Implement drawlist updating for increasing y

game/main.c

@@ -87,6 +87,15 @@ typedef enum {
 	ENTITY_OBJ,
 } entitytag_t;
 
+/*
+ * Common entity header -- this must be at the start of each type of
+ * entity structure.
+ *
+ * The tag is used to distinguish different types of entity when
+ * accessing them through the drawlist.  The prev and next pointers
+ * allow the drawlist to be threaded through all entities as a
+ * doubly-linked list.
+ */
 typedef struct entity {
 	entitytag_t tag;
 	struct entity *prev;
@@ -559,6 +568,87 @@ static inline bool tile_passable(const map_t map, int x, int y)
 	return true;
 }
 
+static inline int dynentity_bottom(dynentity_t *e)
+{
+	return e->pos.y + e->ext.y / 2;
+}
+
+static inline int objentity_bottom(objentity_t *e)
+{
+	return e->pos.y;
+}
+
+static inline int entity_bottom(entity_t *e)
+{
+	switch (e->tag) {
+	case ENTITY_DYN:
+		return dynentity_bottom((dynentity_t *)e);
+	case ENTITY_OBJ:
+		return objentity_bottom((objentity_t *)e);
+	}
+}
+
+static void update_drawlist(dynentity_t *e, entity_t **drawlist)
+{
+	if (e->dir.y == 0)
+		return;
+
+	if (e->dir.y > 0) {
+		// The entity moved down -- check if it needs to be moved
+		// along in the drawlist.
+
+		entity_t *n = e->e.next;
+		if (n == NULL)
+			// The entity is already at the end -- no update required.
+			return;
+
+		const int y = dynentity_bottom(e);
+		int nexty = entity_bottom(n);
+		if (y <= nexty)
+			// The entity is still behind the next entity -- no update
+			// required.
+			return;
+
+		// Update required -- find the earliest entity that is in
+		// front of e, or the end of the list.
+		while (n->next != NULL && y > nexty) {
+			n = n->next;
+			nexty = entity_bottom(n);
+		}
+		assert(n != NULL);
+
+		if (y > nexty) {
+			// The end of the list was reached without finding a
+			// larger or equal y -- move the entity to the end of the
+			// list (after n).
+			e->e.prev->next = e->e.next;
+			e->e.next->prev = e->e.prev;
+
+			e->e.prev = n;
+			e->e.next = NULL;
+			n->next = (entity_t *)e;
+		} else {
+			// An entity with greater or equal y was found -- move the
+			// entity to just before n.
+
+			if (e->e.prev == NULL) {
+				// The entity is at the beginning of the list, we need
+				// to update the drawlist pointer.
+				*drawlist = e->e.next;
+				e->e.next->prev = NULL;
+			} else {
+				e->e.prev->next = e->e.next;
+				e->e.next->prev = e->e.prev;
+			}
+
+			e->e.prev = n->prev;
+			e->e.next = n;
+			n->prev->next = (entity_t *)e;
+			n->prev = (entity_t *)e;
+		}
+	}
+}
+
 static void update_dynentity(gamestate_t *state, dynentity_t *e, double dt)
 {
 	if (0 == e->speed) {
@@ -602,6 +692,8 @@ static void update_dynentity(gamestate_t *state, dynentity_t *e, double dt)
 	if (valid) {
 		e->pos.x = nextx;
 		e->pos.y = nexty;
+
+		update_drawlist(e);
 	}
 }