Class: ExternalBitField

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

Constant Summary collapse

ELEMENT_WIDTH = 
8

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(path) ⇒ ExternalBitField

Returns a new instance of ExternalBitField.



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# File 'lib/bloomfilter/external_bloom_filter.rb', line 15

def initialize(path)
  @size = File.size(path) * ELEMENT_WIDTH
  @field = File.new(path, "r+")
end

Instance Attribute Details

#size ⇒ Object (readonly)

Returns the value of attribute size.



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# File 'lib/bloomfilter/external_bloom_filter.rb', line 4

def size
  @size
end

Class Method Details

.create(path, size) ⇒ Object 



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# File 'lib/bloomfilter/external_bloom_filter.rb', line 8

def self.create(path, size)
  File.open(path, "w") do |file|
    file.write "\000" * (((size - 1) / ELEMENT_WIDTH) + 1)
  end
  ExternalBitField.new(path)
end

Instance Method Details

#[](position) ⇒ Object

Read a bit (1/0)



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# File 'lib/bloomfilter/external_bloom_filter.rb', line 36

def [](position)
  # seek to the appropriate byte position
  @field.seek(position / ELEMENT_WIDTH)
  
  # read the value
  val = @field.read(1)[0]

  # figure out if our bit is flipped or not
  (val & (1 << (position % ELEMENT_WIDTH))) > 0
end

#set(position) ⇒ Object 



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# File 'lib/bloomfilter/external_bloom_filter.rb', line 20

def set(position)
  #debugger
  # seek to the position in the file where we'll be making changes
  @field.seek(position / ELEMENT_WIDTH)

  # read the old value, converted to an integer
  old_val = @field.read(1)[0]
  
  # seek back to our spot
  @field.seek(position / ELEMENT_WIDTH)
  
  # write the new value, as a character again this time
  @field.write( (old_val | 1 << (position % ELEMENT_WIDTH)).chr )
end