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diff --git a/src/include/fst/signed-log-weight.h b/src/include/fst/signed-log-weight.h
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+++ b/src/include/fst/signed-log-weight.h
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+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// 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 2005-2010 Google, Inc.
+// Author: krr@google.com (Kasturi Rangan Raghavan)
+// \file
+// LogWeight along with sign information that represents the value X in the
+// linear domain as <sign(X), -ln(|X|)>
+// The sign is a TropicalWeight:
+//  positive, TropicalWeight.Value() > 0.0, recommended value 1.0
+//  negative, TropicalWeight.Value() <= 0.0, recommended value -1.0
+
+#ifndef FST_LIB_SIGNED_LOG_WEIGHT_H_
+#define FST_LIB_SIGNED_LOG_WEIGHT_H_
+
+#include <fst/float-weight.h>
+#include <fst/pair-weight.h>
+
+
+namespace fst {
+template <class T>
+class SignedLogWeightTpl
+    : public PairWeight<TropicalWeight, LogWeightTpl<T> > {
+ public:
+  typedef TropicalWeight X1;
+  typedef LogWeightTpl<T> X2;
+  using PairWeight<X1, X2>::Value1;
+  using PairWeight<X1, X2>::Value2;
+
+  using PairWeight<X1, X2>::Reverse;
+  using PairWeight<X1, X2>::Quantize;
+  using PairWeight<X1, X2>::Member;
+
+  typedef SignedLogWeightTpl<T> ReverseWeight;
+
+  SignedLogWeightTpl() : PairWeight<X1, X2>() {}
+
+  SignedLogWeightTpl(const SignedLogWeightTpl<T>& w)
+      : PairWeight<X1, X2> (w) { }
+
+  SignedLogWeightTpl(const PairWeight<X1, X2>& w)
+      : PairWeight<X1, X2> (w) { }
+
+  SignedLogWeightTpl(const X1& x1, const X2& x2)
+      : PairWeight<X1, X2>(x1, x2) { }
+
+  static const SignedLogWeightTpl<T> &Zero() {
+    static const SignedLogWeightTpl<T> zero(X1(1.0), X2::Zero());
+    return zero;
+  }
+
+  static const SignedLogWeightTpl<T> &One() {
+    static const SignedLogWeightTpl<T> one(X1(1.0), X2::One());
+    return one;
+  }
+
+  static const SignedLogWeightTpl<T> &NoWeight() {
+    static const SignedLogWeightTpl<T> no_weight(X1(1.0), X2::NoWeight());
+    return no_weight;
+  }
+
+  static const string &Type() {
+    static const string type = "signed_log_" + X1::Type() + "_" + X2::Type();
+    return type;
+  }
+
+  ProductWeight<X1, X2> Quantize(float delta = kDelta) const {
+    return PairWeight<X1, X2>::Quantize();
+  }
+
+  ReverseWeight Reverse() const {
+    return PairWeight<X1, X2>::Reverse();
+  }
+
+  bool Member() const {
+    return PairWeight<X1, X2>::Member();
+  }
+
+  static uint64 Properties() {
+    // not idempotent nor path
+    return kLeftSemiring | kRightSemiring | kCommutative;
+  }
+
+  size_t Hash() const {
+    size_t h1;
+    if (Value2() == X2::Zero() || Value1().Value() > 0.0)
+      h1 = TropicalWeight(1.0).Hash();
+    else
+      h1 = TropicalWeight(-1.0).Hash();
+    size_t h2 = Value2().Hash();
+    const int lshift = 5;
+    const int rshift = CHAR_BIT * sizeof(size_t) - 5;
+    return h1 << lshift ^ h1 >> rshift ^ h2;
+  }
+};
+
+template <class T>
+inline SignedLogWeightTpl<T> Plus(const SignedLogWeightTpl<T> &w1,
+                                  const SignedLogWeightTpl<T> &w2) {
+  if (!w1.Member() || !w2.Member())
+    return SignedLogWeightTpl<T>::NoWeight();
+  bool s1 = w1.Value1().Value() > 0.0;
+  bool s2 = w2.Value1().Value() > 0.0;
+  T f1 = w1.Value2().Value();
+  T f2 = w2.Value2().Value();
+  if (f1 == FloatLimits<T>::kPosInfinity)
+    return w2;
+  else if (f2 == FloatLimits<T>::kPosInfinity)
+    return w1;
+  else if (f1 == f2) {
+    if (s1 == s2)
+      return SignedLogWeightTpl<T>(w1.Value1(), (f2 - log(2.0F)));
+    else
+      return SignedLogWeightTpl<T>::Zero();
+  } else if (f1 > f2) {
+    if (s1 == s2) {
+      return SignedLogWeightTpl<T>(
+        w1.Value1(), (f2 - log(1.0F + exp(f2 - f1))));
+    } else {
+      return SignedLogWeightTpl<T>(
+        w2.Value1(), (f2 - log(1.0F - exp(f2 - f1))));
+    }
+  } else {
+    if (s2 == s1) {
+      return SignedLogWeightTpl<T>(
+        w2.Value1(), (f1 - log(1.0F + exp(f1 - f2))));
+    } else {
+      return SignedLogWeightTpl<T>(
+        w1.Value1(), (f1 - log(1.0F - exp(f1 - f2))));
+    }
+  }
+}
+
+template <class T>
+inline SignedLogWeightTpl<T> Minus(const SignedLogWeightTpl<T> &w1,
+                                   const SignedLogWeightTpl<T> &w2) {
+  SignedLogWeightTpl<T> minus_w2(-w2.Value1().Value(), w2.Value2());
+  return Plus(w1, minus_w2);
+}
+
+template <class T>
+inline SignedLogWeightTpl<T> Times(const SignedLogWeightTpl<T> &w1,
+                                   const SignedLogWeightTpl<T> &w2) {
+  if (!w1.Member() || !w2.Member())
+    return SignedLogWeightTpl<T>::NoWeight();
+  bool s1 = w1.Value1().Value() > 0.0;
+  bool s2 = w2.Value1().Value() > 0.0;
+  T f1 = w1.Value2().Value();
+  T f2 = w2.Value2().Value();
+  if (s1 == s2)
+    return SignedLogWeightTpl<T>(TropicalWeight(1.0), (f1 + f2));
+  else
+    return SignedLogWeightTpl<T>(TropicalWeight(-1.0), (f1 + f2));
+}
+
+template <class T>
+inline SignedLogWeightTpl<T> Divide(const SignedLogWeightTpl<T> &w1,
+                                    const SignedLogWeightTpl<T> &w2,
+                                    DivideType typ = DIVIDE_ANY) {
+  if (!w1.Member() || !w2.Member())
+    return SignedLogWeightTpl<T>::NoWeight();
+  bool s1 = w1.Value1().Value() > 0.0;
+  bool s2 = w2.Value1().Value() > 0.0;
+  T f1 = w1.Value2().Value();
+  T f2 = w2.Value2().Value();
+  if (f2 == FloatLimits<T>::kPosInfinity)
+    return SignedLogWeightTpl<T>(TropicalWeight(1.0),
+      FloatLimits<T>::kNumberBad);
+  else if (f1 == FloatLimits<T>::kPosInfinity)
+    return SignedLogWeightTpl<T>(TropicalWeight(1.0),
+      FloatLimits<T>::kPosInfinity);
+  else if (s1 == s2)
+    return SignedLogWeightTpl<T>(TropicalWeight(1.0), (f1 - f2));
+  else
+    return SignedLogWeightTpl<T>(TropicalWeight(-1.0), (f1 - f2));
+}
+
+template <class T>
+inline bool ApproxEqual(const SignedLogWeightTpl<T> &w1,
+                        const SignedLogWeightTpl<T> &w2,
+                        float delta = kDelta) {
+  bool s1 = w1.Value1().Value() > 0.0;
+  bool s2 = w2.Value1().Value() > 0.0;
+  if (s1 == s2) {
+    return ApproxEqual(w1.Value2(), w2.Value2(), delta);
+  } else {
+    return w1.Value2() == LogWeightTpl<T>::Zero()
+        && w2.Value2() == LogWeightTpl<T>::Zero();
+  }
+}
+
+template <class T>
+inline bool operator==(const SignedLogWeightTpl<T> &w1,
+                       const SignedLogWeightTpl<T> &w2) {
+  bool s1 = w1.Value1().Value() > 0.0;
+  bool s2 = w2.Value1().Value() > 0.0;
+  if (s1 == s2)
+    return w1.Value2() == w2.Value2();
+  else
+    return (w1.Value2() == LogWeightTpl<T>::Zero()) &&
+           (w2.Value2() == LogWeightTpl<T>::Zero());
+}
+
+
+// Single-precision signed-log weight
+typedef SignedLogWeightTpl<float> SignedLogWeight;
+// Double-precision signed-log weight
+typedef SignedLogWeightTpl<double> SignedLog64Weight;
+
+//
+// WEIGHT CONVERTER SPECIALIZATIONS.
+//
+
+template <class W1, class W2>
+bool SignedLogConvertCheck(W1 w) {
+  if (w.Value1().Value() < 0.0) {
+    FSTERROR() << "WeightConvert: can't convert weight from \""
+               << W1::Type() << "\" to \"" << W2::Type();
+    return false;
+  }
+  return true;
+}
+
+// Convert to tropical
+template <>
+struct WeightConvert<SignedLogWeight, TropicalWeight> {
+  TropicalWeight operator()(SignedLogWeight w) const {
+    if (!SignedLogConvertCheck<SignedLogWeight, TropicalWeight>(w))
+      return TropicalWeight::NoWeight();
+    return w.Value2().Value();
+  }
+};
+
+template <>
+struct WeightConvert<SignedLog64Weight, TropicalWeight> {
+  TropicalWeight operator()(SignedLog64Weight w) const {
+    if (!SignedLogConvertCheck<SignedLog64Weight, TropicalWeight>(w))
+      return TropicalWeight::NoWeight();
+    return w.Value2().Value();
+  }
+};
+
+// Convert to log
+template <>
+struct WeightConvert<SignedLogWeight, LogWeight> {
+  LogWeight operator()(SignedLogWeight w) const {
+    if (!SignedLogConvertCheck<SignedLogWeight, LogWeight>(w))
+      return LogWeight::NoWeight();
+    return w.Value2().Value();
+  }
+};
+
+template <>
+struct WeightConvert<SignedLog64Weight, LogWeight> {
+  LogWeight operator()(SignedLog64Weight w) const {
+    if (!SignedLogConvertCheck<SignedLog64Weight, LogWeight>(w))
+      return LogWeight::NoWeight();
+    return w.Value2().Value();
+  }
+};
+
+// Convert to log64
+template <>
+struct WeightConvert<SignedLogWeight, Log64Weight> {
+  Log64Weight operator()(SignedLogWeight w) const {
+    if (!SignedLogConvertCheck<SignedLogWeight, Log64Weight>(w))
+      return Log64Weight::NoWeight();
+    return w.Value2().Value();
+  }
+};
+
+template <>
+struct WeightConvert<SignedLog64Weight, Log64Weight> {
+  Log64Weight operator()(SignedLog64Weight w) const {
+    if (!SignedLogConvertCheck<SignedLog64Weight, Log64Weight>(w))
+      return Log64Weight::NoWeight();
+    return w.Value2().Value();
+  }
+};
+
+// Convert to signed log
+template <>
+struct WeightConvert<TropicalWeight, SignedLogWeight> {
+  SignedLogWeight operator()(TropicalWeight w) const {
+    TropicalWeight x1 = 1.0;
+    LogWeight x2 = w.Value();
+    return SignedLogWeight(x1, x2);
+  }
+};
+
+template <>
+struct WeightConvert<LogWeight, SignedLogWeight> {
+  SignedLogWeight operator()(LogWeight w) const {
+    TropicalWeight x1 = 1.0;
+    LogWeight x2 = w.Value();
+    return SignedLogWeight(x1, x2);
+  }
+};
+
+template <>
+struct WeightConvert<Log64Weight, SignedLogWeight> {
+  SignedLogWeight operator()(Log64Weight w) const {
+    TropicalWeight x1 = 1.0;
+    LogWeight x2 = w.Value();
+    return SignedLogWeight(x1, x2);
+  }
+};
+
+template <>
+struct WeightConvert<SignedLog64Weight, SignedLogWeight> {
+  SignedLogWeight operator()(SignedLog64Weight w) const {
+    TropicalWeight x1 = w.Value1();
+    LogWeight x2 = w.Value2().Value();
+    return SignedLogWeight(x1, x2);
+  }
+};
+
+// Convert to signed log64
+template <>
+struct WeightConvert<TropicalWeight, SignedLog64Weight> {
+  SignedLog64Weight operator()(TropicalWeight w) const {
+    TropicalWeight x1 = 1.0;
+    Log64Weight x2 = w.Value();
+    return SignedLog64Weight(x1, x2);
+  }
+};
+
+template <>
+struct WeightConvert<LogWeight, SignedLog64Weight> {
+  SignedLog64Weight operator()(LogWeight w) const {
+    TropicalWeight x1 = 1.0;
+    Log64Weight x2 = w.Value();
+    return SignedLog64Weight(x1, x2);
+  }
+};
+
+template <>
+struct WeightConvert<Log64Weight, SignedLog64Weight> {
+  SignedLog64Weight operator()(Log64Weight w) const {
+    TropicalWeight x1 = 1.0;
+    Log64Weight x2 = w.Value();
+    return SignedLog64Weight(x1, x2);
+  }
+};
+
+template <>
+struct WeightConvert<SignedLogWeight, SignedLog64Weight> {
+  SignedLog64Weight operator()(SignedLogWeight w) const {
+    TropicalWeight x1 = w.Value1();
+    Log64Weight x2 = w.Value2().Value();
+    return SignedLog64Weight(x1, x2);
+  }
+};
+
+}  // namespace fst
+
+#endif  // FST_LIB_SIGNED_LOG_WEIGHT_H_