Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System/compmod/system/collections/specialized/fixedstringlookup.cs

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+//------------------------------------------------------------------------------
+// <copyright file="FixedStringLookup.cs" company="Microsoft">
+//     Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>                                                                
+//------------------------------------------------------------------------------
+
+namespace System.Collections.Specialized {
+
+    using System;
+    using System.Collections;
+    using System.Diagnostics;
+    using System.Globalization;
+    
+    // This class provides a very efficient way to lookup an entry in a list of strings,
+    // providing that they are declared in a particular way.
+    
+    // It requires the set of strings to be orderded into an array of arrays of strings.
+    // The first indexer must the length of the string, so that each sub-array is of the
+    // same length. The contained array must be in alphabetical order. Furthermore, if the 
+    // table is to be searched case-insensitively, the strings must all be lower case.
+    internal static class FixedStringLookup {
+        
+        // Returns whether the match is found in the lookup table
+        internal static bool Contains(string[][] lookupTable, string value, bool ignoreCase) {
+            int length = value.Length;
+            if (length <= 0 || length - 1 >= lookupTable.Length) {
+                return false;
+            }
+
+            string[] subArray = lookupTable[length - 1];
+            if (subArray == null) {
+                return false;
+            }
+            return Contains(subArray, value, ignoreCase);            
+        }
+
+#if DEBUG
+
+        internal static void VerifyLookupTable(string[][] lookupTable, bool ignoreCase) {
+            for (int i = 0; i < lookupTable.Length; i++) {
+                string[] subArray = lookupTable[i];
+                if (subArray != null) {
+                    string lastValue = null;
+                    for (int j = 0; j < subArray.Length; j++) {
+                        string value = subArray[j];
+                        // Must all be the length of the hashed position
+                        Debug.Assert(value.Length == i + 1, "Lookup table contains an item in the wrong subtable.  Item name: " + value);
+                        if (lastValue != null) {
+                            // Must be sorted within the sub array;
+                            Debug.Assert(string.Compare(lastValue, value, ((ignoreCase) ? StringComparison.OrdinalIgnoreCase : StringComparison.Ordinal)) < 0, 
+                                         String.Format(CultureInfo.InvariantCulture, "Lookup table is out of order.  Items {0} and {1} are reversed", lastValue, value));
+                        }
+                        lastValue = value;
+                    }
+                }
+            }
+        }
+
+#endif
+
+        // This routine finds a hit within a single sorted array, with the assumption that the
+        // value and all the strings are of the same length.
+        private static bool Contains(string[] array, string value, bool ignoreCase) {
+            int min = 0;
+            int max = array.Length;
+            int pos = 0;
+            char searchChar;
+            while (pos < value.Length) {            
+                if (ignoreCase) {
+                    searchChar = char.ToLower(value[pos], CultureInfo.InvariantCulture);
+                } else {
+                    searchChar = value[pos];
+                }
+                if ((max - min) <= 1) {
+                    // we are down to a single item, so we can stay on this row until the end.
+                    if (searchChar != array[min][pos]) {
+                        return false;
+                    }
+                    pos++;
+                    continue;
+                }
+
+                // There are multiple items to search, use binary search to find one of the hits
+                if (!FindCharacter(array, searchChar, pos, ref min, ref max)) {
+                    return false;
+                }
+                // and move to next char
+                pos++;
+            }
+            return true;
+        }
+
+        // Do a binary search on the character array at the specific position and constrict the ranges appropriately.
+        private static bool FindCharacter(string[] array, char value, int pos, ref int min, ref int max) {
+            int index = min;
+            while (min < max) {
+                index = (min + max) / 2;
+                char comp = array[index][pos];
+                if (value == comp) {
+                    // We have a match. Now adjust to any adjacent matches
+                    int newMin = index;
+                    while (newMin > min && array[newMin - 1][pos] == value) {
+                        newMin--;
+                    }
+                    min = newMin;
+
+                    int newMax = index + 1;
+                    while (newMax < max && array[newMax][pos] == value) {
+                        newMax++;
+                    }
+                    max = newMax;
+                    return true;
+                }
+                if (value < comp) {
+                    max = index;
+                }
+                else {
+                    min = index + 1;
+                }
+            }
+            return false;
+        }
+    }
+}