Fix DiscreteChar.Sample() (#153)

Due to refactoring mistake `sampleProb / prob * intervalLength`
turned into `sampleProb / (prob * intervalLength)` which is obviosly incorrect.

Fixed that + added a rudimentary test for `DiscreteChar.Sample()`

src/Runtime/Distributions/DiscreteChar.cs

@@ -859,7 +859,7 @@ namespace Microsoft.ML.Probabilistic.Distributions
                 sampleProb -= prob.Value;
                 if (sampleProb < 0)
                 {
-                    return (char)(interval.StartInclusive - sampleProb / (prob * intervalLength).Value);
+                    return (char)(interval.StartInclusive - sampleProb / interval.Probability.Value);
                 }
             }
 

test/Tests/Strings/DiscreteCharTest.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using Microsoft.ML.Probabilistic.Math;
+
 namespace Microsoft.ML.Probabilistic.Tests
 {
     using System;
@@ -64,6 +66,31 @@ namespace Microsoft.ML.Probabilistic.Tests
             Assert.True(false);
         }
 
+        [Fact]
+        [Trait("Category", "StringInference")]
+        public void SampleFromUniformCharDistribution()
+        {
+            // Make test deterministic
+            Rand.Restart(7);
+
+            // 10 chars in distribution
+            const int numChars = 10;
+            const int numSamples = 100000;
+            var dist = DiscreteChar.UniformInRanges("aj");
+
+            var hist = Vector.Zero(numChars);
+            for (var i = 0; i < numSamples; ++i)
+            {
+                hist[dist.Sample() - 'a'] += 1;
+            }
+
+            hist = hist * (1.0 / numSamples);
+            var unif = Vector.Constant(numChars, 1.0 / numChars);
+            var maxDiff = hist.MaxDiff(unif);
+
+            Assert.True(maxDiff < 0.01);
+        }
+
         /// <summary>
         /// Tests the support of a character distribution.
         /// </summary>