PKWAREのCRC32のテーブル

PROGRAMING

ここから拝借。
これ仕様書に載ってんのかなぁ？

$__crc32_table = [] # Lookup table array

def __crc32_reflect(ref, ch) # Reflects CRC bits in the lookup table
  value = 0

  # Swap bit 0 for bit 7, bit 1 for bit 6, etc.
  (1..ch).each do |i|
    if (ref & 1).nonzero?
      value |= (1 << (ch - i))
    end

    ref = ((ref >> 1) & 0x7fffffff)
  end

  return value;
end

def __crc32_init_table() #  Builds lookup table array
  # This is the official polynomial used by
  # CRC-32 in PKZip, WinZip and Ethernet.
  polynomial = 0x04c11db7;

  # 256 values representing ASCII character codes.
  256.times do |i|
    $__crc32_table[i] = (__crc32_reflect(i, 8) << 24);

    8.times do |j|
      $__crc32_table[i] = (($__crc32_table[i] << 1) ^ (($__crc32_table[i] & (1 << 31)).nonzero? ? polynomial : 0))
    end

    $__crc32_table[i] = __crc32_reflect($__crc32_table[i], 32)
  end
end

__crc32_init_table()

puts '%x' % $__crc32_table[?x]