4 files changed, 90 insertions, 0 deletions
diff --git a/utils/Makefile.am b/utils/Makefile.am
index a22b6727..c0ce3509 100644
--- a/utils/Makefile.am
+++ b/utils/Makefile.am
@@ -27,6 +27,7 @@ libutils_a_SOURCES = \
   citycrc.h \
   corpus_tools.h \
   dict.h \
+  exp_semiring.h \
   fast_sparse_vector.h \
   fdict.h \
   feature_vector.h \
@@ -49,6 +50,7 @@ libutils_a_SOURCES = \
   show.h \
   small_vector.h \
   sparse_vector.h \
+  star.h \
   static_utoa.h \
   stringlib.h \
   tdict.h \
diff --git a/utils/exp_semiring.h b/utils/exp_semiring.h
new file mode 100644
index 00000000..7572ccf5
--- /dev/null
+++ b/utils/exp_semiring.h
@@ -0,0 +1,64 @@
+#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
diff --git a/utils/logval.h b/utils/logval.h
index ec1f6acd..7f1e1024 100644
--- a/utils/logval.h
+++ b/utils/logval.h
@@ -11,6 +11,7 @@
 #include <cassert>
 #include "semiring.h"
 #include "show.h"
+#include "star.h"
 
 //TODO: template for supporting negation or not - most uses are for nonnegative "probs" only; probably some 10-20% speedup available
 template <class T>
@@ -242,4 +243,15 @@ bool operator>=(const LogVal<T>& lhs, const LogVal<T>& rhs) {
 template <class T>
 std::size_t hash_value(const LogVal<T>& x) { return x.hash_impl(); }
 
+template <class T>
+LogVal<T> star(LogVal<T> x) {
+  if (x.is_0()) return x;
+  if (x.v_ >= 0) {
+    x.v_ = std::numeric_limits<T>::infinity();
+  } else {
+    x.v_ = -log1p(-x.as_float());
+  }
+  return x;
+}
+
 #endif
diff --git a/utils/star.h b/utils/star.h
new file mode 100644
index 00000000..3295112c
--- /dev/null
+++ b/utils/star.h
@@ -0,0 +1,12 @@
+#ifndef _STAR_H_
+#define _STAR_H_
+
+template <typename T>
+T star(const T& x) {
+  if (!x) return T();
+  if (x > T(1)) return std::numeric_limits<T>::infinity();
+  if (x < -T(1)) return -std::numeric_limits<T>::infinity();
+  return T(1) / (T(1) - x);
+}
+
+#endif