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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
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