Relaxed the requirements on the feature extractor interface and also

added some tests to make sure the code really does work with the
relaxed interface.

dlib/svm/sequence_labeler_abstract.h

@@ -137,7 +137,10 @@ namespace dlib
                     set_feature(55,1);
                   Therefore, the first argument to set_feature is the index of the feature 
                   to be set while the second argument is the value the feature should take.
-                - This function only calls set_feature() once for each feature index.
+                  Additionally, note that calling set_feature() multiple times with the same 
+                  feature index does NOT overwrite the old value, it adds to the previous 
+                  value.  For example, if you call set_feature(55) 3 times then it will
+                  result in feature 55 having a value of 3.
                 - This function only calls set_feature() with feature_index values < num_features()
         !*/
 

dlib/test/sequence_labeler.cpp

@@ -62,6 +62,47 @@ namespace
         }
     };
 
+    class feature_extractor_partial
+    {
+    public:
+        typedef unsigned long sample_type; 
+
+        unsigned long num_features() const
+        {
+            return num_label_states*num_label_states + num_label_states*num_sample_states;
+        }
+
+        unsigned long order() const 
+        { 
+            return 1; 
+        }
+
+        unsigned long num_labels() const 
+        { 
+            return num_label_states; 
+        }
+
+        template <typename feature_setter, typename EXP>
+        void get_features (
+            feature_setter& set_feature,
+            const std::vector<sample_type>& x,
+            const matrix_exp<EXP>& y,
+            unsigned long position
+        ) const
+        {
+            if (y.size() > 1)
+            {
+                set_feature(y(1)*num_label_states + y(0), 0.5);
+                set_feature(y(1)*num_label_states + y(0), 0.5);
+            }
+
+            set_feature(num_label_states*num_label_states +
+                        y(0)*num_sample_states + x[position],0.4);
+            set_feature(num_label_states*num_label_states +
+                        y(0)*num_sample_states + x[position],0.6);
+        }
+    };
+
     bool called_rejct_labeling = false;
     class feature_extractor2
     {
@@ -324,6 +365,53 @@ namespace
         DLIB_TEST(std::abs(accuracy - 0.882) < 0.01);
     }
 
+// ----------------------------------------------------------------------------------------
+
+    void test2()
+    {
+        /*
+            The point of this test is to make sure calling set_feature() multiple
+            times works the way it is supposed to.
+        */
+
+        print_spinner();
+        std::vector<std::vector<unsigned long> > samples;
+        std::vector<std::vector<unsigned long> > labels;
+
+        matrix<double> transition_probabilities(num_label_states, num_label_states);
+        transition_probabilities = 0.05, 0.90, 0.05,
+        0.05, 0.05, 0.90,
+        0.90, 0.05, 0.05;
+
+        matrix<double> emission_probabilities(num_label_states,num_sample_states);
+        emission_probabilities = 0.5, 0.5, 0.0,
+        0.0, 0.5, 0.5,
+        0.5, 0.0, 0.5;
+
+
+        make_dataset(transition_probabilities,emission_probabilities, 
+                     samples, labels, 1000);
+
+        dlog << LINFO << "samples.size(): "<< samples.size();
+
+        structural_sequence_labeling_trainer<feature_extractor> trainer;
+        structural_sequence_labeling_trainer<feature_extractor_partial> trainer_part;
+        trainer.set_c(4);
+        trainer_part.set_c(4);
+        trainer.set_num_threads(4);
+        trainer_part.set_num_threads(4);
+
+
+
+        // Learn to do sequence labeling from the dataset
+        sequence_labeler<feature_extractor> labeler = trainer.train(samples, labels);
+        sequence_labeler<feature_extractor_partial> labeler_part = trainer_part.train(samples, labels);
+
+        // Both feature extractors should be equivalent.
+        DLIB_TEST(length(labeler.get_weights() - labeler_part.get_weights()) < 1e-10);
+
+    }
+
 // ----------------------------------------------------------------------------------------
 
     class sequence_labeler_tester : public tester
@@ -342,6 +430,8 @@ namespace
             DLIB_TEST(called_rejct_labeling == false);
             do_test<feature_extractor2>();
             DLIB_TEST(called_rejct_labeling == true);
+
+            test2();
         }
     } a;