count_intersection.rb

#!/usr/bin/env ruby

# Given 2 sorted arrays with unique values and arbitrary size, count the
# intersection values between them

array_a = [1, 2, 4, 5, 7, 8]
array_b = [0, 1, 3, 4, 6, 7, 9]
index_a = index_b = count = 0

loop do
  break if array_a[index_a].nil? || array_b[index_b].nil?

  check = array_a[index_a] <=> array_b[index_b]

  count   += 1 if check.zero?
  index_a += 1 if check <= 0
  index_b += 1 if check >= 0
end

puts count