1 files changed, 110 insertions, 0 deletions
diff --git a/lib/nlp_ruby/bleu.rb b/lib/nlp_ruby/bleu.rb
new file mode 100644
index 0000000..42be45e
--- /dev/null
+++ b/lib/nlp_ruby/bleu.rb
@@ -0,0 +1,110 @@
+module BLEU
+
+
+class BLEU::NgramCounts
+  attr_accessor :sum, :clipped, :ref_len, :hyp_len, :n
+
+  def initialize(n)
+    @n = 0
+    @sum = []
+    @clipped = []
+    @ref_len = 0.0
+    @hyp_len = 0.0
+    grow(n)
+  end
+
+  def grow(n)
+    (n-@n).times {
+      @sum << 0.0
+      @clipped << 0.0
+    }
+    @n = n
+  end
+
+  def plus_eq(other)
+    if other.n > @n then grow(other.n) end
+    0.upto(other.n-1) { |m|
+      @sum[m] += other.sum[m]
+      @clipped[m] += other.clipped[m]
+    }
+    @ref_len += other.ref_len
+    @hyp_len += other.hyp_len
+  end
+
+  def to_s
+    return "n=#{n} sum=#{sum} clipped=#{clipped} ref_len=#{ref_len} hyp_len=#{hyp_len}"
+  end
+end
+
+class BLEU::Ngrams
+  def initialize
+    @h_ = {}
+    @h_.default = 0
+  end
+
+  def add(k)
+    if k.class == Array then k = k.join ' ' end
+    @h_[k] += 1
+  end
+
+  def get_count(k)
+    if k.class == Array then k = k.join ' ' end
+    return @h_[k]
+  end
+
+  def each
+    @h_.each_pair { |k,v|
+      yield k.split, v
+    }
+  end
+
+  def to_s
+    @h_.to_s
+  end
+end
+
+def BLEU::get_counts hypothesis, reference, n, times=1
+  p = NgramCounts.new n
+  r = Ngrams.new
+  ngrams(reference, n) { |ng| r.add ng }
+  h = Ngrams.new
+  ngrams(hypothesis, n) { |ng| h.add ng }
+  h.each { |ng,count|
+    sz = ng.size-1
+    p.sum[sz] += count * times
+    p.clipped[sz] += [r.get_count(ng), count].min * times
+  }
+  p.ref_len = tokenize(reference.strip).size * times
+  p.hyp_len = tokenize(hypothesis.strip).size * times
+  return p
+end
+
+def BLEU::brevity_penalty(c, r)
+  if c > r then return 1.0 end
+  return Math.exp(1-r/c)
+end
+
+def BLEU::bleu(counts, n, debug=false)
+  corpus_stats = NgramCounts.new n
+  counts.each { |i| corpus_stats.plus_eq i }
+  sum = 0.0
+  w = 1.0/n
+  0.upto(n-1) { |m|
+    STDERR.write "#{m+1} #{corpus_stats.clipped[m]} / #{corpus_stats.sum[m]}\n" if debug
+    return 0.0 if corpus_stats.clipped[m] == 0 or corpus_stats.sum == 0
+    sum += w * Math.log(corpus_stats.clipped[m] / corpus_stats.sum[m])
+  }
+  if debug
+    STDERR.write "BP #{brevity_penalty(corpus_stats.hyp_len, corpus_stats.ref_len)}\n"
+    STDERR.write "sum #{Math.exp(sum)}\n"
+  end
+  return brevity_penalty(corpus_stats.hyp_len, corpus_stats.ref_len) * Math.exp(sum)
+end
+
+def BLEU::hbleu counts, n, debug=false
+  (100*bleu(counts, n, debug)).round(3)
+end
+
+
+end
+