Class: Digest::CRC

Inherits:
Class
  • Object
show all
Includes:
Instance
Defined in:
lib/digest/crc.rb

Overview

Base class for all CRC algorithms.

Direct Known Subclasses

CRC1, CRC15, CRC16, CRC24, CRC32, CRC5, CRC64, CRC8

Constant Summary collapse

INIT_CRC =

The initial value of the CRC checksum

0x00
XOR_MASK =

The XOR mask to apply to the resulting CRC checksum

0x00
WIDTH =

The bit width of the CRC checksum

0
REFLECT_INPUT =

Define true or false whether the input direction is bit reversed or not of the CRC checksum

nil
TABLE =

Default place holder CRC table

[].freeze

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize ⇒ CRC

Initializes the CRC checksum.



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# File 'lib/digest/crc.rb', line 85

def initialize
  @init_crc      = self.class.const_get(:INIT_CRC)
  @xor_mask      = self.class.const_get(:XOR_MASK)
  @width         = self.class.const_get(:WIDTH)
  @reflect_input = self.class.const_get(:REFLECT_INPUT)
  @table         = self.class.const_get(:TABLE)

  reset
end

Class Method Details

.checksum(data) ⇒ Integer

Calculates the CRC checksum.

Parameters:

  • data (String) —

    The given data.

Returns:

  • (Integer) —

    The CRC checksum.



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# File 'lib/digest/crc.rb', line 35

def self.checksum(data)
  crc = self.new
  crc << data

  return crc.checksum
end

.implementation ⇒ :c_ext, :pure

Determines whether the library is using the optimized C extensions implementation, or the pure-Ruby implementation.

Returns:

  • (:c_ext, :pure)

Since:

  • 0.7.0



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# File 'lib/digest/crc.rb', line 74

def self.implementation
  if instance_method(:update).source_location.nil?
    :c_ext
  else
    :pure
  end
end

.pack(crc) ⇒ String

Packs the given CRC checksum.

Parameters:

  • crc (Integer) —

    The raw CRC checksum.

Returns:

  • (String) —

    The packed CRC checksum.



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# File 'lib/digest/crc.rb', line 51

def self.pack(crc)
  unless (width = self::WIDTH) > 0
    raise(NotImplementedError, "#{self} is incompleted as CRC")
  end

  bitclass   = width + (-width & 0x07)
  byte_count = bitclass / 8

  crc &= ~(-1 << width)

  result = [crc].pack("Q>")
  result[0, result.bytesize - byte_count] = ""
  result
end

Instance Method Details

#<<(data) ⇒ Object

See Also:



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# File 'lib/digest/crc.rb', line 176

def <<(data)
  update(data)
  return self
end

#block_length ⇒ 1

The input block length.

Returns:

  • (1) 


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# File 'lib/digest/crc.rb', line 100

def block_length
  1
end

#checksum ⇒ Integer

The resulting CRC checksum.

Returns:

  • (Integer) —

    The resulting CRC checksum.



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# File 'lib/digest/crc.rb', line 197

def checksum
  @crc ^ @xor_mask
end

#digest_length ⇒ Integer

The length of the digest.

Returns:

  • (Integer) —

    The length in bytes.



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# File 'lib/digest/crc.rb', line 110

def digest_length
  (@width / 8.0).ceil
end

#finish ⇒ Object

Finishes the CRC checksum calculation.

See Also:



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# File 'lib/digest/crc.rb', line 206

def finish
  self.class.pack(checksum)
end

#reset ⇒ Integer

Resets the CRC checksum.

Returns:

  • (Integer) —

    The default value of the CRC checksum.



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# File 'lib/digest/crc.rb', line 187

def reset
  @crc = @init_crc
end

#update(data) ⇒ Object

Updates the CRC checksum with the given data.

Parameters:

  • data (String) —

    The data to update the CRC checksum with.

Raises:

  • (NotImplementedError) —

    If WIDTH, TABLE, or REFLECT_INPUT constants are not set properly.



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# File 'lib/digest/crc.rb', line 123

def update(data)
  unless @width >= 1
    raise(NotImplementedError, "incompleted #{self.class} as CRC (expected WIDTH to be 1 or more)")
  end

  if @table.empty?
    raise(NotImplementedError, "incompleted #{self.class} as CRC (expected TABLE to be not empty)")
  end

  if @reflect_input.nil?
    raise(NotImplementedError, "incompleted #{self.class} as CRC (expected REFLECT_INPUT to be not nil)")
  end

  table = @table
  crc   = @crc

  if @reflect_input
    if @width > 8
      data.each_byte do |b|
        crc = table[b ^ (0xff & crc)] ^ (crc >> 8)
      end
    else
      data.each_byte do |b|
        # Omit (crc >> 8) since bits upper than the lower 8 bits are always 0
        crc = table[b ^ (0xff & crc)]
      end
    end
  else
    if @width > 8
      higher_bit_off = @width - 8
      remain_mask    = ~(-1 << higher_bit_off)

      data.each_byte do |b|
        crc = table[b ^ (0xff & (crc >> higher_bit_off))] ^ ((remain_mask & crc) << 8)
      end
    else
      padding = 8 - @width

      data.each_byte do |b|
        # Omit (crc << 8) since bits lower than the upper 8 bits are always 0
        crc = table[b ^ (0xff & (crc << padding))]
      end
    end
  end

  @crc = crc

  self
end