class RuboCop::StringUtil::Jaro

def self.distance(*args)
  new(*args).distance
end

def compute_distance
  common_chars_a, common_chars_b = find_common_characters
  matched_count = common_chars_a.size
  return 0.0 if matched_count.zero?
  transposition_count =
    count_transpositions(common_chars_a, common_chars_b)
  sum = (matched_count / shorter.size.to_f) +
        (matched_count / longer.size.to_f) +
        ((matched_count - transposition_count / 2) / matched_count.to_f)
  sum / 3.0
end

def count_transpositions(common_chars_a, common_chars_b)
  common_chars_a.size.times.count do |index|
    common_chars_a[index] != common_chars_b[index]
  end
end

def distance
  @distance ||= compute_distance
end

def find_common_characters
  common_chars_of_shorter = Array.new(shorter.size)
  common_chars_of_longer = Array.new(longer.size)
  # In Ruby 1.9 String#chars returns Enumerator rather than Array.
  longer_chars = longer.each_char.to_a
  shorter.each_char.with_index do |shorter_char, shorter_index|
    matching_index_range(shorter_index).each do |longer_index|
      longer_char = longer_chars[longer_index]
      next unless shorter_char == longer_char
      common_chars_of_shorter[shorter_index] = shorter_char
      common_chars_of_longer[longer_index] = longer_char
      # Mark the matching character as already used
      longer_chars[longer_index] = nil
      break
    end
  end
  [common_chars_of_shorter, common_chars_of_longer].map(&:compact)
end

def initialize(a, b)
  if a.size < b.size
    @shorter, @longer = a, b
  else
    @shorter, @longer = b, a
  end
end

def matching_index_range(origin)
  min = origin - matching_window
  min = 0 if min < 0
  max = origin + matching_window
  min..max
end

def matching_window
  @matching_window ||= (longer.size / 2).to_i - 1
end