/usr/include/ngram/ngram-witten-bell.h is in libngram-dev 1.3.2-3.
This file is owned by root:root, with mode 0o644.
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// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// Copyright 2005-2016 Brian Roark and Google, Inc.
// Witten-Bell derived class for smoothing.
#ifndef NGRAM_NGRAM_WITTEN_BELL_H_
#define NGRAM_NGRAM_WITTEN_BELL_H_
#include <vector>
#include <ngram/ngram-make.h>
namespace ngram {
class NGramWittenBell : public NGramMake<StdArc> {
public:
// Construct NGramMake object, consisting of the FST and some
// information about the states under the assumption that the FST is a model.
// Ownership of the FST is retained by the caller.
explicit NGramWittenBell(StdMutableFst *infst, bool backoff = false,
Label backoff_label = 0, double norm_eps = kNormEps,
bool check_consistency = false,
double parameter = 1.0)
: NGramMake(infst, backoff, backoff_label, norm_eps, check_consistency),
parameter_(parameter) {}
// Smooth model according to 'method' and parameters.
bool MakeNGramModel() { return NGramMake::MakeNGramModel(); }
protected:
// No discount, hence hi order mass is count
double CalculateHiOrderMass(const std::vector<double> &discounts,
double nlog_count) const override {
return nlog_count;
}
// Return Normalization constant for Witten Bell:
// -log c(h) + K |{w:c(hw)>0}|
double CalculateTotalMass(double nlog_count, StateId st) override {
double ngcount = GetFst().NumArcs(st) - 1;
if (GetFst().Final(st).Value() != StdArc::Weight::Zero().Value())
++ngcount; // count </s> if p > 0
// Count mass allocated to lower order estimates: K |{w:c(hw)>0}|
double low_order_mass = -log(ngcount) - log(parameter_);
return NegLogSum(nlog_count, low_order_mass);
}
private:
double parameter_; // Witten-Bell K
};
} // namespace ngram
#endif // NGRAM_NGRAM_WITTEN_BELL_H_
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