Add Computus in Rust (#695)

Author: fanninpm

contents/computus/code/rust/gauss_easter.rs

@@ -0,0 +1,69 @@
+fn computus(year: usize, servois: bool) -> String {
+    // Year's position on the 19 year metonic cycle
+    let a = year % 19;
+
+    // Century index
+    let k = year / 100; // NOTE: dividing integers always truncates the result
+
+    // Shift of metonic cycle, add a day offset every 300 years
+    let p = (13 + 8 * k) / 25;
+
+    // Correction for non-observed leap days
+    let q = k / 4;
+
+    // Correction to starting point of calculation each century
+    let m = (15 - p + k - q) % 30;
+
+    // Number of days from March 21st until the full moon
+    let d = (19 * a + m) % 30;
+
+    if servois {
+        return ((21 + d) % 31).to_string();
+    }
+
+    // Finding the next Sunday
+    // Century-based offset in weekly calculation
+    let n = (4 + k - q) % 7;
+
+    // Correction for leap days
+    let b = year % 4;
+    let c = year % 7;
+
+    // Days from d to next Sunday
+    let temp_e = ((2 * b + 4 * c + 6 * d + n) % 7) as isize;
+
+    // Historical corrections for April 26 and 25
+    let e = if (d == 29 && temp_e == 6) || (d == 28 && temp_e == 6 && a > 10) {
+        -1
+    } else {
+        temp_e
+    };
+
+    // Determination of the correct month for Easter
+    if (22 + d) as isize + e > 31 {
+        format!("April {}", d as isize + e - 9)
+    } else {
+        format!("March {}", 22 + d as isize + e)
+    }
+}
+
+fn main() {
+    // Here, we will output the date of the Paschal full moon
+    // (using Servois notation), and Easter for 2020-2030
+
+    let years = 2020..=2030;
+
+    println!(
+        "The following are the dates of the Paschal full moon (using \
+            Servois notation) and the date of Easter for 2020-2030 AD:"
+    );
+    println!("Year\tServois number\tEaster");
+    years.for_each(|year| {
+        println!(
+            "{}\t{:<14}\t{}",
+            year,
+            computus(year, true),
+            computus(year, false),
+        )
+    });
+}

contents/computus/computus.md

@@ -285,6 +285,8 @@ For now, we have the code outputting a tuple of $$d$$ and $$e$$, so users can us
 [import, lang:"python"](code/python/gauss_easter.py)
 {% sample lang="crystal" %}
 [import, lang:"crystal"](code/crystal/gauss_easter.cr)
+{% sample lang="rust" %}
+[import, lang:"rust"](code/rust/gauss_easter.rs)
 {% sample lang="ps1" %}
 [import, lang:"powershell"](code/powershell/gauss_easter.ps1)
 {% sample lang="c" %}