Class: Origami::Filter::Utils::BitReader

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

Overview

Class used to read a String as a stream of bits. Internally used by some filters.

Constant Summary collapse

BRUIJIN_TABLE = 
::Array.new(32)

Instance Method Summary collapse

Constructor Details

#initialize(data) ⇒ BitReader

Returns a new instance of BitReader.



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# File 'lib/origami/filters.rb', line 138

def initialize(data)
  @data = data
  reset
end

Instance Method Details

#clo ⇒ Object

Used for bit scanning. Count leading ones. Does not advance read pointer.



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# File 'lib/origami/filters.rb', line 254

def clo
  count = 0
  if @ptr_bit != 0
    bits = peek(8 - @ptr_bit) 
    count = clz32(~(bits << (32 - (8 - @ptr_bit))) & 0xff)

    return count if count < (8 - @ptr_bit)
  end

  delta = 0
  while @data.size > @ptr_byte + delta * 4
    word = @data[@ptr_byte + delta * 4, 4] # next 32 bits
    z = clz32(~((word << (4 - word.size)).unpack("N")[0]) & 0xffff_ffff)
    
    count += z
    delta += 1

    return count if z < 32 - ((4 - word.size) << 3)
  end

  count
end

#clz ⇒ Object

Used for bit scanning. Counts leading zeros. Does not advance read pointer.



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# File 'lib/origami/filters.rb', line 227

def clz
  count = 0
  if @ptr_bit != 0
    bits = peek(8 - @ptr_bit) 
    count = clz32(bits << (32 - (8 - @ptr_bit)))

    return count if count < (8 - @ptr_bit)
  end

  delta = 0
  while @data.size > @ptr_byte + delta * 4
    word = @data[@ptr_byte + delta * 4, 4] # next 32 bits
    z = clz32((word << (4 - word.size)).unpack("N")[0])
    
    count += z
    delta += 1

    return count if z < 32 - ((4 - word.size) << 3)
  end

  count
end

#eod? ⇒ Boolean

Returns true if end of data has been reached.

Returns:



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# File 'lib/origami/filters.rb', line 154

def eod?
  @ptr_byte >= @data.size
end

#peek(length) ⇒ Object

Reads length bits as a Fixnum. Does not advance read pointer.



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# File 'lib/origami/filters.rb', line 198

def peek(length)
  return BitReaderError, "Invalid read length" unless length > 0 
  return BitReaderError, "Insufficient data" if self.pos + length > self.size

  n = 0
  ptr_byte, ptr_bit = @ptr_byte, @ptr_bit

  while length > 0
    byte = @data[ptr_byte].ord
    
    if length > 8 - ptr_bit
      length -= 8 - ptr_bit
      n |= ( byte & ((1 << (8 - ptr_bit)) - 1) ) << length

      ptr_byte += 1
      ptr_bit = 0
    else
      n |= (byte >> (8 - ptr_bit - length)) & ((1 << length) - 1)
      length = 0
    end
  end

  n
end

#pos ⇒ Object

Returns the read pointer position in bits.



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# File 'lib/origami/filters.rb', line 161

def pos
  (@ptr_byte << 3) + @ptr_bit
end

#pos=(bits) ⇒ Object

Sets the read pointer position in bits.

Raises:



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# File 'lib/origami/filters.rb', line 175

def pos=(bits)
  raise BitReaderError, "Pointer position out of data" if bits > self.size

  pbyte = bits >> 3
  pbit = bits - (pbyte << 3)
  @ptr_byte, @ptr_bit = pbyte, pbit
  
  bits
end

#read(length) ⇒ Object

Reads length bits as a Fixnum and advances read pointer.



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# File 'lib/origami/filters.rb', line 188

def read(length)
  n = self.peek(length)
  self.pos += length

  n
end

#reset ⇒ Object

Resets the read pointer.



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# File 'lib/origami/filters.rb', line 146

def reset
  @ptr_byte, @ptr_bit = 0, 0
  self
end

#size ⇒ Object

Returns the data size in bits.



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# File 'lib/origami/filters.rb', line 168

def size
  @data.size << 3
end