| | 1 | | using System; |
| | 2 | |
|
| | 3 | | namespace ICSharpCode.SharpZipLib.Checksum |
| | 4 | | { |
| | 5 | | /// <summary> |
| | 6 | | /// Computes Adler32 checksum for a stream of data. An Adler32 |
| | 7 | | /// checksum is not as reliable as a CRC32 checksum, but a lot faster to |
| | 8 | | /// compute. |
| | 9 | | /// |
| | 10 | | /// The specification for Adler32 may be found in RFC 1950. |
| | 11 | | /// ZLIB Compressed Data Format Specification version 3.3) |
| | 12 | | /// |
| | 13 | | /// |
| | 14 | | /// From that document: |
| | 15 | | /// |
| | 16 | | /// "ADLER32 (Adler-32 checksum) |
| | 17 | | /// This contains a checksum value of the uncompressed data |
| | 18 | | /// (excluding any dictionary data) computed according to Adler-32 |
| | 19 | | /// algorithm. This algorithm is a 32-bit extension and improvement |
| | 20 | | /// of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073 |
| | 21 | | /// standard. |
| | 22 | | /// |
| | 23 | | /// Adler-32 is composed of two sums accumulated per byte: s1 is |
| | 24 | | /// the sum of all bytes, s2 is the sum of all s1 values. Both sums |
| | 25 | | /// are done modulo 65521. s1 is initialized to 1, s2 to zero. The |
| | 26 | | /// Adler-32 checksum is stored as s2*65536 + s1 in most- |
| | 27 | | /// significant-byte first (network) order." |
| | 28 | | /// |
| | 29 | | /// "8.2. The Adler-32 algorithm |
| | 30 | | /// |
| | 31 | | /// The Adler-32 algorithm is much faster than the CRC32 algorithm yet |
| | 32 | | /// still provides an extremely low probability of undetected errors. |
| | 33 | | /// |
| | 34 | | /// The modulo on unsigned long accumulators can be delayed for 5552 |
| | 35 | | /// bytes, so the modulo operation time is negligible. If the bytes |
| | 36 | | /// are a, b, c, the second sum is 3a + 2b + c + 3, and so is position |
| | 37 | | /// and order sensitive, unlike the first sum, which is just a |
| | 38 | | /// checksum. That 65521 is prime is important to avoid a possible |
| | 39 | | /// large class of two-byte errors that leave the check unchanged. |
| | 40 | | /// (The Fletcher checksum uses 255, which is not prime and which also |
| | 41 | | /// makes the Fletcher check insensitive to single byte changes 0 - |
| | 42 | | /// 255.) |
| | 43 | | /// |
| | 44 | | /// The sum s1 is initialized to 1 instead of zero to make the length |
| | 45 | | /// of the sequence part of s2, so that the length does not have to be |
| | 46 | | /// checked separately. (Any sequence of zeroes has a Fletcher |
| | 47 | | /// checksum of zero.)" |
| | 48 | | /// </summary> |
| | 49 | | /// <see cref="ICSharpCode.SharpZipLib.Zip.Compression.Streams.InflaterInputStream"/> |
| | 50 | | /// <see cref="ICSharpCode.SharpZipLib.Zip.Compression.Streams.DeflaterOutputStream"/> |
| | 51 | | public sealed class Adler32 : IChecksum |
| | 52 | | { |
| | 53 | | #region Instance Fields |
| | 54 | | /// <summary> |
| | 55 | | /// largest prime smaller than 65536 |
| | 56 | | /// </summary> |
| 1 | 57 | | readonly static uint BASE = 65521; |
| | 58 | |
|
| | 59 | | /// <summary> |
| | 60 | | /// The CRC data checksum so far. |
| | 61 | | /// </summary> |
| | 62 | | uint checkValue; |
| | 63 | | #endregion |
| | 64 | |
|
| | 65 | | /// <summary> |
| | 66 | | /// Initialise a default instance of <see cref="Adler32"></see> |
| | 67 | | /// </summary> |
| 709 | 68 | | public Adler32() |
| | 69 | | { |
| 709 | 70 | | Reset(); |
| 709 | 71 | | } |
| | 72 | |
|
| | 73 | | /// <summary> |
| | 74 | | /// Resets the Adler32 data checksum as if no update was ever called. |
| | 75 | | /// </summary> |
| | 76 | | public void Reset() |
| | 77 | | { |
| 1143 | 78 | | checkValue = 1; |
| 1143 | 79 | | } |
| | 80 | |
|
| | 81 | | /// <summary> |
| | 82 | | /// Returns the Adler32 data checksum computed so far. |
| | 83 | | /// </summary> |
| | 84 | | public long Value { |
| | 85 | | get { |
| 18 | 86 | | return checkValue; |
| | 87 | | } |
| | 88 | | } |
| | 89 | |
|
| | 90 | | /// <summary> |
| | 91 | | /// Updates the checksum with the byte b. |
| | 92 | | /// </summary> |
| | 93 | | /// <param name="bval"> |
| | 94 | | /// The data value to add. The high byte of the int is ignored. |
| | 95 | | /// </param> |
| | 96 | | public void Update(int bval) |
| | 97 | | { |
| | 98 | | // We could make a length 1 byte array and call update again, but I |
| | 99 | | // would rather not have that overhead |
| 0 | 100 | | uint s1 = checkValue & 0xFFFF; |
| 0 | 101 | | uint s2 = checkValue >> 16; |
| | 102 | |
|
| 0 | 103 | | s1 = (s1 + ((uint)bval & 0xFF)) % BASE; |
| 0 | 104 | | s2 = (s1 + s2) % BASE; |
| | 105 | |
|
| 0 | 106 | | checkValue = (s2 << 16) + s1; |
| 0 | 107 | | } |
| | 108 | |
|
| | 109 | | /// <summary> |
| | 110 | | /// Updates the Adler32 data checksum with the bytes taken from |
| | 111 | | /// a block of data. |
| | 112 | | /// </summary> |
| | 113 | | /// <param name="buffer">Contains the data to update the checksum with.</param> |
| | 114 | | public void Update(byte[] buffer) |
| | 115 | | { |
| 2 | 116 | | if (buffer == null) { |
| 1 | 117 | | throw new ArgumentNullException(nameof(buffer)); |
| | 118 | | } |
| | 119 | |
|
| 1 | 120 | | Update(buffer, 0, buffer.Length); |
| 1 | 121 | | } |
| | 122 | |
|
| | 123 | | /// <summary> |
| | 124 | | /// Update Adler32 data checksum based on a portion of a block of data |
| | 125 | | /// </summary> |
| | 126 | | /// <param name = "buffer">Contains the data to update the CRC with.</param> |
| | 127 | | /// <param name = "offset">The offset into the buffer where the data starts</param> |
| | 128 | | /// <param name = "count">The number of data bytes to update the CRC with.</param> |
| | 129 | | public void Update(byte[] buffer, int offset, int count) |
| | 130 | | { |
| 6204 | 131 | | if (buffer == null) { |
| 1 | 132 | | throw new ArgumentNullException(nameof(buffer)); |
| | 133 | | } |
| | 134 | |
|
| 6203 | 135 | | if (offset < 0) { |
| 1 | 136 | | throw new ArgumentOutOfRangeException(nameof(offset), "cannot be less than zero"); |
| | 137 | | } |
| | 138 | |
|
| 6202 | 139 | | if (offset >= buffer.Length) { |
| 1 | 140 | | throw new ArgumentOutOfRangeException(nameof(offset), "not a valid index into buffer"); |
| | 141 | | } |
| | 142 | |
|
| 6201 | 143 | | if (count < 0) { |
| 1 | 144 | | throw new ArgumentOutOfRangeException(nameof(count), "cannot be less than zero"); |
| | 145 | | } |
| | 146 | |
|
| 6200 | 147 | | if (offset + count > buffer.Length) { |
| 1 | 148 | | throw new ArgumentOutOfRangeException(nameof(count), "exceeds buffer size"); |
| | 149 | | } |
| | 150 | |
|
| | 151 | | //(By Per Bothner) |
| 6199 | 152 | | uint s1 = checkValue & 0xFFFF; |
| 6199 | 153 | | uint s2 = checkValue >> 16; |
| | 154 | |
|
| 13890 | 155 | | while (count > 0) { |
| | 156 | | // We can defer the modulo operation: |
| | 157 | | // s1 maximally grows from 65521 to 65521 + 255 * 3800 |
| | 158 | | // s2 maximally grows by 3800 * median(s1) = 2090079800 < 2^31 |
| 7691 | 159 | | int n = 3800; |
| 7691 | 160 | | if (n > count) { |
| 6199 | 161 | | n = count; |
| | 162 | | } |
| 7691 | 163 | | count -= n; |
| 8355525 | 164 | | while (--n >= 0) { |
| 8347834 | 165 | | s1 = s1 + (uint)(buffer[offset++] & 0xff); |
| 8347834 | 166 | | s2 = s2 + s1; |
| | 167 | | } |
| 7691 | 168 | | s1 %= BASE; |
| 7691 | 169 | | s2 %= BASE; |
| | 170 | | } |
| | 171 | |
|
| 6199 | 172 | | checkValue = (s2 << 16) | s1; |
| 6199 | 173 | | } |
| | 174 | | } |
| | 175 | | } |