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- using System;
- using System.Collections.Generic;
- namespace SharpCompress.Compressors.Xz
- {
- internal static class Crc32
- {
- public const UInt32 DefaultPolynomial = 0xedb88320u;
- public const UInt32 DefaultSeed = 0xffffffffu;
- private static UInt32[] defaultTable;
- public static UInt32 Compute(byte[] buffer)
- {
- return Compute(DefaultSeed, buffer);
- }
- public static UInt32 Compute(UInt32 seed, byte[] buffer)
- {
- return Compute(DefaultPolynomial, seed, buffer);
- }
- public static UInt32 Compute(UInt32 polynomial, UInt32 seed, byte[] buffer)
- {
- return ~CalculateHash(InitializeTable(polynomial), seed, buffer, 0, buffer.Length);
- }
- private static UInt32[] InitializeTable(UInt32 polynomial)
- {
- if (polynomial == DefaultPolynomial && defaultTable != null)
- return defaultTable;
- var createTable = new UInt32[256];
- for (var i = 0; i < 256; i++)
- {
- var entry = (UInt32)i;
- for (var j = 0; j < 8; j++)
- if ((entry & 1) == 1)
- entry = (entry >> 1) ^ polynomial;
- else
- entry = entry >> 1;
- createTable[i] = entry;
- }
- if (polynomial == DefaultPolynomial)
- defaultTable = createTable;
- return createTable;
- }
- private static UInt32 CalculateHash(UInt32[] table, UInt32 seed, IList<byte> buffer, int start, int size)
- {
- var crc = seed;
- for (var i = start; i < size - start; i++)
- crc = (crc >> 8) ^ table[buffer[i] ^ crc & 0xff];
- return crc;
- }
- }
- }
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