Made find_min_single_variable() more stable. Fixes https://github.com/davisking/dlib/issues/1224.

dlib/optimization/optimization_line_search.h

@@ -785,13 +785,16 @@ namespace dlib
             // make sure one side of the bracket isn't super huge compared to the other
             // side.  If it is then contract it.
             const double bracket_ratio = abs(p1-p2)/abs(p2-p3);
-            if ( !( bracket_ratio < 10 && bracket_ratio > 0.1) )
-            {
             // Force p_min to be on a reasonable side.  But only if lagrange_poly_min_extrap()
             // didn't put it on a good side already.
-                if (bracket_ratio > 1 && p_min > p2)
+            if (bracket_ratio >= 10)
+            { 
+                if (p_min > p2)
                     p_min = (p1+p2)/2;
-                else if (p_min < p2)
+            }
+            else if (bracket_ratio <= 0.1) 
+            {
+                if (p_min < p2)
                     p_min = (p2+p3)/2;
             }
 

dlib/test/optimization.cpp

@@ -1202,6 +1202,23 @@ namespace
 
     }
 
+// ----------------------------------------------------------------------------------------
+
+    void test_find_min_single_variable()
+    {
+        auto f = [](double x) { return (x-0.2)*(x-0.2); };
+        double x = 0.8;
+        try
+        {
+            find_min_single_variable(f, x, 0, 1, 1e-9);
+            DLIB_TEST(std::abs(x-0.2) < 1e-7);
+        }
+        catch(optimize_single_variable_failure&)
+        {
+            DLIB_TEST(false);
+        }
+    }
+
 // ----------------------------------------------------------------------------------------
 
     class optimization_tester : public tester
@@ -1223,6 +1240,7 @@ namespace
             test_poly_min_extract_2nd();
             optimization_test();
             test_solve_trust_region_subproblem_bounded();
+            test_find_min_single_variable();
         }
     } a;