Class: String

Inherits:
Object
  • Object
show all
Defined in:
lib/PatchedPNG.rb

Overview

:nodoc:

Instance Method Summary collapse

Instance Method Details

#png_crc ⇒ Object

Calculates a CRC using the algorithm in the PNG specification.



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# File 'lib/PatchedPNG.rb', line 11

def png_crc
  unless defined? @@crc then
    @@crc = Array.new(256)
    256.times do |n|
      c = n
      8.times do
        c = (c & 1 == 1) ? 0xedb88320 ^ (c >> 1) : c >> 1
      end
      @@crc[n] = c
    end
  end

  c = 0xffffffff
  each_byte do |b|
    c = @@crc[(c^b) & 0xff] ^ (c >> 8)
  end
  return c ^ 0xffffffff
end