Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.Web/Util/CodePageUtils.cs

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+//------------------------------------------------------------------------------
+// <copyright file="CodePageUtils.cs" company="Microsoft">
+//     Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>                                                                
+//------------------------------------------------------------------------------
+
+namespace System.Web.Util {
+    using System.Collections;
+    
+    //
+    // Utility class to help with determining if a given code page
+    // is ASCII compatible (preserves 0-127 Unicode characters as is)
+    //
+
+    internal static class CodePageUtils {
+
+        /*      The following array of ASCII compatible code pages
+                is generated by running this code on a machine that
+                has _many_ codepages installed:
+
+                using System;
+                using System.Collections;
+                using System.Text;
+                public class qq {
+                    public static void Main(string[] args) {
+                        ArrayList list = new ArrayList();
+                        byte[] bb = new byte[128]; for (int i = 0; i < 128; i++) bb[i] = (byte)i;
+                        String asciiString = Encoding.ASCII.GetString(bb);
+                        for (int i = 1; i < 100000; i++) {
+                            try {
+                                Encoding e = Encoding.GetEncoding(i);
+                                byte[] xx = e.GetBytes(asciiString);
+                                if (xx.Length == 128) {
+                                    bool good = true;
+                                    for (int j = 0; j < 128; j++) { if (bb[j] != xx[j]) { good = false; break; } }
+                                    if (good) list.Add(i);
+                                }
+                            }
+                            catch {}
+                        }
+                        int n = list.Count;
+                        Console.Write("private const int[] _asciiCompatCodePages = new int[" + n + "] {\r\n    ");
+                        for (int i = 0; i < n; i++) {
+                            Console.Write("{0,5}", list[i]);
+                            if (i < n-1) Console.Write(", ");
+                            if (((i+1) % 10) == 0) Console.Write("\r\n    ");
+                        }
+                        Console.Write("\r\n};\r\n");
+                    }
+                }
+
+        */
+
+        private static int[] _asciiCompatCodePages = new int[79] {
+              437,   708,   720,   737,   775,   850,   852,   855,   857,   858,
+              860,   861,   862,   863,   864,   865,   866,   869,   874,   932,
+              936,   949,   950,  1250,  1251,  1252,  1253,  1254,  1255,  1256,
+             1257,  1258,  1361, 10000, 10001, 10002, 10003, 10004, 10005, 10006,
+            10007, 10008, 10010, 10017, 10029, 10079, 10081, 10082, 20000, 20001,
+            20002, 20003, 20004, 20005, 20127, 20866, 20932, 20936, 20949, 21866,
+            28591, 28592, 28593, 28594, 28595, 28596, 28597, 28598, 28599, 28605,
+            38598, 50220, 50221, 50222, 50225, 50227, 51932, 51949, 65001
+        };
+
+        internal /*public*/ static bool IsAsciiCompatibleCodePage(int codepage) {
+            //alternatives to binary search considered
+            //Hashtable: static initialization increases startup up, perf relative
+            //Byte array would consume ~8K, but lookups constant
+            //with 80 entries, binary search limited to 7 indexes into array
+            int lo = 0;
+            int hi = 78;
+            while(lo <= hi) {
+                int i = (lo + hi) >> 1;
+                int c;
+                c = _asciiCompatCodePages[i] - codepage;
+                if (c == 0) return true; //i is the index of the item
+                if (c < 0) {
+                    lo = i + 1;
+                }
+                else {
+                    hi = i - 1;
+                }
+            }
+            return false; 
+            //lo is the index of the item immediately after 
+            //~lo returned in some implementations for false indicator with additional info
+        }
+
+        internal const int CodePageUT8 = 65001;
+    }
+
+
+
+}
+
+
+