2 files changed, 82 insertions, 1 deletions

diff --git a/internal/engine/tick.go b/internal/engine/tick.go
index ff1bace..251373c 100644
--- a/internal/engine/tick.go
+++ b/internal/engine/tick.go
@@ -2,6 +2,7 @@ package engine
 
 import (
 	"math/rand"
+	"sort"
 	"sync"
 	"time"
 )
@@ -85,6 +86,17 @@ func (e *Engine) Stop() {
 	}
 }
 
+// processTick fires every due subscriber for the current engine tick.
+//
+// Subscribers are invoked in ascending subscription-ID order, i.e. the order in
+// which they were registered with Subscribe. IDs are allocated monotonically
+// (e.nextID++), so subscription order is a stable, deterministic sequence:
+// the bootstrap master subscriber (cmd/thoi/main.go) subscribes at startup as
+// ID 1 and therefore always fires first each tick, followed by each
+// subsequently-registered tick (combat rounds, aggro rolls, scheduled respawns,
+// etc.) in the order they registered. This guarantees predictable per-tick
+// ordering across runs — critical for features whose behaviour depends on the
+// relative ordering of combat resolution vs. movement vs. aggro checks.
 func (e *Engine) processTick() {
 	e.mu.Lock()
 	snapshot := make(map[uint64]*subscriber, len(e.subscribers))
@@ -93,7 +105,14 @@ func (e *Engine) processTick() {
 	}
 	e.mu.Unlock()
 
-	for id, sub := range snapshot {
+	ids := make([]uint64, 0, len(snapshot))
+	for id := range snapshot {
+		ids = append(ids, id)
+	}
+	sort.Slice(ids, func(i, j int) bool { return ids[i] < ids[j] })
+
+	for _, id := range ids {
+		sub := snapshot[id]
 		sub.ticks++
 		if sub.ticks >= sub.interval {
 			sub.ticks = 0
diff --git a/internal/engine/tick_test.go b/internal/engine/tick_test.go
new file mode 100644
index 0000000..520ef19
--- /dev/null
+++ b/internal/engine/tick_test.go
@@ -0,0 +1,62 @@
+package engine
+
+import (
+	"sync"
+	"testing"
+)
+
+// TestProcessTickFiresInSubscriptionOrder asserts the engine invokes due
+// tick subscribers in ascending subscription-ID order, i.e. the order in
+// which they registered. This is the determinism contract that callers
+// (notably the game's master subscriber at cmd/thoi/main.go, which subscribes
+// first as ID 1 and must fire before combat/aggro/respawn per-tick callbacks)
+// increasingly rely on.
+func TestProcessTickFiresInSubscriptionOrder(t *testing.T) {
+	e := New()
+	var mu sync.Mutex
+	var order []int
+
+	// Three interval-1 subscribers. Each appends its own index. All return
+	// true to stay subscribed (we Unsubscribe at the end).
+	id1 := e.Subscribe(1, func() bool {
+		mu.Lock()
+		order = append(order, 1)
+		mu.Unlock()
+		return true
+	})
+	id2 := e.Subscribe(1, func() bool {
+		mu.Lock()
+		order = append(order, 2)
+		mu.Unlock()
+		return true
+	})
+	id3 := e.Subscribe(1, func() bool {
+		mu.Lock()
+		order = append(order, 3)
+		mu.Unlock()
+		return true
+	})
+
+	if id1 >= id2 || id2 >= id3 {
+		t.Fatalf("ID monotonicity broken: got %d, %d, %d", id1, id2, id3)
+	}
+
+	// processTick is synchronous: it invokes every due interval-1 subscriber
+	// inline in turn before returning. So `order` is fully populated here.
+	e.processTick()
+
+	mu.Lock()
+	got := append([]int(nil), order...)
+	mu.Unlock()
+
+	e.Unsubscribe(id1)
+	e.Unsubscribe(id2)
+	e.Unsubscribe(id3)
+
+	if len(got) != 3 {
+		t.Fatalf("expected 3 subscriber firings, got %d: %v", len(got), got)
+	}
+	if got[0] != 1 || got[1] != 2 || got[2] != 3 {
+		t.Errorf("subscribers fired out of registration order: got %v, want [1 2 3]", got)
+	}
+}