ruby function for Levenshtein word distance

levenshtein.rb

def distance(a,b)
  matrix = Array.new(a.size + 1) { Array.new(b.size + 1) }
  (0..a.size).each { |i| matrix[i][0] = i }
  (0..b.size).each { |j| matrix[0][j] = j }
    (1..a.size).each do |i|
      (1..b.size).each do |j|
        if a[i-1] == b[j-1]
          matrix[i][j] = matrix[i-1][j-1]
        else
          matrix[i][j] = [matrix[i-1][j] + 1,matrix[i][j-1] + 1,matrix[i-1][j-1] + 1].min
        end
      end
    end
  matrix[a.size][b.size]
end