Find the longest common prefix of a bunch of arrays, or a bunch of strings

prefix.rb

def common_array_prefix(*arrays)
  arrays.reduce do |a, b|
    index = a.zip(b).index { |x, y| x != y }
    a[0...index]
  end
end

def common_string_prefix(*strings)
  common_array_prefix(*strings.map(&:chars)).join("")
end