Add checking if block can be placed

src/block_data.c

@@ -70,3 +70,14 @@ int block_connects(block_id block)
             return 0;
     }
 }
+
+int block_can_be_placed_on(block_id ground, block_id block)
+{
+    switch (block)
+    {
+        case SAPLING:
+            return ground == GRASS;
+        default:
+            return 1;
+    }
+}

src/block_data.h

@@ -14,6 +14,7 @@ extern bounding_box block_box;
 
 int block_is_opaque(block_id block);
 int block_is_obstacle(block_id block);
+int block_can_be_placed_on(block_id ground, block_id block);
 int block_connects(block_id block);
 
 #endif

src/world.c

@@ -345,8 +345,9 @@ void world_tick(world *w)
 
             vec3 position = {w->selected_block_x, w->selected_block_y - 0.5f, w->selected_block_z};
             bounding_box_update(&block_box, &position);
+            block_id ground = world_get_block(w, position.x, position.y, position.z);
 
-            if (!is_colliding(&block_box, &w->player.box))
+            if (!is_colliding(&block_box, &w->player.box) && block_can_be_placed_on(ground, w->selected_block))
             {
                 world_set_block(w, w->selected_block_x, w->selected_block_y, w->selected_block_z, w->selected_block);
                 w->block_changed = 1;
@@ -378,9 +379,12 @@ void world_draw(world *w, double delta_time, double time_since_tick)
     vec3 player_delta;
     multiply_v3f(&player_delta, &w->player.velocity, tick_delta_time);
 
-    if (w->noclip_mode) {
+    if (w->noclip_mode)
+    {
         add_v3(&w->player.position, &w->player.position, &player_delta);
-    } else {
+    }
+    else
+    {
         entity_move(&w->player, w, &player_delta);
 
         w->player.on_ground = 0;