Avoid Copying Fit Models (#219)

* Modify Prediction to avoid copying the model and fit when used
in a chained call such as:

  model.fit(dataset).predict(features);

* use _t helpers

include/albatross/src/core/declarations.hpp

@@ -23,6 +23,19 @@ using mapbox::util::variant;
 
 namespace albatross {
 
+/*
+ * We frequently inspect for definitions of functions which
+ * must be defined for const references to objects
+ * (so that repeated evaluations return the same thing
+ *  and so the computations are not repeatedly copying.)
+ * This type conversion utility will turn a type `T` into `const T&`
+ */
+template <class T> struct const_ref {
+  typedef std::add_lvalue_reference_t<std::add_const_t<T>> type;
+};
+
+template <typename T> using const_ref_t = typename const_ref<T>::type;
+
 /*
  * Model
  */
@@ -35,6 +48,10 @@ template <typename T> struct PredictTypeIdentity;
 template <typename ModelType, typename FeatureType, typename FitType>
 class Prediction;
 
+template <typename ModelType, typename FeatureType, typename FitType>
+using PredictionReference =
+    Prediction<const_ref_t<ModelType>, FeatureType, const_ref_t<FitType>>;
+
 template <typename ModelType, typename FitType> class FitModel;
 
 template <typename Derived> class Fit {};

include/albatross/src/core/fit_model.hpp

@@ -30,23 +30,32 @@ template <typename ModelType, typename Fit> class FitModel {
   FitModel(const ModelType &model, Fit &&fit)
       : model_(model), fit_(std::move(fit)) {}
 
+  // When FitModel is an lvalue we store a reference to the fit
+  // inside the resulting Prediction class.
+  template <typename PredictFeatureType>
+  const PredictionReference<ModelType, PredictFeatureType, Fit>
+  predict(const std::vector<PredictFeatureType> &features) const & {
+    return PredictionReference<ModelType, PredictFeatureType, Fit>(model_, fit_,
+                                                                   features);
+  }
+
+  // When FitModel is an rvalue the Fit will be a temporary so
+  // we move it into the Prediction class to be stored there.
   template <typename PredictFeatureType>
   Prediction<ModelType, PredictFeatureType, Fit>
-  predict(const std::vector<PredictFeatureType> &features) const {
-    return Prediction<ModelType, PredictFeatureType, Fit>(model_, fit_,
-                                                          features);
+  predict(const std::vector<PredictFeatureType> &features) && {
+    return Prediction<ModelType, PredictFeatureType, Fit>(
+        std::move(model_), std::move(fit_), features);
   }
 
   template <typename PredictFeatureType>
-  Prediction<ModelType, Measurement<PredictFeatureType>, Fit>
-  predict_with_measurement_noise(
+  auto predict_with_measurement_noise(
       const std::vector<PredictFeatureType> &features) const {
     std::vector<Measurement<PredictFeatureType>> measurements;
     for (const auto &f : features) {
       measurements.emplace_back(Measurement<PredictFeatureType>(f));
     }
-    return Prediction<ModelType, Measurement<PredictFeatureType>, Fit>(
-        model_, fit_, measurements);
+    return predict(measurements);
   }
 
   Fit get_fit() const { return fit_; }

include/albatross/src/core/prediction.hpp

@@ -113,14 +113,17 @@ class JointPredictor {
 template <typename ModelType, typename FeatureType, typename FitType>
 class Prediction {
 
+  using PlainModelType = typename std::decay<ModelType>::type;
+  using PlainFitType = typename std::decay<FitType>::type;
+
 public:
-  Prediction(const ModelType &model, const FitType &fit,
+  Prediction(const PlainModelType &model, const PlainFitType &fit,
              const std::vector<FeatureType> &features)
       : model_(model), fit_(fit), features_(features) {}
 
-  Prediction(const ModelType &model, const FitType &fit,
-             std::vector<FeatureType> &&features)
-      : model_(model), fit_(fit), features_(std::move(features)) {}
+  Prediction(PlainModelType &&model, PlainFitType &&fit,
+             const std::vector<FeatureType> &features)
+      : model_(std::move(model)), fit_(std::move(fit)), features_(features) {}
 
   // Mean
   template <typename DummyType = FeatureType,

include/albatross/src/details/traits.hpp

@@ -15,19 +15,6 @@
 
 namespace albatross {
 
-/*
- * We frequently inspect for definitions of functions which
- * must be defined for const references to objects
- * (so that repeated evaluations return the same thing
- *  and so the computations are not repeatedly copying.)
- * This type conversion utility will turn a type `T` into `const T&`
- */
-template <class T> struct const_ref {
-  typedef
-      typename std::add_lvalue_reference<typename std::add_const<T>::type>::type
-          type;
-};
-
 /*
  * This little trick was borrowed from cereal, you can think of it as
  * a function that will always return false ... but that doesn't