comment on router lookahead 6d arrqy

Author: amin1377

vpr/src/route/router_lookahead_map.cpp

@@ -1006,6 +1006,8 @@ static Cost_Entry get_nearby_cost_entry(int from_layer_num, int x, int y, int to
     if (std::isnan(copy_entry.delay) && std::isnan(copy_entry.congestion)) {
         if (copy_x == 0 && copy_y == 0) {
             copy_entry = Cost_Entry(0., 0.); //(0, 0) entry is invalid so set zero to terminate recursion
+            // set zero if the source and sink nodes are on the same layer. If they are not, it means that there is no connection from the source node to
+            // the other layer. This means that the connection should be set to a very large number
             if (from_layer_num == to_layer_num) {
                 copy_entry = Cost_Entry(0., 0.);
             } else {

vpr/src/route/router_lookahead_map.h

@@ -57,6 +57,8 @@ class Cost_Entry {
  * in the x/y direction */
 typedef vtr::NdMatrix<Cost_Entry, 6> t_wire_cost_map; //[0..num_layers][0..1][[0..num_seg_types-1][0..num_layers][0..device_ctx.grid.width()-1][0..device_ctx.grid.height()-1]
                                                       //[0..1] entry distinguish between CHANX/CHANY start nodes respectively
+                                                      // The first index is the layer number that the node under consideration is on, and the forth index
+                                                      // is the layer number that the target node is on.
 
 void read_router_lookahead(const std::string& file);
 void write_router_lookahead(const std::string& file);