Implement the 003-remove-combatant feature that adds the possibility to remove a combatant from an encounter

packages/domain/src/remove-combatant.ts

@@ -0,0 +1,65 @@
+import type { DomainEvent } from "./events.js";
+import type { CombatantId, DomainError, Encounter } from "./types.js";
+
+export interface RemoveCombatantSuccess {
+	readonly encounter: Encounter;
+	readonly events: DomainEvent[];
+}
+
+/**
+ * Pure function that removes a combatant from an encounter by ID.
+ *
+ * Adjusts activeIndex to preserve turn integrity:
+ * - Removed after active → unchanged
+ * - Removed before active → decrement
+ * - Removed is active, mid-list → same index (next slides in)
+ * - Removed is active, at end → wrap to 0
+ * - Only combatant removed → 0
+ *
+ * roundNumber is never changed.
+ */
+export function removeCombatant(
+	encounter: Encounter,
+	id: CombatantId,
+): RemoveCombatantSuccess | DomainError {
+	const removedIdx = encounter.combatants.findIndex((c) => c.id === id);
+
+	if (removedIdx === -1) {
+		return {
+			kind: "domain-error",
+			code: "combatant-not-found",
+			message: `No combatant found with ID "${id}"`,
+		};
+	}
+
+	const removed = encounter.combatants[removedIdx];
+	const newCombatants = encounter.combatants.filter((_, i) => i !== removedIdx);
+
+	let newActiveIndex: number;
+	if (newCombatants.length === 0) {
+		newActiveIndex = 0;
+	} else if (removedIdx < encounter.activeIndex) {
+		newActiveIndex = encounter.activeIndex - 1;
+	} else if (removedIdx > encounter.activeIndex) {
+		newActiveIndex = encounter.activeIndex;
+	} else {
+		// removedIdx === activeIndex
+		newActiveIndex =
+			removedIdx >= newCombatants.length ? 0 : encounter.activeIndex;
+	}
+
+	return {
+		encounter: {
+			combatants: newCombatants,
+			activeIndex: newActiveIndex,
+			roundNumber: encounter.roundNumber,
+		},
+		events: [
+			{
+				type: "CombatantRemoved",
+				combatantId: removed.id,
+				name: removed.name,
+			},
+		],
+	};
+}