Fix values equiprobability for generating random BigInteger.

TheSquidCombatant · Oct 16, 2023 · 16d0ea4 · 16d0ea4
1 parent 4fe9e78
commit 16d0ea4
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Showing 2 changed files with 41 additions and 4 deletions.
diff --git a/TheSquid.Numerics.Extensions.Tests/NextBigIntegerExtensionTests.cs b/TheSquid.Numerics.Extensions.Tests/NextBigIntegerExtensionTests.cs
@@ -17,7 +17,7 @@ public class NextBigIntegerExtensionTests
         [DataRow(10000, 100)]
         [DataRow(100000, 10)]
         [DataRow(1000000, 1)]
-        public void NextBigIntegerRandomTest(int iterationsCount, int borderValueLength)
+        public void NextBigIntegerRandomRangeTest(int iterationsCount, int borderValueLength)
         {
             var random = new Random(DateTime.Now.Millisecond);
 
@@ -50,5 +50,41 @@ public void NextBigIntegerRandomTest(int iterationsCount, int borderValueLength)
                 Assert.IsTrue(result <= max, message, min, max, result);
             }
         }
+
+        /// <summary>
+        /// Checks that any value within a given range can be generated.
+        /// </summary>
+        [TestMethod]
+        [DataRow(1000000, 10, 100)]
+        [DataRow(1000000, 100, 100)]
+        [DataRow(1000000, 1000, 100)]
+        public void NextBigIntegerRandomEquiprobabilityTest(int iterationsCount, int randomRangeSize, int maxDigitsCount)
+        {
+            var random = new Random(DateTime.Now.Millisecond);
+            var maxValueRaw = new StringBuilder(random.Next(1, 10).ToString(), maxDigitsCount);
+
+            for (int i = 1; i < maxDigitsCount; ++i)
+            {
+                maxValueRaw.Append(random.Next(0, 10));
+            }
+
+            var maxValue = BigInteger.Parse(maxValueRaw.ToString());
+            var minValue = maxValue - randomRangeSize + 1;
+            var resultCounter = new int[randomRangeSize];
+
+            for (int i = 0; i < iterationsCount; ++i)
+            {
+                var result = random.NextBigInteger(minValue, maxValue);
+                var index = (int)(result - minValue);
+                ++resultCounter[index];
+            }
+
+            const string message = "value={0}, count={1}";
+
+            for (int i = 0; i < randomRangeSize; ++i)
+            {
+                Assert.IsTrue(resultCounter[i] > 0, message, i + minValue, resultCounter[i]);
+            }
+        }
     }
 }
diff --git a/TheSquid.Numerics.Extensions/NextBigIntegerExtension.cs b/TheSquid.Numerics.Extensions/NextBigIntegerExtension.cs
@@ -32,12 +32,13 @@ public static BigInteger NextBigInteger(this Random random, BigInteger min, BigI
             const string maxCannotBeLessMessage = "Max value can not be less then min value.";
             if (max < min) throw new ArgumentOutOfRangeException(maxCannotBeLessMessage);
             var residual = max - min;
+            if (residual == 0) return max;
             var buffer = residual.ToByteArray();
             random.NextBytes(buffer);
             var multiplier = new BigInteger(buffer);
-            var addendum = residual & multiplier;
-            if (addendum < 0) addendum *= -1;
-            return min + addendum;
+            if (multiplier < 0) multiplier *= -1;
+            if (multiplier > residual) multiplier %= residual;
+            return min + multiplier;
         }
     }
 }