a tile can have more than one entity now

but there can only be one solid entity per tile

internal/sim/entity.go

@@ -10,6 +10,7 @@ type entity struct {
 	ID       int      `json:"id"`
 	Position math.Vec `json:"pos"`
 	Glyph    rune     `json:"glyph"`
+	solid    bool     `json:"-"`
 	behavior
 }
 

internal/sim/player.go

@@ -155,18 +155,19 @@ func sendResponse(conn *websocket.Conn, res wire.Response) error {
 type spawnPlayer struct{}
 
 func (s spawnPlayer) exec(w *world, r *room, p *player, seq int) result {
+	e := entity{
+		ID:       <-w.nextID,
+		Glyph:    '@',
+		solid:    true,
+		behavior: doNothing{},
+	}
+	p.avatar = &e
+
 	for n, t := range r.tiles {
-		if t.here == nil {
-			x, y := n%r.Width, n/r.Width
-			e := entity{
-				ID:       <-w.nextID,
-				Position: math.Vec{x, y},
-				Glyph:    '@',
-				behavior: doNothing{},
-			}
-			p.avatar = &e
-			t.here = &e
-			break
+		x, y := n%r.Width, n/r.Width
+		e.Position = math.Vec{x, y}
+		if t.addEntity(&e) {
+			return result{}
 		}
 	}
 	return result{}
@@ -186,11 +187,10 @@ func (m *Move) exec(w *world, r *room, p *player, seq int) result {
 
 	currentTile := r.getTile(pos)
 	nextTile := r.getTile(target)
-	if nextTile.here != nil {
+	if !nextTile.addEntity(p.avatar) {
 		return result{reply: wire.Errorf("target cell (%d, %d) is occupied", target.X, target.Y)}
 	}
-
-	currentTile.here, nextTile.here = nil, p.avatar
+	currentTile.removeEntity(p.avatar.ID)
 	p.avatar.Position = target
 	return result{reply: wire.OK{}}
 }

internal/sim/room.go

@@ -17,8 +17,7 @@ type room struct {
 func (r *room) allEntities() map[int]wire.Entity {
 	all := make(map[int]wire.Entity, 4)
 	for _, t := range r.tiles {
-		if t.here != nil {
-			e := t.here
+		for _, e := range t.here {
 			all[e.ID] = wire.Entity{
 				ID:       e.ID,
 				Position: e.Position,
@@ -42,11 +41,7 @@ func (r *room) addEntity(e *entity) bool {
 	if t == nil {
 		return false
 	}
-	if t.here != nil {
-		return false
-	}
-	t.here = e
-	return true
+	return t.addEntity(e)
 }
 
 func (r *room) addPlayer(p *player) {

internal/sim/tile.go

@@ -2,5 +2,27 @@ package sim
 
 type tile struct {
 	floor floor
-	here  *entity
+	here  []*entity
+}
+
+func (t *tile) addEntity(e *entity) bool {
+	if e.solid {
+		for _, other := range t.here {
+			if other.solid {
+				return false
+			}
+		}
+	}
+	t.here = append(t.here, e)
+	return true
+}
+
+func (t *tile) removeEntity(id int) {
+	here := t.here[:0]
+	for _, e := range t.here {
+		if e.ID != id {
+			here = append(here, e)
+		}
+	}
+	t.here = here
 }

internal/sim/world.go

@@ -43,6 +43,7 @@ func newWorld(log *blammo.Log) *world {
 		ID:       -1,
 		Position: math.Vec{5, 5},
 		Glyph:    'd',
+		solid:    true,
 		behavior: doNothing{},
 	})
 
@@ -203,8 +204,8 @@ func (w *world) tick(d time.Duration) {
 	// run all object effects
 	for _, r := range w.rooms {
 		for _, t := range r.tiles {
-			if t.here != nil {
-				t.here.update(d)
+			for _, e := range t.here {
+				e.update(d)
 			}
 		}