Add doomsday algorithm (rust-lang#391)

Author: jerr-it

src/math/doomsday.rs

@@ -0,0 +1,36 @@
+const T: [i32; 12] = [0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4];
+
+pub fn doomsday(y: i32, m: i32, d: i32) -> i32 {
+    let y = if m < 3 { y - 1 } else { y };
+    (y + y / 4 - y / 100 + y / 400 + T[(m - 1) as usize] + d) % 7
+}
+
+pub fn get_week_day(y: i32, m: i32, d: i32) -> String {
+    let day = doomsday(y, m, d);
+    let day_str = match day {
+        0 => "Sunday",
+        1 => "Monday",
+        2 => "Tuesday",
+        3 => "Wednesday",
+        4 => "Thursday",
+        5 => "Friday",
+        6 => "Saturday",
+        _ => "Unknown",
+    };
+
+    day_str.to_string()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn doomsday_test() {
+        assert_eq!(get_week_day(1990, 3, 21), "Wednesday");
+        assert_eq!(get_week_day(2000, 8, 24), "Thursday");
+        assert_eq!(get_week_day(2000, 10, 13), "Friday");
+        assert_eq!(get_week_day(2001, 4, 18), "Wednesday");
+        assert_eq!(get_week_day(2002, 3, 19), "Tuesday");
+    }
+}

src/math/mod.rs

@@ -2,6 +2,7 @@ mod amicable_numbers;
 mod armstrong_number;
 mod baby_step_giant_step;
 mod chinese_remainder_theorem;
+mod doomsday;
 mod extended_euclidean_algorithm;
 mod fast_fourier_transform;
 mod fast_power;
@@ -36,6 +37,7 @@ pub use self::amicable_numbers::amicable_pairs_under_n;
 pub use self::armstrong_number::is_armstrong_number;
 pub use self::baby_step_giant_step::baby_step_giant_step;
 pub use self::chinese_remainder_theorem::chinese_remainder_theorem;
+pub use self::doomsday::get_week_day;
 pub use self::extended_euclidean_algorithm::extended_euclidean_algorithm;
 pub use self::fast_fourier_transform::{
     fast_fourier_transform, fast_fourier_transform_input_permutation,