Update only cells affected by change

+ some more minor optimizations

Makefile

@@ -1,5 +1,5 @@
 CXX = g++
-CXXFLAGS = -Wall
+CXXFLAGS = -Wall -O3
 
 q: main.o
 	$(CXX) $(CXXFLAGS) -o q main.o

main.cpp

@@ -15,15 +15,15 @@ class Board {
 public:
     int values[BOARD_SIZE];
 
-    int coordinates_to_id(int x, int y) {
+    inline int coordinates_to_id(int x, int y) {
         return y * BOARD_DIMENSION + x;
     }
 
-    int get(int x, int y) {
+    inline int get(int x, int y) {
         return values[coordinates_to_id(x, y)];
     }
 
-    void set(int x, int y, int value) {
+    inline void set(int x, int y, int value) {
         values[coordinates_to_id(x, y)] = value;
     }
 };
@@ -39,33 +39,31 @@ public:
 
     Cell(Board *board, int x, int y) : board(board), x(x), y(y), value(board->get(x, y)) {}
 
-    void set_value(int new_value) {
+    inline void set_value(int new_value) {
         value = new_value;
         update_possible_values();
     }
 
     void update_possible_values() {
         // Start off with all possible numbers
-        std::list<int> possibilities{1, 2, 3, 4, 5, 6, 7, 8 ,9};
+        possible_values = {1, 2, 3, 4, 5, 6, 7, 8 ,9};
 
         // Remove all from this row
         for (int dx = 0; dx < BOARD_DIMENSION; dx++) {
-            possibilities.remove(board->get(dx, y));
+            possible_values.remove(board->get(dx, y));
         }
 
         // Remove all from this column
         for (int dy = 0; dy < BOARD_DIMENSION; dy++) {
-            possibilities.remove(board->get(x, dy));
+            possible_values.remove(board->get(x, dy));
         }
 
         // Remove all from this quadrant
         for (int dy = 0; dy < 3; dy++) {
             for (int dx = 0; dx < 3; dx++) {
-                possibilities.remove(board->get((x - x % 3) + dx, (y - y % 3) + dy));
+                possible_values.remove(board->get((x - x % 3) + dx, (y - y % 3) + dy));
             }
         }
-
-        possible_values = possibilities;
     }
 
     bool operator<(const Cell &other) {
@@ -117,6 +115,25 @@ public:
         std::sort_heap(heap.begin(), heap.end());
     }
 
+    // Update the possible values of all cells in the heap which are affected by
+    //  the value of the cell at the given position
+    void update_affected_cells(int x, int y) {
+        int x_quadrant = x - x % 3;
+        int y_quadrant = y - y % 3;
+        
+        for (Cell &cell : heap) {
+            if (cell.x == x // Column
+                || cell.y == y // Row
+                || (cell.x - cell.x % 3 == x_quadrant // Quadrant
+                 && cell.y - cell.y % 3 == y_quadrant)) {
+                
+                cell.update_possible_values();
+            }
+        }
+
+        std::sort_heap(heap.begin(), heap.end());
+    }
+
     // Returns the best cell in the heap (the one with the most obvious choice)
     Cell get_next_best_empty_cell() {
 		std::pop_heap(heap.begin(), heap.end());
@@ -164,7 +181,7 @@ public:
         for (int value : cell.possible_values) {
             // Apply this value
             board->set(cell.x, cell.y, value);
-            update_all_cells();  // TODO: We'd only need to re-rate the neighbours
+            update_affected_cells(cell.x, cell.y);
 
             // Recurse; if we find a solution, we're done and can return true
             if (solve_recurse()) {
@@ -173,7 +190,7 @@ public:
 
             // Seems like this was a dead end -- reset this move
             board->set(cell.x, cell.y, 0);
-            update_all_cells();  // TODO: We could save and reuse the rating from before
+            update_affected_cells(cell.x, cell.y);
         }
 
         // Put the cell back into the heap -- it couldn't be used for any valid solution, so we'll need it somewhere else later