 16),
137        (2029, 4, 8),
138    ] {
139        assert_eq!(easter(y), crate::gregorian::fixed_from_gregorian(y, m, d));
140    }
141}
calendrical_calculations/julian.rs

calendrical_calculations/
julian.rs
