Bisect a proc that takes two numeric arguments and raises an exception when the error condition is between them

bisect.rb

def bisect(start_num, end_num, test_proc)
  # calling test_proc(start_num, end_num) should raise an exception on failure
  legit_exceptions = [SystemExit, NoMethodError]
  initial_fail = nil

  begin
    test_proc.call(start_num, end_num)
  rescue Exception => e
    e.reraise if legit_exceptions.include?(e.class)
    initial_fail = true
  end
  raise("test_proc should raise an exception when called with #{start_num} and #{end_num}!") unless initial_fail

  n1  = start_num
  n2  = end_num

  while true
    return start_num if start_num == end_num
    break if n2 == n1 + 1

    mid = (n1 + n2) / 2

    begin
      test_proc.call(mid, n2)
      
      # if we get here, the test passed
      n2 = mid
    rescue Exception => e
      e.reraise if legit_exceptions.include?(e.class)

      # if we get here, the test failed
      n1 = mid
    end
  end

  # if we get here, n2 = n1 + 1 (imperfect bisection) -- test each individually
  begin
    test_proc.call(n1, n1)
  rescue Exception => e
    e.reraise if legit_exceptions.include?(e.class)
    return n1
  end
  
  begin
    test_proc.call(n2, n2)
  rescue Exception => e
    e.reraise if legit_exceptions.include?(e.class)
    return n2
  end

  return "Inconclusive -- got to #{n1}, #{n2}"  
end

# test it -- should return 63
bisect(0, 100, lambda {|a,b| puts "#{a}, #{b}" ;  raise('fail') if a <=63 and b >= 63})