csharp/common/reedsolomon/GF256.cs

   1 /*\r
   2 * Copyright 2007 ZXing authors\r
   3 *\r
   4 * Licensed under the Apache License, Version 2.0 (the "License");\r
   5 * you may not use this file except in compliance with the License.\r
   6 * You may obtain a copy of the License at\r
   7 *\r
   8 *      http://www.apache.org/licenses/LICENSE-2.0\r
   9 *\r
  10 * Unless required by applicable law or agreed to in writing, software\r
  11 * distributed under the License is distributed on an "AS IS" BASIS,\r
  12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.\r
  13 * See the License for the specific language governing permissions and\r
  14 * limitations under the License.\r
  15 */\r
  16 using System;\r
  17 namespace com.google.zxing.common.reedsolomon\r
  18 {\r
  19         \r
  20         /// <summary> <p>This class contains utility methods for performing mathematical operations over\r
  21         /// the Galois Field GF(256). Operations use a given primitive polynomial in calculations.</p>\r
  22         /// \r
  23         /// <p>Throughout this package, elements of GF(256) are represented as an <code>int</code>\r
  24         /// for convenience and speed (but at the cost of memory).\r
  25         /// Only the bottom 8 bits are really used.</p>\r
  26         /// \r
  27         /// </summary>\r
  28         /// <author>  Sean Owen\r
  29         /// </author>\r
  30         /// <author>www.Redivivus.in (suraj.supekar@redivivus.in) - Ported from ZXING Java Source \r
  31         /// </author>\r
  32         public sealed class GF256\r
  33         {\r
  34                 internal GF256Poly Zero\r
  35                 {\r
  36                         get\r
  37                         {\r
  38                                 return zero;\r
  39                         }\r
  40                         \r
  41                 }\r
  42                 internal GF256Poly One\r
  43                 {\r
  44                         get\r
  45                         {\r
  46                                 return one;\r
  47                         }\r
  48                         \r
  49                 }\r
  50                 \r
  51                 //UPGRADE_NOTE: Final was removed from the declaration of 'QR_CODE_FIELD '. "ms-help://MS.VSCC.v80/dv_commoner/local/redirect.htm?index='!DefaultContextWindowIndex'&keyword='jlca1003'"\r
  52                 public static readonly GF256 QR_CODE_FIELD = new GF256(0x011D); // x^8 + x^4 + x^3 + x^2 + 1\r
  53                 //UPGRADE_NOTE: Final was removed from the declaration of 'DATA_MATRIX_FIELD '. "ms-help://MS.VSCC.v80/dv_commoner/local/redirect.htm?index='!DefaultContextWindowIndex'&keyword='jlca1003'"\r
  54                 public static readonly GF256 DATA_MATRIX_FIELD = new GF256(0x012D); // x^8 + x^5 + x^3 + x^2 + 1\r
  55                 \r
  56                 //UPGRADE_NOTE: Final was removed from the declaration of 'expTable '. "ms-help://MS.VSCC.v80/dv_commoner/local/redirect.htm?index='!DefaultContextWindowIndex'&keyword='jlca1003'"\r
  57                 private int[] expTable;\r
  58                 //UPGRADE_NOTE: Final was removed from the declaration of 'logTable '. "ms-help://MS.VSCC.v80/dv_commoner/local/redirect.htm?index='!DefaultContextWindowIndex'&keyword='jlca1003'"\r
  59                 private int[] logTable;\r
  60                 //UPGRADE_NOTE: Final was removed from the declaration of 'zero '. "ms-help://MS.VSCC.v80/dv_commoner/local/redirect.htm?index='!DefaultContextWindowIndex'&keyword='jlca1003'"\r
  61                 private GF256Poly zero;\r
  62                 //UPGRADE_NOTE: Final was removed from the declaration of 'one '. "ms-help://MS.VSCC.v80/dv_commoner/local/redirect.htm?index='!DefaultContextWindowIndex'&keyword='jlca1003'"\r
  63                 private GF256Poly one;\r
  64                 \r
  65                 /// <summary> Create a representation of GF(256) using the given primitive polynomial.\r
  66                 /// \r
  67                 /// </summary>\r
  68                 /// <param name="primitive">irreducible polynomial whose coefficients are represented by\r
  69                 /// the bits of an int, where the least-significant bit represents the constant\r
  70                 /// coefficient\r
  71                 /// </param>\r
  72                 private GF256(int primitive)\r
  73                 {\r
  74                         expTable = new int[256];\r
  75                         logTable = new int[256];\r
  76                         int x = 1;\r
  77                         for (int i = 0; i < 256; i++)\r
  78                         {\r
  79                                 expTable[i] = x;\r
  80                                 x <<= 1; // x = x * 2; we're assuming the generator alpha is 2\r
  81                                 if (x >= 0x100)\r
  82                                 {\r
  83                                         x ^= primitive;\r
  84                                 }\r
  85                         }\r
  86                         for (int i = 0; i < 255; i++)\r
  87                         {\r
  88                                 logTable[expTable[i]] = i;\r
  89                         }\r
  90                         // logTable[0] == 0 but this should never be used\r
  91                         zero = new GF256Poly(this, new int[]{0});\r
  92                         one = new GF256Poly(this, new int[]{1});\r
  93                 }\r
  94                 \r
  95                 /// <returns> the monomial representing coefficient * x^degree\r
  96                 /// </returns>\r
  97                 internal GF256Poly buildMonomial(int degree, int coefficient)\r
  98                 {\r
  99                         if (degree < 0)\r
 100                         {\r
 101                                 throw new System.ArgumentException();\r
 102                         }\r
 103                         if (coefficient == 0)\r
 104                         {\r
 105                                 return zero;\r
 106                         }\r
 107                         int[] coefficients = new int[degree + 1];\r
 108                         coefficients[0] = coefficient;\r
 109                         return new GF256Poly(this, coefficients);\r
 110                 }\r
 111                 \r
 112                 /// <summary> Implements both addition and subtraction -- they are the same in GF(256).\r
 113                 /// \r
 114                 /// </summary>\r
 115                 /// <returns> sum/difference of a and b\r
 116                 /// </returns>\r
 117                 internal static int addOrSubtract(int a, int b)\r
 118                 {\r
 119                         return a ^ b;\r
 120                 }\r
 121                 \r
 122                 /// <returns> 2 to the power of a in GF(256)\r
 123                 /// </returns>\r
 124                 internal int exp(int a)\r
 125                 {\r
 126                         return expTable[a];\r
 127                 }\r
 128                 \r
 129                 /// <returns> base 2 log of a in GF(256)\r
 130                 /// </returns>\r
 131                 internal int log(int a)\r
 132                 {\r
 133                         if (a == 0)\r
 134                         {\r
 135                                 throw new System.ArgumentException();\r
 136                         }\r
 137                         return logTable[a];\r
 138                 }\r
 139                 \r
 140                 /// <returns> multiplicative inverse of a\r
 141                 /// </returns>\r
 142                 internal int inverse(int a)\r
 143                 {\r
 144                         if (a == 0)\r
 145                         {\r
 146                                 throw new System.ArithmeticException();\r
 147                         }\r
 148                         return expTable[255 - logTable[a]];\r
 149                 }\r
 150                 \r
 151                 /// <param name="a">\r
 152                 /// </param>\r
 153                 /// <param name="b">\r
 154                 /// </param>\r
 155                 /// <returns> product of a and b in GF(256)\r
 156                 /// </returns>\r
 157                 internal int multiply(int a, int b)\r
 158                 {\r
 159                         if (a == 0 || b == 0)\r
 160                         {\r
 161                                 return 0;\r
 162                         }\r
 163                         if (a == 1)\r
 164                         {\r
 165                                 return b;\r
 166                         }\r
 167                         if (b == 1)\r
 168                         {\r
 169                                 return a;\r
 170                         }\r
 171                         return expTable[(logTable[a] + logTable[b]) % 255];\r
 172                 }\r
 173         }\r
 174 }