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annotate Checksums/Adler32.cs @ 1:94e25b786321

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Re #311: can't read ZIP file packed by Linux app Archive Manager/File Roller Initial commit of clean SharpZipLib 0860 source. Only change is build paths.
author IBBoard <dev@ibboard.co.uk>
date Sat, 30 Oct 2010 14:03:17 +0000
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children
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1
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1 // Adler32.cs - Computes Adler32 data checksum of a data stream
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2 // Copyright (C) 2001 Mike Krueger
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3 //
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4 // This file was translated from java, it was part of the GNU Classpath
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5 // Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.
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6 //
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7 // This program is free software; you can redistribute it and/or
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8 // modify it under the terms of the GNU General Public License
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9 // as published by the Free Software Foundation; either version 2
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10 // of the License, or (at your option) any later version.
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11 //
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12 // This program is distributed in the hope that it will be useful,
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13 // but WITHOUT ANY WARRANTY; without even the implied warranty of
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14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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15 // GNU General Public License for more details.
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16 //
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17 // You should have received a copy of the GNU General Public License
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18 // along with this program; if not, write to the Free Software
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19 // Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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20 //
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21 // Linking this library statically or dynamically with other modules is
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22 // making a combined work based on this library. Thus, the terms and
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23 // conditions of the GNU General Public License cover the whole
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24 // combination.
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25 //
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26 // As a special exception, the copyright holders of this library give you
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27 // permission to link this library with independent modules to produce an
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28 // executable, regardless of the license terms of these independent
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29 // modules, and to copy and distribute the resulting executable under
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30 // terms of your choice, provided that you also meet, for each linked
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31 // independent module, the terms and conditions of the license of that
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32 // module. An independent module is a module which is not derived from
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33 // or based on this library. If you modify this library, you may extend
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34 // this exception to your version of the library, but you are not
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35 // obligated to do so. If you do not wish to do so, delete this
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36 // exception statement from your version.
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37
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38 using System;
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39
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40 namespace ICSharpCode.SharpZipLib.Checksums
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41 {
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42
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43 /// <summary>
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44 /// Computes Adler32 checksum for a stream of data. An Adler32
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45 /// checksum is not as reliable as a CRC32 checksum, but a lot faster to
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46 /// compute.
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47 ///
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48 /// The specification for Adler32 may be found in RFC 1950.
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49 /// ZLIB Compressed Data Format Specification version 3.3)
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50 ///
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51 ///
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52 /// From that document:
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53 ///
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54 /// "ADLER32 (Adler-32 checksum)
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55 /// This contains a checksum value of the uncompressed data
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56 /// (excluding any dictionary data) computed according to Adler-32
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57 /// algorithm. This algorithm is a 32-bit extension and improvement
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58 /// of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073
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59 /// standard.
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60 ///
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61 /// Adler-32 is composed of two sums accumulated per byte: s1 is
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62 /// the sum of all bytes, s2 is the sum of all s1 values. Both sums
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63 /// are done modulo 65521. s1 is initialized to 1, s2 to zero. The
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64 /// Adler-32 checksum is stored as s2*65536 + s1 in most-
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65 /// significant-byte first (network) order."
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66 ///
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67 /// "8.2. The Adler-32 algorithm
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68 ///
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69 /// The Adler-32 algorithm is much faster than the CRC32 algorithm yet
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70 /// still provides an extremely low probability of undetected errors.
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71 ///
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72 /// The modulo on unsigned long accumulators can be delayed for 5552
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73 /// bytes, so the modulo operation time is negligible. If the bytes
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74 /// are a, b, c, the second sum is 3a + 2b + c + 3, and so is position
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75 /// and order sensitive, unlike the first sum, which is just a
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76 /// checksum. That 65521 is prime is important to avoid a possible
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77 /// large class of two-byte errors that leave the check unchanged.
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78 /// (The Fletcher checksum uses 255, which is not prime and which also
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79 /// makes the Fletcher check insensitive to single byte changes 0 -
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80 /// 255.)
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81 ///
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82 /// The sum s1 is initialized to 1 instead of zero to make the length
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83 /// of the sequence part of s2, so that the length does not have to be
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84 /// checked separately. (Any sequence of zeroes has a Fletcher
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85 /// checksum of zero.)"
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86 /// </summary>
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87 /// <see cref="ICSharpCode.SharpZipLib.Zip.Compression.Streams.InflaterInputStream"/>
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88 /// <see cref="ICSharpCode.SharpZipLib.Zip.Compression.Streams.DeflaterOutputStream"/>
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89 public sealed class Adler32 : IChecksum
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90 {
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91 /// <summary>
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92 /// largest prime smaller than 65536
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93 /// </summary>
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94 const uint BASE = 65521;
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95
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96 /// <summary>
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97 /// Returns the Adler32 data checksum computed so far.
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98 /// </summary>
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99 public long Value {
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100 get {
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101 return checksum;
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102 }
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103 }
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104
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105 /// <summary>
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106 /// Creates a new instance of the Adler32 class.
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107 /// The checksum starts off with a value of 1.
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108 /// </summary>
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109 public Adler32()
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110 {
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111 Reset();
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112 }
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113
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114 /// <summary>
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115 /// Resets the Adler32 checksum to the initial value.
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116 /// </summary>
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117 public void Reset()
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118 {
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119 checksum = 1;
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120 }
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121
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122 /// <summary>
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123 /// Updates the checksum with a byte value.
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124 /// </summary>
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125 /// <param name="value">
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parents:
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126 /// The data value to add. The high byte of the int is ignored.
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127 /// </param>
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128 public void Update(int value)
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129 {
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130 // We could make a length 1 byte array and call update again, but I
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131 // would rather not have that overhead
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132 uint s1 = checksum & 0xFFFF;
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133 uint s2 = checksum >> 16;
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134
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135 s1 = (s1 + ((uint)value & 0xFF)) % BASE;
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136 s2 = (s1 + s2) % BASE;
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137
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138 checksum = (s2 << 16) + s1;
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139 }
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140
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141 /// <summary>
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142 /// Updates the checksum with an array of bytes.
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143 /// </summary>
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144 /// <param name="buffer">
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145 /// The source of the data to update with.
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146 /// </param>
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147 public void Update(byte[] buffer)
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148 {
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149 if ( buffer == null ) {
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150 throw new ArgumentNullException("buffer");
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151 }
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152
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153 Update(buffer, 0, buffer.Length);
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154 }
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155
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156 /// <summary>
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157 /// Updates the checksum with the bytes taken from the array.
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158 /// </summary>
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159 /// <param name="buffer">
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160 /// an array of bytes
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161 /// </param>
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162 /// <param name="offset">
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163 /// the start of the data used for this update
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164 /// </param>
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165 /// <param name="count">
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166 /// the number of bytes to use for this update
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167 /// </param>
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168 public void Update(byte[] buffer, int offset, int count)
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169 {
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170 if (buffer == null) {
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171 throw new ArgumentNullException("buffer");
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172 }
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173
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174 if (offset < 0) {
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175 #if NETCF_1_0
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176 throw new ArgumentOutOfRangeException("offset");
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177 #else
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178 throw new ArgumentOutOfRangeException("offset", "cannot be negative");
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179 #endif
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180 }
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181
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182 if ( count < 0 )
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183 {
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184 #if NETCF_1_0
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185 throw new ArgumentOutOfRangeException("count");
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186 #else
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187 throw new ArgumentOutOfRangeException("count", "cannot be negative");
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188 #endif
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189 }
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190
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191 if (offset >= buffer.Length)
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192 {
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193 #if NETCF_1_0
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194 throw new ArgumentOutOfRangeException("offset");
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195 #else
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196 throw new ArgumentOutOfRangeException("offset", "not a valid index into buffer");
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197 #endif
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198 }
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199
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200 if (offset + count > buffer.Length)
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parents:
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201 {
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202 #if NETCF_1_0
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parents:
diff changeset
203 throw new ArgumentOutOfRangeException("count");
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diff changeset
204 #else
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diff changeset
205 throw new ArgumentOutOfRangeException("count", "exceeds buffer size");
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parents:
diff changeset
206 #endif
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207 }
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parents:
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208
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parents:
diff changeset
209 //(By Per Bothner)
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parents:
diff changeset
210 uint s1 = checksum & 0xFFFF;
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diff changeset
211 uint s2 = checksum >> 16;
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parents:
diff changeset
212
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parents:
diff changeset
213 while (count > 0) {
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parents:
diff changeset
214 // We can defer the modulo operation:
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parents:
diff changeset
215 // s1 maximally grows from 65521 to 65521 + 255 * 3800
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216 // s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31
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parents:
diff changeset
217 int n = 3800;
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diff changeset
218 if (n > count) {
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219 n = count;
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parents:
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220 }
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parents:
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221 count -= n;
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parents:
diff changeset
222 while (--n >= 0) {
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parents:
diff changeset
223 s1 = s1 + (uint)(buffer[offset++] & 0xff);
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diff changeset
224 s2 = s2 + s1;
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225 }
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226 s1 %= BASE;
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227 s2 %= BASE;
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parents:
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228 }
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parents:
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229
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230 checksum = (s2 << 16) | s1;
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231 }
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parents:
diff changeset
232
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parents:
diff changeset
233 #region Instance Fields
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parents:
diff changeset
234 uint checksum;
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parents:
diff changeset
235 #endregion
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diff changeset
236 }
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237 }