Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.Web/Security/AntiXss/EncoderUtil.cs

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+//------------------------------------------------------------------------------
+// <copyright file="EncoderUtil.cs" company="Microsoft">
+//     Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>
+//------------------------------------------------------------------------------
+
+namespace System.Web.Security.AntiXss {
+    using System;
+    using System.Text;
+
+    /// <summary>
+    /// Provides helper methods common to all Anti-XSS encoders.
+    /// </summary>
+    internal static class EncoderUtil {
+
+        /// <summary>
+        /// Gets an appropriately-sized StringBuilder for the output of an encoding routine.
+        /// </summary>
+        /// <param name="inputLength">The length (in characters) of the input string.</param>
+        /// <param name="worstCaseOutputCharsPerInputChar">The worst-case ratio of output characters per input character.</param>
+        /// <returns>A StringBuilder appropriately-sized to hold the output string.</returns>
+        internal static StringBuilder GetOutputStringBuilder(int inputLength, int worstCaseOutputCharsPerInputChar) {
+            // We treat 32KB byte size (16k chars) as a soft upper boundary for the length of any StringBuilder
+            // that we allocate. We'll try to avoid going above this boundary if we can avoid it so that we
+            // don't allocate objects on the LOH.
+            const int upperBound = 16 * 1024;
+
+            int charsToAllocate;
+            if (inputLength >= upperBound) {
+                // We know that the output will contain at least as many characters as the input, so if the
+                // input length exceeds the soft upper boundary just preallocate the entire builder and hope for
+                // a best-case outcome.
+                charsToAllocate = inputLength;
+            }
+            else {
+                // Allocate the worst-case if we can, but don't exceed the soft upper boundary.
+                long worstCaseTotalChars = (long)inputLength * worstCaseOutputCharsPerInputChar; // don't overflow Int32
+                charsToAllocate = (int)Math.Min(upperBound, worstCaseTotalChars);
+            }
+
+            // Once we have chosen an initial value for the StringBuilder size, the StringBuilder type will
+            // efficiently allocate additionally blocks if necessary.
+            return new StringBuilder(charsToAllocate);
+        }
+
+    }
+}