singleton cache capacity is a prime number (#425)

* singleton cache prime

* fallback

---------

Author: bitfaster

BitFaster.Caching.UnitTests/ConcurrentDictionarySizeTests.cs

@@ -18,7 +18,7 @@ public ConcurrentDictionarySizeTests(ITestOutputHelper testOutputHelper)
         [InlineData(8, 11)]
         [InlineData(12, 17)]
         [InlineData(196, 197)]
-        [InlineData(500, 197)]
+        [InlineData(7199370, 7199370)]
         public void NextPrimeGreaterThan(int input, int nextPrime)
         {
             ConcurrentDictionarySize.NextPrimeGreaterThan(input).Should().Be(nextPrime);

BitFaster.Caching/ConcurrentDictionarySize.cs

@@ -15,7 +15,7 @@ internal static int NextPrimeGreaterThan(int min)
                 }
             }
 
-            return 197;
+            return min;
         }
 
         /// <summary>
@@ -71,7 +71,11 @@ internal static int Estimate(int desiredSize)
 #else
         internal static ReadOnlySpan<int> Primes => new int[] {
 #endif
-            7, 11, 17, 23, 29, 37, 47, 59, 71, 89, 107, 131, 163, 197
+            3, 7, 11, 17, 23, 29, 37, 47, 59, 71, 89, 107, 131, 163, 197, 239, 293, 353, 431, 521, 631, 761, 919,
+            1103, 1327, 1597, 1931, 2333, 2801, 3371, 4049, 4861, 5839, 7013, 8419, 10103, 12143, 14591,
+            17519, 21023, 25229, 30293, 36353, 43627, 52361, 62851, 75431, 90523, 108631, 130363, 156437,
+            187751, 225307, 270371, 324449, 389357, 467237, 560689, 672827, 807403, 968897, 1162687, 1395263,
+            1674319, 2009191, 2411033, 2893249, 3471899, 4166287, 4999559, 5999471, 7199369
         };
 
 #if NETSTANDARD2_0

BitFaster.Caching/SingletonCache.cs

@@ -33,7 +33,7 @@ public SingletonCache()
         /// <param name="comparer">The IEqualityComparer implementation to use when comparing keys.</param>
         public SingletonCache(int concurrencyLevel, int capacity, IEqualityComparer<TKey> comparer)
         {
-            this.cache = new ConcurrentDictionary<TKey, ReferenceCount<TValue>>(concurrencyLevel, capacity, comparer);
+            this.cache = new ConcurrentDictionary<TKey, ReferenceCount<TValue>>(concurrencyLevel, ConcurrentDictionarySize.NextPrimeGreaterThan(capacity), comparer);
         }
 
         /// <summary>