Add Meeus' Julian Easter algorithm (TheAlgorithms#363)

Co-authored-by: Kerry Russell <[email protected]>

Author: kamikkels

Algorithms.Tests/Other/JulianEasterTests.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Globalization;
+using Algorithms.Other;
+using NUnit.Framework;
+
+namespace Algorithms.Tests.Other
+{
+    /// <summary>
+    ///     A class for testing the Meeus's Julian Easter algorithm.
+    /// </summary>
+    public static class JulianEasterTest
+    {
+        private static readonly JulianCalendar Calendar = new();
+
+        [TestCaseSource(nameof(CalculateCases))]
+        public static void CalculateTest(int year, DateTime expected)
+        {
+            var result = JulianEaster.Calculate(year);
+
+            Assert.AreEqual(expected, result);
+        }
+
+        private static readonly object[] CalculateCases =
+        {
+            new object[] { 1800, new DateTime(1800, 04, 08, Calendar) },
+            new object[] { 1950, new DateTime(1950, 03, 27, Calendar) },
+            new object[] { 1991, new DateTime(1991, 03, 25, Calendar) },
+            new object[] { 2000, new DateTime(2000, 04, 17, Calendar) },
+            new object[] { 2199, new DateTime(2199, 04, 07, Calendar) }
+        };
+    }
+}

Algorithms/Other/JulianEaster.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Globalization;
+
+namespace Algorithms.Other
+{
+    /// <summary>
+    ///     Date of Easter calculated with Meeus's Julian algorithm.
+    ///     The algorithm is described in Jean Meeus' <a href="https://archive.org/details/astronomicalalgorithmsjeanmeeus1991/page/n73/mode/2up">Astronomical Algorithms (1991, p. 69)</a>.
+    /// </summary>
+    public static class JulianEaster
+    {
+        /// <summary>
+        ///     Calculates the date of Easter.
+        /// </summary>
+        /// <param name="year">Year to calculate the date of Easter.</param>
+        /// <returns>Date of Easter as a DateTime.</returns>
+        public static DateTime Calculate(int year)
+        {
+            var a = year % 4;
+            var b = year % 7;
+            var c = year % 19;
+            var d = (19 * c + 15) % 30;
+            var e = (2 * a + 4 * b - d + 34) % 7;
+            var month = (int)Math.Floor((d + e + 114) / 31M);
+            var day = ((d + e + 114) % 31) + 1;
+
+            DateTime easter = new(year, month, day, new JulianCalendar());
+
+            return easter;
+        }
+    }
+}

README.md

@@ -191,6 +191,7 @@ find more than one implementation for the same objective but using different alg
     * [Gauss Optimization](./Algorithms/Other/GaussOptimization.cs)
     * [Decisions Convolutions](./Algorithms/Other/DecisionsConvolutions.cs)
     * [Welford's Variance](./Algorithms/Other/WelfordsVariance.cs)
+    * [Julian Easter](./Algorithms/Other/JulianEaster.cs)
   * [Problems](./Algorithms/Problems)
     * [Stable Marriage](./Algorithms/Problems/StableMarriage)
       * [Gale-Shapley](./Algorithms/Problems/StableMarriage/GaleShapley.cs)