#!/usr/bin/env ruby

require 'trollop'


def ngrams_it(s, n, fix=false)
  a = s.strip.split
  a.each_with_index { |tok, i|
    tok.strip!
    0.upto([n-1, a.size-i-1].min) { |m|
      yield a[i..i+m] if !(fix||(a[i..i+m].size>n))
    }
  }
end

def brevity_penalty hypothesis, reference
  a = hypothesis.split; b = reference.split
  return 1.0 if a.size>b.size
  return Math.exp(1.0 - ((b.size.to_f+1)/a.size));
end

def per_sentence_bleu hypothesis, reference, n=4
  h_ng = {}; r_ng = {}
  (1).upto(n) {|i| h_ng[i] = []; r_ng[i] = []}
  ngrams_it(hypothesis, n) {|i| h_ng[i.size] << i}
  ngrams_it(reference, n) {|i| r_ng[i.size] << i}
  m = [n, reference.split.size].min
  weight = 1.0/m
  add = 0.0
  sum = 0
  (1).upto(m) { |i|
    counts_clipped = 0
    counts_sum = h_ng[i].size
    h_ng[i].uniq.each {|j| counts_clipped += r_ng[i].count(j)}
    add = 1.0 if i >= 2
    sum += weight * Math.log((counts_clipped + add)/(counts_sum + add));
  }
  return brevity_penalty(hypothesis, reference) * Math.exp(sum)
end

def main
  opts = Trollop::options do
    opt :input, "input", :type => :string, :default => '-'
    opt :references, "references", :type => :string, :required => true
  end
 
  refs = File.new(opts[:references], 'r').readlines.map{|i|i.strip} 
  i = -1
  if opts[:input] == '-'
    input = STDIN 
  else
    input = File.new opts[:input], 'r'
  end
  while line = input.gets
    i += 1
    if line.strip == ''
      puts 0.0
      next
    end
    puts per_sentence_bleu line.strip, refs[i]
  end
  input.close if opts[:input]!='-'
end


main