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diff --git a/utils/exp_semiring.h b/utils/exp_semiring.h
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+#ifndef _EXP_SEMIRING_H_
+#define _EXP_SEMIRING_H_
+
+#include <iostream>
+#include "star.h"
+
+// 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 <class PType, class 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 <class P, class R>
+std::ostream& operator<<(std::ostream& o, const PRPair<P,R>& x) {
+ return o << '<' << x.p << ", " << x.r << '>';
+}
+
+template <class P, class 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 <class P, class 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 <class P, class R>
+const PRPair<P,R> star(const PRPair<P,R>& x) {
+ const P pstar = star(x.p);
+ return PRPair<P,R>(pstar, pstar * x.r * pstar);
+}
+
+#endif