Implement board value placement heuristics

board.c

@@ -209,3 +209,77 @@ board_is_valid (struct board *board)
         return false;
   return true;
 }
+
+
+void
+board_update_marks (
+  struct board *board,
+  board_pos x,
+  board_pos y
+)
+{
+  if (is_in_bounds (x, y))
+  {
+    struct board_element *elem = BOARD_ELEM (board, x, y);
+
+    /* Mark all values as impossible */
+    elem->potential = 0;
+
+    /* Check x-axis */
+    for (board_pos check_x = 0; check_x < 9; ++check_x)
+      if (check_x != x && board_has_value (board, check_x, y))
+        elem->potential |= 1 << BOARD_ELEM (board, check_x, y)->value;
+
+    /* Check y-axis */
+    for (board_pos check_y = 0; check_y < 9; ++check_y)
+      if (check_y != y && board_has_value (board, x, check_y))
+        elem->potential |= 1 << BOARD_ELEM (board, x, check_y)->value;
+
+    /* Invert matches */
+    elem->potential ^= 0x1FF;
+  }
+  else ERROR("Invalid parameters to function board_update_marks()");
+}
+
+
+void
+board_update_all_marks (struct board *board)
+{
+  for (board_pos y = 0; y < 9; ++y)
+    for (board_pos x = 0; x < 9; ++x)
+      if (! board_has_value (board, x, y))
+        board_update_marks (board, x, y);
+}
+
+
+bool
+board_place (
+  struct board *board,
+  board_pos x,
+  board_pos y,
+  element_value value
+)
+{
+  if (is_in_bounds (x, y) && is_valid_value (value))
+  {
+    if (board_can_place_value (board, x, y, value))
+    {
+      /* Unmark x-axis */
+      for (board_pos unmark_x = 0; unmark_x < 9; ++unmark_x)
+        if (unmark_x != x && ! board_has_value (board, unmark_x, y))
+          board_unmark (board, unmark_x, y, value);
+
+      /* Unmark y-axis */
+      for (board_pos unmark_y = 0; unmark_y < 9; ++unmark_y)
+        if (unmark_y != y && ! board_has_value (board, x, unmark_y))
+          board_unmark (board, x, unmark_y, value);
+
+      /* Set value */
+      board_set (board, x, y, value); 
+      
+      return true;
+    }
+    else return false;
+  }
+  else ERROR("Invalid parameters to function board_place()");
+}

board.h

@@ -142,3 +142,38 @@ board_can_place_value (
  */
 bool
 board_is_valid (struct board *board);
+
+
+/**
+ * Check row and column of given position and mark potential values that could
+ * be set at that position and still leave the board in a valid state
+ */
+void
+board_update_marks (
+  struct board *board,
+  board_pos x,
+  board_pos y
+);
+
+
+/**
+ * Refreshes marks of all board elements without a value
+ */
+void
+board_update_all_marks (struct board *board);
+
+
+/**
+ * Attempt to set value at the given position
+ * If value can be placed, this updates all elements without a value on the
+ * same row or column that do not have a value so that their markers reflect
+ * the addition of this value and returns true
+ * If value cannot be placed, just return false
+ */
+bool
+board_place (
+  struct board *board,
+  board_pos x,
+  board_pos y,
+  element_value value
+);