Longest non-repeating substring

longest_nonrepeating_substring.rb

def longest_nonrepeating_substring(str)
    max_len = 0
    look_up = {}
    len = 0
    last_repeating_index = 0 # indicates beginning of unique string
    (0...str.size).each do |index|
      if (p = look_up[str[index]]) # repeat char found at index
        if p > last_repeating_index 
          len = index - p
          last_repeating_index = p
        else
          len = index - last_repeating_index
        end
      else
        len += 1
      end
      look_up[str[index]] = index
      max_len = [max_len, len].max
    end
    max_len
end