Allow dumping and loading bitarrays

README.md

@@ -61,8 +61,28 @@ ba = BitArray.new(16, ["0000111111110000"].pack('B*'), reverse_byte: false)
 ba.to_s # "0000111111110000"
 ```
 
+Saving and loading `BitArray`:
+
+```ruby
+ba = BitArray.new(16, ["0000111111110000"].pack('B*'))
+ba.dump(File.new("bitarray.dat", "w"))
+#=> #<File:bitarray.dat>
+ba = BitArray.load(File.open("bitarray.dat"))
+ba.to_s # "1111000000001111"
+```
+
+Read-only access without loading it into memory:
+
+```ruby
+ba = BitArray.new(16, ["0000111111110000"].pack('B*'))
+ba.dump(File.new("bitarray.dat", "w"))
+ba_ro = BitArrayFile.new(filename: "bitarray.dat")
+ba_ro[0] # 1
+ba_ro[4] # 0
+```
 
 ## History
+- 1.4 in 2022 (cleanups, add unions, dump/load, and BitArrayFile)
 - 1.3 in 2022 (cleanups and a minor perf tweak)
 - 1.2 in 2018 (Added option to skip reverse the bits for each byte by @dalibor)
 - 1.1 in 2018 (fixed a significant bug)

lib/bitarray.rb

@@ -1 +1,2 @@
 require_relative "bitarray/bit_array"
+require_relative "bitarray/bit_array_file"

lib/bitarray/bit_array.rb

@@ -2,7 +2,8 @@ class BitArray
   attr_reader :field, :reverse_byte, :size
   include Enumerable
 
-  VERSION = "1.3.0"
+  VERSION = "1.4.0"
+  HEADER_LENGTH = 8 + 1 # QC (@size, @reverse_byte)
 
   def initialize(size, field = nil, reverse_byte: true)
     @size = size
@@ -24,6 +25,26 @@ def [](position)
     (@field.getbyte(position >> 3) & (1 << (byte_position(position) % 8))) > 0 ? 1 : 0
   end
 
+  def ==(rhs)
+    @size == rhs.size && @reverse_byte == rhs.reverse_byte && @field == rhs.field
+  end
+
+  # Allows joining (union) two bitarrays of identical size.
+  # The resulting bitarray will contain any bit set in either constituent arrays.
+  # |= is implicitly defined, so you can do source_ba |= other_ba
+  def |(rhs)
+    raise ArgumentError.new("Bitarray sizes must be identical") if @size != rhs.size
+    raise ArgumentError.new("Reverse byte settings must be identical") if @reverse_byte != rhs.reverse_byte
+
+    combined = BitArray.new(@size, @field, reverse_byte: @reverse_byte)
+    rhs.field.each_byte.inject(0) do |byte_pos, byte|
+      combined.field.setbyte(byte_pos, combined.field.getbyte(byte_pos) | byte)
+      byte_pos + 1
+    end
+
+    combined
+  end
+
   # Iterate over each bit
   def each
     return to_enum(:each) unless block_given?
@@ -55,4 +76,18 @@ def total_set
   private def byte_position(position)
     @reverse_byte ? position : 7 - position
   end
+
+  # Save contents to an io device such as a file
+  def dump(io)
+    io.write([@size, @reverse_byte ? 1 : 0].pack("QC"))
+    io.write(@field.b)
+    io
+  end
+
+  # Load bitarray from an io device such as a file
+  def self.load(io)
+    size, reverse_byte = io.read(9).unpack("QC")
+    field = io.read
+    new(size, field, reverse_byte: reverse_byte == 1)
+  end
 end

lib/bitarray/bit_array_file.rb

@@ -0,0 +1,38 @@
+require_relative "bit_array"
+
+# Read-only access to a BitArray dumped to disk.
+# This is considerably slower than using the RAM-based BitArray, but
+# avoids the memory requirements and initial setup time.
+class BitArrayFile
+  HEADER_LENGTH = BitArray::HEADER_LENGTH
+
+  attr_reader :io, :reverse_byte, :size
+
+  def initialize(filename: nil, io: nil)
+    if io
+      @io = io
+    elsif filename
+      @io = File.open(filename, "r")
+    else
+      raise ArgumentError.new("Must specify a filename or io argument")
+    end
+
+    @io.seek(0)
+    @size, @reverse_byte = @io.read(9).unpack("QC")
+    @reverse_byte = @reverse_byte != 0
+  end
+
+  # Read a bit (1/0)
+  def [](position)
+    seek_to(position >> 3)
+    (@io.getbyte & (1 << (byte_position(position) % 8))) > 0 ? 1 : 0
+  end
+
+  private def byte_position(position)
+    @reverse_byte ? position : 7 - position
+  end
+
+  private def seek_to(position)
+    @io.seek(position + HEADER_LENGTH)
+  end
+end

test/test_bit_array.rb

@@ -1,4 +1,5 @@
 require "minitest/autorun"
+require "tempfile"
 require_relative "../lib/bitarray"
 
 class TestBitArray < Minitest::Test
@@ -79,6 +80,50 @@ def test_total_set
     ba[9] = 1
     assert_equal 3, ba.total_set
   end
+
+  def test_dump_load
+    ba_dump = BitArray.new(35)
+    [1, 5, 6, 7, 10, 16, 33].each { |i| ba_dump[i] = 1}
+    Tempfile.create("bit_array.dat") do |io|
+      ba_dump.dump(io)
+      io.rewind
+      ba_load = BitArray.load(io)
+
+      assert_equal ba_dump, ba_load
+    end
+  end
+
+  def test_union
+    set_bits = [1, 5, 6, 7, 10, 16, 33].shuffle
+
+    ba_lhs = BitArray.new(35)
+    set_bits[0..3].each { |i| ba_lhs[i] = 1}
+    ba_rhs = BitArray.new(35)
+    # Deliberately overlap a little
+    set_bits[3..-1].each { |i| ba_rhs[i] = 1}
+    ba_expected = BitArray.new(35)
+    set_bits.each { |i| ba_expected[i] = 1}
+
+    assert_equal ba_lhs | ba_rhs, ba_expected
+  end
+
+  def test_union_unequal_sizes
+    ba_lhs = BitArray.new(4)
+    ba_rhs = BitArray.new(5)
+
+    assert_raises ArgumentError do
+      ba_lhs | ba_rhs
+    end
+  end
+
+  def test_union_unequal_reverse_bytes
+    ba_lhs = BitArray.new(4, reverse_byte: true)
+    ba_rhs = BitArray.new(4, reverse_byte: false)
+
+    assert_raises ArgumentError do
+      ba_lhs | ba_rhs
+    end
+  end
 end
 
 class TestBitArrayWhenNonReversedByte < Minitest::Test

test/test_bit_array_file.rb

@@ -0,0 +1,65 @@
+require "minitest/autorun"
+require "tempfile"
+require_relative "../lib/bitarray"
+
+class TestBitArrayFile < Minitest::Test
+  def setup
+    ba = BitArray.new(35)
+    [1, 5, 6, 7, 10, 16, 33].each { |i| ba[i] = 1}
+    @file = Tempfile.new("bit_array_file.dat")
+    ba.dump(@file)
+    @file.rewind
+  end
+
+  def teardown
+    @file.close
+    @file.unlink
+  end
+
+  def test_from_filename
+    baf = BitArrayFile.new(filename: @file.path)
+    for i in 0...35
+      expected = [1, 5, 6, 7, 10, 16, 33].include?(i) ? 1 : 0
+      assert_equal expected, baf[i]
+    end
+  end
+
+  def test_from_io
+    baf = BitArrayFile.new(io: @file)
+    for i in 0...35
+      expected = [1, 5, 6, 7, 10, 16, 33].include?(i) ? 1 : 0
+      assert_equal expected, baf[i]
+    end
+  end
+end
+
+class TestBitArrayFileWhenNonReversedByte < Minitest::Test
+  def setup
+    ba = BitArray.new(35, reverse_byte: false)
+    [1, 5, 6, 7, 10, 16, 33].each { |i| ba[i] = 1}
+    @file = Tempfile.new("bit_array_file.dat")
+    ba.dump(@file)
+    @file.rewind
+  end
+
+  def teardown
+    @file.close
+    @file.unlink
+  end
+
+  def test_from_filename
+    baf = BitArrayFile.new(filename: @file.path)
+    for i in 0...35
+      expected = [1, 5, 6, 7, 10, 16, 33].include?(i) ? 1 : 0
+      assert_equal expected, baf[i]
+    end
+  end
+
+  def test_from_io
+    baf = BitArrayFile.new(io: @file)
+    for i in 0...35
+      expected = [1, 5, 6, 7, 10, 16, 33].include?(i) ? 1 : 0
+      assert_equal expected, baf[i]
+    end
+  end
+end