range_intersection.rb

#!/usr/bin/env ruby

# From my blog post at http://www.postal-code.com/binarycode/2009/06/06/better-range-intersection-in-ruby/

class Range
  def intersection(other)
    raise ArgumentError, 'value must be a Range' unless other.kind_of?(Range)

    my_min, my_max = first, exclude_end? ? max : last
    other_min, other_max = other.first, other.exclude_end? ? other.max : other.last

    new_min = self === other_min ? other_min : other === my_min ? my_min : nil
    new_max = self === other_max ? other_max : other === my_max ? my_max : nil

    new_min && new_max ? new_min..new_max : nil
  end

  alias_method :&, :intersection
end


if __FILE__ == $0
  def do_test( range, other, expected )
    result = range.intersection(other)
    result_status = ( result == expected ) ? "passed" : "failed"
    puts "#{range.inspect} #{other.inspect} result #{result_status}: #{result.inspect} (#{expected.inspect})"
  end
  
  # Test an inclusive range
  range = 5..10

  tests = {
    1..4   => nil,     # before
    11..15 => nil,     # after
    1..6   => 5..6,    # overlap_begin
    9..15  => 9..10,   # overlap_end
    1..5   => 5..5,    # overlap_begin_edge
    10..15 => 10..10,  # overlap_end_edge
    5..10  => 5..10,   # overlap_all
    6..9   => 6..9,    # overlap_inner

    1...5  => nil,     # before             (exclusive range)
    1...7  => 5..6,    # overlap_begin      (exclusive range)
    1...6  => 5..5,    # overlap_begin_edge (exclusive range)
    5...11 => 5..10,   # overlap_all        (exclusive range)
    6...10 => 6..9,    # overlap_inner      (exclusive range)
  }

  tests.each do |other, expected|
    do_test( range, other, expected )
  end
  
  
  # Test an exclusive range
  # This covers a bug found by Montgomery Kosma in the original code
  range = 1...10
  
  tests = {
    5..15 => 5..9       #overlap_end
  }
  
  tests.each do |other, expected|
    do_test( range, other, expected )
  end
end