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# NAME: BitField
# AUTHOR: Peter Cooper
# LICENSE: MIT ( http://www.opensource.org/licenses/mit-license.php )
# COPYRIGHT: (c) 2007 Peter Cooper (http://www.petercooper.co.uk/)
# VERSION: v4
# HISTORY: v4 (better support for loading and dumping fields)
# v3 (supports dynamic bitwidths for array elements.. now doing 32 bit widths default)
# v2 (now uses 1 << y, rather than 2 ** y .. it's 21.8 times faster!)
# v1 (first release)
#
# DESCRIPTION: Basic, pure Ruby bit field. Pretty fast (for what it is) and memory efficient.
# I've written a pretty intensive test suite for it and it passes great.
# Works well for Bloom filters (the reason I wrote it).
#
# Create a bit field 1000 bits wide
# bf = BitField.new(1000)
#
# Setting and reading bits
# bf[100] = 1
# bf[100] .. => 1
# bf[100] = 0
#
# More
# bf.to_s = "10101000101010101" (example)
# bf.total_set .. => 10 (example - 10 bits are set to "1")
class BitField
attr_reader :size
attr_accessor :field
include Enumerable
ELEMENT_WIDTH = 32
def initialize(size)
@size = size
@field = Array.new(((size - 1) / ELEMENT_WIDTH) + 1, 0)
end
# Set a bit (1/0)
def []=(position, value)
value == 1 ? @field[position / ELEMENT_WIDTH] |= 1 << (position % ELEMENT_WIDTH) : @field[position / ELEMENT_WIDTH] ^= 1 << (position % ELEMENT_WIDTH)
end
# Read a bit (1/0)
def [](position)
@field[position / ELEMENT_WIDTH] & 1 << (position % ELEMENT_WIDTH) > 0 ? 1 : 0
end
# Iterate over each bit
def each(&block)
@size.times { |position| yield self[position] }
end
# Returns the field as a string like "0101010100111100," etc.
def to_s
inject("") { |a, b| a + b.to_s }
end
# Returns the total number of bits that are set
# (The technique used here is about 6 times faster than using each or inject direct on the bitfield)
def total_set
@field.inject(0) { |a, byte| a += byte & 1 and byte >>= 1 until byte == 0; a }
end
end
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