General improvements and fixes in StringBuilder (#191)

Author: josesimoes

Tests/NFUnitTestStringBuilder/StringBuilderTests.cs

@@ -443,6 +443,74 @@ public void Clear_AtMaxCapacity_CapacityStaysUnchanged()
             builder.Clear();
             Assert.AreEqual(14, builder.Capacity);
         }
+
+        [TestMethod]
+        [DataRow(1)]
+        [DataRow(10000)]
+        public void Clear_AppendAndInsertBeforeClearManyTimes_CapacityStaysWithinRange(int times)
+        {
+            var builder = new StringBuilder();
+            var originalCapacity = builder.Capacity;
+            var s = new string(' ', 10);
+            int oldLength = 0;
+            for (int i = 0; i < times; i++)
+            {
+                builder.Append(s);
+                builder.Append(s);
+                builder.Append(s);
+                builder.Insert(0, s, 1);
+                builder.Insert(0, s, 1);
+                oldLength = builder.Length;
+
+                builder.Clear();
+            }
+            Assert.IsTrue(builder.Capacity >= 1 && builder.Capacity <= oldLength * 1.2);
+        }
+
+        [TestMethod]
+        public void Clear_InitialCapacityMuchLargerThanLength_CapacityReducedToInitialCapacity()
+        {
+            var builder = new StringBuilder(100);
+            var initialCapacity = builder.Capacity;
+            builder.Append(new string('a', 40));
+            builder.Insert(0, new string('a', 10), 1);
+            builder.Insert(0, new string('a', 10), 1);
+            builder.Insert(0, new string('a', 10), 1);
+            var oldCapacity = builder.Capacity;
+            var oldLength = builder.Length;
+            builder.Clear();
+            Assert.AreNotEqual(oldCapacity, builder.Capacity);
+            Assert.AreEqual(initialCapacity, builder.Capacity);
+            Assert.IsFalse(builder.Capacity >= 1 && builder.Capacity <= oldLength * 1.2);
+
+            // find max between initial capacity and 1.2 * old length
+            int maxCapacity = initialCapacity > (int)(oldLength * 1.2) ? initialCapacity : (int)(oldLength * 1.2);
+
+            Assert.IsTrue(builder.Capacity >= 1 && builder.Capacity <= maxCapacity);
+        }
+
+        [TestMethod]
+        public void Clear_StringBuilderHasTwoChunks_OneChunkIsEmpty_ClearReducesCapacity()
+        {
+            var sb = new StringBuilder(string.Empty);
+            int initialCapacity = sb.Capacity;
+            for (int i = 0; i < initialCapacity; i++)
+            {
+                sb.Append('a');
+            }
+
+            sb.Insert(0, new char[] { 'a' }, 0, 1);
+
+            while (sb.Length > 1)
+            {
+                sb.Remove(1, 1);
+            }
+
+            int oldCapacity = sb.Capacity;
+            sb.Clear();
+            Assert.AreEqual(oldCapacity - 1, sb.Capacity);
+            Assert.AreEqual(initialCapacity, sb.Capacity);
+        }
     }
 
     static class RandomExtension