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|
require 'digest'
module Digest
#
# Base class for all CRC algorithms.
#
class CRC < Digest::Class
include Digest::Instance
# The initial value of the CRC checksum
INIT_CRC = 0x00
# The XOR mask to apply to the resulting CRC checksum
XOR_MASK = 0x00
# The bit width of the CRC checksum
WIDTH = 0
# Define true or false whether the input direction is bit reversed or not of the CRC checksum
REFLECT_INPUT = nil
# Default place holder CRC table
TABLE = [].freeze
#
# Calculates the CRC checksum.
#
# @param [String] data
# The given data.
#
# @return [Integer]
# The CRC checksum.
#
def self.checksum(data)
crc = self.new
crc << data
return crc.checksum
end
#
# Packs the given CRC checksum.
#
# @param [Integer] crc
# The raw CRC checksum.
#
# @return [String]
# The packed CRC checksum.
#
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
#
# Determines whether the library is using the optimized C extensions
# implementation, or the pure-Ruby implementation.
#
# @return [:c_ext, :pure]
#
# @since 0.7.0
#
def self.implementation
if instance_method(:update).source_location.nil?
:c_ext
else
:pure
end
end
#
# Initializes the CRC checksum.
#
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
#
# The input block length.
#
# @return [1]
#
def block_length
1
end
#
# The length of the digest.
#
# @return [Integer]
# The length in bytes.
#
def digest_length
(@width / 8.0).ceil
end
#
# Updates the CRC checksum with the given data.
#
# @param [String] data
# The data to update the CRC checksum with.
#
# @raise [NotImplementedError]
# If WIDTH, TABLE, or REFLECT_INPUT constants are not set properly.
#
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
#
# @see #update
#
def <<(data)
update(data)
return self
end
#
# Resets the CRC checksum.
#
# @return [Integer]
# The default value of the CRC checksum.
#
def reset
@crc = @init_crc
end
#
# The resulting CRC checksum.
#
# @return [Integer]
# The resulting CRC checksum.
#
def checksum
@crc ^ @xor_mask
end
#
# Finishes the CRC checksum calculation.
#
# @see pack
#
def finish
self.class.pack(checksum)
end
end
end
|
