From d7c003d1901bd2f4789af3aff9658984bb25d87e Mon Sep 17 00:00:00 2001
From: Davis King <davis@dlib.net>
Date: Sat, 3 Sep 2016 07:14:07 -0400
Subject: [PATCH] Added visit_layers_backwards(),
 visit_layers_backwards_range(), and visit_layers_range().

---
 dlib/dnn/core.h          | 77 +++++++++++++++++++++++++++++++++++++
 dlib/dnn/core_abstract.h | 83 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 160 insertions(+)

diff --git a/dlib/dnn/core.h b/dlib/dnn/core.h
index e231878a5..6cf6cee64 100644
--- a/dlib/dnn/core.h
+++ b/dlib/dnn/core.h
@@ -3337,6 +3337,39 @@ namespace dlib
             }
         };
 
+        template <size_t i, size_t num>
+        struct vl_loop_backwards
+        {
+            template <
+                typename net_type,
+                typename visitor
+                >
+            static void visit(
+                net_type& net,
+                visitor&& v
+            )
+            {
+                vl_loop<i+1, num>::visit(net,v);
+                v(i, layer<i>(net));
+            }
+        };
+
+        template <size_t num>
+        struct vl_loop_backwards<num,num>
+        {
+            template <
+                typename net_type,
+                typename visitor
+                >
+            static void visit(
+                net_type&,
+                visitor&& 
+            )
+            {
+                // Base case of recursion.  Don't do anything.
+            }
+        };
+
     }
 
     template <
@@ -3351,6 +3384,50 @@ namespace dlib
         impl::vl_loop<0, net_type::num_layers>::visit(net, v);
     }
 
+    template <
+        typename net_type,
+        typename visitor
+        >
+    void visit_layers_backwards(
+        net_type& net,
+        visitor v
+    )
+    {
+        impl::vl_loop_backwards<0, net_type::num_layers>::visit(net, v);
+    }
+
+    template <
+        size_t begin,
+        size_t end,
+        typename net_type,
+        typename visitor
+        >
+    void visit_layers_range(
+        net_type& net,
+        visitor v
+    )
+    {
+        static_assert(begin <= end, "Invalid range");
+        static_assert(end <= net_type::num_layers, "Invalid range");
+        impl::vl_loop<begin,end>::visit(net, v);
+    }
+
+    template <
+        size_t begin,
+        size_t end,
+        typename net_type,
+        typename visitor
+        >
+    void visit_layers_backwards_range(
+        net_type& net,
+        visitor v
+    )
+    {
+        static_assert(begin <= end, "Invalid range");
+        static_assert(end <= net_type::num_layers, "Invalid range");
+        impl::vl_loop_backwards<begin,end>::visit(net, v);
+    }
+
 // ----------------------------------------------------------------------------------------
 
 }
diff --git a/dlib/dnn/core_abstract.h b/dlib/dnn/core_abstract.h
index 759079817..82868a7e4 100644
--- a/dlib/dnn/core_abstract.h
+++ b/dlib/dnn/core_abstract.h
@@ -1491,6 +1491,89 @@ namespace dlib
                     v(i, layer<i>(net));
     !*/
 
+    template <
+        typename net_type,
+        typename visitor
+        >
+    void visit_layers_backwards(
+        net_type& net,
+        visitor v
+    );
+    /*!
+        requires
+            - net_type is an object of type add_layer, add_loss_layer, add_skip_layer, or
+              add_tag_layer.
+            - v is a function object with a signature equivalent to: 
+                v(size_t idx, any_net_type& t)
+              That is, it must take a size_t and then any of the network types such as
+              add_layer, add_loss_layer, etc.
+        ensures
+            - Loops over all the layers in net and calls v() on them.  The loop happens in
+              the reverse order of visit_layers().  To be specific, this function
+              essentially performs the following:
+
+                for (size_t i = net_type::num_layers; i != 0; --i)
+                    v(i-1, layer<i-1>(net));
+    !*/
+
+// ----------------------------------------------------------------------------------------
+
+    template <
+        size_t begin,
+        size_t end,
+        typename net_type,
+        typename visitor
+        >
+    void visit_layers_range(
+        net_type& net,
+        visitor v
+    );
+    /*!
+        requires
+            - net_type is an object of type add_layer, add_loss_layer, add_skip_layer, or
+              add_tag_layer.
+            - v is a function object with a signature equivalent to: 
+                v(size_t idx, any_net_type& t)
+              That is, it must take a size_t and then any of the network types such as
+              add_layer, add_loss_layer, etc.
+            - begin <= end <= net_type::num_layers
+        ensures
+            - Loops over the layers in the range [begin,end) in net and calls v() on them.
+              The loop happens in the reverse order of visit_layers().  To be specific,
+              this function essentially performs the following:
+
+                for (size_t i = begin; i < end; ++i)
+                    v(i, layer<i>(net));
+    !*/
+
+    template <
+        size_t begin,
+        size_t end,
+        typename net_type,
+        typename visitor
+        >
+    void visit_layers_backwards_range(
+        net_type& net,
+        visitor v
+    );
+    /*!
+        requires
+            - net_type is an object of type add_layer, add_loss_layer, add_skip_layer, or
+              add_tag_layer.
+            - v is a function object with a signature equivalent to: 
+                v(size_t idx, any_net_type& t)
+              That is, it must take a size_t and then any of the network types such as
+              add_layer, add_loss_layer, etc.
+            - begin <= end <= net_type::num_layers
+        ensures
+            - Loops over the layers in the range [begin,end) in net and calls v() on them.
+              The loop happens in the reverse order of visit_layers_range().  To be specific,
+              this function essentially performs the following:
+
+                for (size_t i = end; i != begin; --i)
+                    v(i-1, layer<i-1>(net));
+    !*/
+
 // ----------------------------------------------------------------------------------------
 
     struct layer_test_results