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#ifndef _EXP_SEMIRING_H_
#define _EXP_SEMIRING_H_
#include <iostream>
// this file implements the first-order expectation semiring described
// in Li & Eisner (EMNLP 2009)
// requirements:
// RType * RType ==> RType
// PType * PType ==> PType
// RType * PType ==> RType
// good examples:
// PType scalar, RType vector
// BAD examples:
// PType vector, RType scalar
template <typename PType, typename RType>
struct PRPair {
PRPair() : p(), r() {}
// Inside algorithm requires that T(0) and T(1)
// return the 0 and 1 values of the semiring
explicit PRPair(double x) : p(x), r() {}
PRPair(const PType& p, const RType& r) : p(p), r(r) {}
PRPair& operator+=(const PRPair& o) {
p += o.p;
r += o.r;
return *this;
}
PRPair& operator*=(const PRPair& o) {
r = (o.r * p) + (o.p * r);
p *= o.p;
return *this;
}
PType p;
RType r;
};
template <typename P, typename R>
std::ostream& operator<<(std::ostream& o, const PRPair<P,R>& x) {
return o << '<' << x.p << ", " << x.r << '>';
}
template <typename P, typename R>
const PRPair<P,R> operator+(const PRPair<P,R>& a, const PRPair<P,R>& b) {
PRPair<P,R> result = a;
result += b;
return result;
}
template <typename P, typename R>
const PRPair<P,R> operator*(const PRPair<P,R>& a, const PRPair<P,R>& b) {
PRPair<P,R> result = a;
result *= b;
return result;
}
template <typename P, typename PWeightFunction, typename R, typename RWeightFunction>
struct PRWeightFunction {
explicit PRWeightFunction(const PWeightFunction& pwf = PWeightFunction(),
const RWeightFunction& rwf = RWeightFunction()) :
pweight(pwf), rweight(rwf) {}
PRPair<P,R> operator()(const Hypergraph::Edge& e) const {
const P p = pweight(e);
const R r = rweight(e);
return PRPair<P,R>(p, r * p);
}
const PWeightFunction pweight;
const RWeightFunction rweight;
};
#endif
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