multiple-string.rb

#!/usr/bin/env ruby

class String

def *(str)
  maxshift = self.length + str.length - 2

  results = Array.new(maxshift+1, 0)

  self.split(//).map(&:to_i).each_with_index do |i, at1|
    str.split(//).map(&:to_i).each_with_index do |j, at2|
      results[maxshift - at1 - at2] += i * j
    end
  end

  results.each_with_index do |i, shift|
    if i > 9
      results[shift+1] = results[shift+1].to_i + (i / 10)
      results[shift] = i % 10
    end
  end

  results.reverse.join
end

end

for i in 1..1000000
  a = (i.to_s * i.to_s).to_i
  b = i * i
  unless a == b
    puts a
    puts b
  end
end