Added 2 RandChoice methods in Simulation class (#27)

* Add 2 RandChoice methods in Simulation class
* Add unit tests

Author: CrystalWindSnake

src/SimSharp/Core/Environment.cs

@@ -9,6 +9,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -620,6 +621,69 @@ public TimeSpan RandWeibull(IRandom random, TimeSpan alpha, TimeSpan beta) {
     public TimeSpan RandWeibull(TimeSpan alpha, TimeSpan beta) {
       return RandWeibull(Random, alpha, beta);
     }
+
+
+    /// <summary>
+    /// Generates a random sample from a given source
+    /// </summary>
+    /// <typeparam name="T">The type of the element in parameter source</typeparam>
+    /// <exception cref="ArgumentException">
+    /// Thrown when <paramref name="source"/> and <paramref name="weights"/> have different size.
+    /// or when <paramref name="weights"/> contains an invalid or negative value.
+    /// or when <paramref name="weights"/> sum equals zero or an invalid value.
+    /// </exception>
+    /// <param name="random">The random number generator to use.</param>
+    /// <param name="source">a random sample is generated from its elements.</param>
+    /// <param name="weights">The weight associated with each entry in source.</param>
+    /// <returns>The generated random samples</returns>
+    public T RandChoice<T>(IRandom random, IList<T> source, IList<double> weights) {
+      if (source.Count != weights.Count) {
+        throw new ArgumentException("source and weights must have same size");
+      }
+
+      double totalW = 0;
+      foreach (var w in weights) {
+        if (w < 0) {
+          throw new ArgumentException("weight values must be non-negative", nameof(weights));
+        }
+        totalW += w;
+      }
+
+      if (double.IsNaN(totalW) || double.IsInfinity(totalW))
+        throw new ArgumentException("Not a valid weight", nameof(weights));
+      if (totalW == 0)
+        throw new ArgumentException("total weight must be greater than 0", nameof(weights));
+
+      var rnd = random.NextDouble();
+      double aggWeight = 0;
+      int idx = 0;
+      foreach (var w in weights) {
+        if (w > 0) {
+          aggWeight += (w / totalW);
+          if (rnd <= aggWeight) {
+            break;
+          }
+        }
+        idx++;
+      }
+      return source[idx];
+    }
+    /// <summary>
+    /// Generates a random sample from a given source
+    /// </summary>
+    /// <typeparam name="T">The type of the element in parameter source</typeparam>
+    /// <exception cref="ArgumentException">
+    /// Thrown when <paramref name="source"/> and <paramref name="weights"/> have different size.
+    /// or when <paramref name="weights"/> contains an invalid or negative value.
+    /// or when <paramref name="weights"/> sum equals zero
+    /// </exception>
+    /// <param name="source">a random sample is generated from its elements.</param>
+    /// <param name="weights">The weight associated with each entry in source.</param>
+    /// <returns>The generated random samples</returns>
+    public T RandChoice<T>(IList<T> source, IList<double> weights) {
+      return RandChoice(Random, source, weights);
+    }
+
     #endregion
 
     #region Random timeouts

src/Tests/RandomTest.cs

@@ -14,5 +14,41 @@ public void PcgReproducibilityTest() {
       var seq2 = Enumerable.Range(0, 1000).Select(x => pcg.Next()).ToArray();
       Assert.Equal(seq, seq2);
     }
+
+    [Theory]
+    [InlineData(new double[] { double.NaN, 0.3 })]
+    [InlineData(new double[] { 1.0 / 0.0, 0.3 })]
+    [InlineData(new double[] { 0, 0 })]
+    [InlineData(new double[] { -1, 10 })]
+    [InlineData(new double[] { double.MaxValue, double.MaxValue })]
+    public void RandChoiceTestArgumentException(double[] weights) {
+      var env = new Simulation(15);
+      Assert.Throws<System.ArgumentException>(
+        () => env.RandChoice(new[] { "a", "b" }, weights));
+    }
+
+    [Fact]
+    public void RandChoiceTestContainZeroWeight() {
+      var env = new Simulation(15);
+      var source = new[] { "a", "b", "c" };
+      var res1 = Enumerable.Range(1, 100)
+        .Select(_ => env.RandChoice(source, new[] { 0.7, 0.2, 0 }));
+      Assert.DoesNotContain("c", res1);
+    }
+
+    [Fact]
+    public void RandChoiceTestTotalWeightMoreThanOne() {
+      var source = new[] { "a", "b", "c" };
+
+      var env1 = new Simulation(15);
+      var res1 = Enumerable.Range(1, 100)
+        .Select(_ => env1.RandChoice(source, new[] { 0.5, 0.3, 0.2 }));
+
+      var env2 = new Simulation(15); new Simulation(15);
+      var res2 = Enumerable.Range(1, 100)
+        .Select(_ => env2.RandChoice(source, new[] { 5d, 3, 2 }));
+
+      Assert.Equal(res1, res2);
+    }
   }
 }