Holidays: Christmas and Easter beside Halloween, two weeks before to a few days after; decorations in the dungeon and around the title

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/holiday.rs

@@ -0,0 +1,109 @@
+// Holidays of the real date: the dungeon and the title menu dress up for them (pumpkins,
+// fir trees and presents, painted eggs; see render3d's room decor and menu's draw_title).
+// Every holiday runs from about two weeks before its day to a few days after it.
+
+pub const NONE: u8 = 0;
+pub const HALLOWEEN: u8 = 1;
+pub const CHRISTMAS: u8 = 2;
+pub const EASTER: u8 = 3;
+
+// days since 1970-01-01 of a date (proleptic Gregorian)
+fn days_from_civil(y: i64, m: i64, d: i64) -> i64 {
+    let y = if m <= 2 { y - 1 } else { y };
+    let era = y.div_euclid(400);
+    let yoe = y - era * 400;
+    let mp = (m + 9) % 12;
+    let doy = (153 * mp + 2) / 5 + d - 1;
+    let doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;
+    era * 146097 + doe - 719468
+}
+
+// Easter Sunday of a year (month, day): the anonymous Gregorian algorithm
+pub fn easter(y: i64) -> (i64, i64) {
+    let a = y % 19;
+    let b = y / 100;
+    let c = y % 100;
+    let d = b / 4;
+    let e = b % 4;
+    let f = (b + 8) / 25;
+    let g = (b - f + 1) / 3;
+    let h = (19 * a + b - d - g + 15) % 30;
+    let i = c / 4;
+    let k = c % 4;
+    let l = (32 + 2 * e + 2 * i - h - k) % 7;
+    let m = (a + 11 * h + 22 * l) / 451;
+    let n = h + l - 7 * m + 114;
+    (n / 31, n % 31 + 1)
+}
+
+// the holiday on a date: Halloween 17 Oct - 3 Nov, Christmas 10 Dec - 1 Jan, Easter from two
+// weeks before Easter Sunday to the Wednesday after it
+pub fn holiday_on(y: i64, m: i64, d: i64) -> u8 {
+    match (m, d) {
+        (10, 17..=31) | (11, 1..=3) => return HALLOWEEN,
+        (12, 10..=31) | (1, 1) => return CHRISTMAS,
+        _ => {}
+    }
+    let (em, ed) = easter(y);
+    let off = days_from_civil(y, m, d) - days_from_civil(y, em, ed);
+    if (-14..=3).contains(&off) {
+        EASTER
+    } else {
+        NONE
+    }
+}
+
+// today's holiday (worked out once); on the desktop DUNGEONCRAFT_HOLIDAY=halloween / christmas /
+// easter / none shows one at any time
+pub fn current() -> u8 {
+    thread_local! {
+        static HOLIDAY: std::cell::Cell<u8> = const { std::cell::Cell::new(255) };
+    }
+    HOLIDAY.with(|h| {
+        if h.get() == 255 {
+            #[cfg(not(target_arch = "wasm32"))]
+            let forced = std::env::var("DUNGEONCRAFT_HOLIDAY").ok().and_then(|v| match v.to_lowercase().as_str() {
+                "halloween" => Some(HALLOWEEN),
+                "christmas" | "xmas" => Some(CHRISTMAS),
+                "easter" => Some(EASTER),
+                "none" => Some(NONE),
+                _ => None,
+            });
+            #[cfg(target_arch = "wasm32")]
+            let forced: Option<u8> = None;
+            let (y, m, d, _, _) = crate::cloud::civil(macroquad::miniquad::date::now() as i64);
+            h.set(forced.unwrap_or_else(|| holiday_on(y, m, d)));
+        }
+        h.get()
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn easter_dates() {
+        assert_eq!(easter(2024), (3, 31));
+        assert_eq!(easter(2025), (4, 20));
+        assert_eq!(easter(2026), (4, 5));
+        assert_eq!(easter(2027), (3, 28));
+    }
+
+    #[test]
+    fn holiday_windows() {
+        assert_eq!(holiday_on(2026, 10, 16), NONE);
+        assert_eq!(holiday_on(2026, 10, 17), HALLOWEEN);
+        assert_eq!(holiday_on(2026, 11, 3), HALLOWEEN);
+        assert_eq!(holiday_on(2026, 11, 7), NONE);
+        assert_eq!(holiday_on(2026, 12, 9), NONE);
+        assert_eq!(holiday_on(2026, 12, 10), CHRISTMAS);
+        assert_eq!(holiday_on(2027, 1, 1), CHRISTMAS);
+        assert_eq!(holiday_on(2027, 1, 2), NONE);
+        // Easter 2026: 5 April
+        assert_eq!(holiday_on(2026, 3, 21), NONE);
+        assert_eq!(holiday_on(2026, 3, 22), EASTER);
+        assert_eq!(holiday_on(2026, 4, 8), EASTER);
+        assert_eq!(holiday_on(2026, 4, 9), NONE);
+    }
+}