Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.Xml/System/Xml/Bits.cs

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+//------------------------------------------------------------------------------
+// <copyright file="Bits.cs" company="Microsoft">
+//     Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>
+// <owner current="true" primary="true">[....]</owner>
+//------------------------------------------------------------------------------
+
+namespace System.Xml {
+    using System;
+    using System.Diagnostics;
+
+    /// <summary>
+    /// Contains static utility methods used to manipulate bits in a word.
+    /// </summary>
+    internal static class Bits {
+        private static readonly uint MASK_0101010101010101 = 0x55555555;
+        private static readonly uint MASK_0011001100110011 = 0x33333333;
+        private static readonly uint MASK_0000111100001111 = 0x0f0f0f0f;
+        private static readonly uint MASK_0000000011111111 = 0x00ff00ff;
+        private static readonly uint MASK_1111111111111111 = 0x0000ffff;
+
+        /// <summary>
+        /// Returns the number of 1 bits in an unsigned integer.  Counts bits by divide-and-conquer method,
+        /// first computing 16 2-bit counts, then 8 4-bit counts, then 4 8-bit counts, then 2 16-bit counts,
+        /// and finally 1 32-bit count.
+        /// </summary>
+        public static int Count(uint num) {
+            num = (num & MASK_0101010101010101) + ((num >> 1) & MASK_0101010101010101);
+            num = (num & MASK_0011001100110011) + ((num >> 2) & MASK_0011001100110011);
+            num = (num & MASK_0000111100001111) + ((num >> 4) & MASK_0000111100001111);
+            num = (num & MASK_0000000011111111) + ((num >> 8) & MASK_0000000011111111);
+            num = (num & MASK_1111111111111111) + (num >> 16);
+
+            return (int) num;
+        }
+
+        /// <summary>
+        /// Returns true if the unsigned integer has exactly one bit set.
+        /// </summary>
+        public static bool ExactlyOne(uint num) {
+            return num != 0 && (num & (num - 1)) == 0;
+        }
+
+#if !SILVERLIGHT // These methods are not used in Silverlight
+        /// <summary>
+        /// Returns true if the unsigned integer has more than one bit set.
+        /// </summary>
+        public static bool MoreThanOne(uint num) {
+            return (num & (num - 1)) != 0;
+        }
+
+        /// <summary>
+        /// Clear the least significant bit that is set and return the result.
+        /// </summary>
+        public static uint ClearLeast(uint num) {
+            return num & (num - 1);
+        }
+#endif
+
+        /// <summary>
+        /// Compute the 1-based position of the least sigificant bit that is set, and return it (return 0 if no bits are set).
+        /// (e.g. 0x1001100 will return 3, since the 3rd bit is set).
+        /// </summary>
+        public static int LeastPosition(uint num) {
+            if (num == 0) return 0;
+            return Count(num ^ (num - 1));
+        }
+    }
+}