Implement the 004-edit-combatant feature that adds the possibility to change a combatants name

packages/domain/src/edit-combatant.ts

@@ -0,0 +1,62 @@
+import type { DomainEvent } from "./events.js";
+import type { CombatantId, DomainError, Encounter } from "./types.js";
+
+export interface EditCombatantSuccess {
+	readonly encounter: Encounter;
+	readonly events: DomainEvent[];
+}
+
+/**
+ * Pure function that renames a combatant in an encounter by ID.
+ *
+ * FR-001: Accepts Encounter, CombatantId, and newName; returns next state + events.
+ * FR-002: Emits a CombatantUpdated event with combatantId, oldName, newName.
+ * FR-004: Rejects empty/whitespace-only names with DomainError.
+ * FR-005: Preserves activeIndex and roundNumber.
+ * FR-006: Preserves combatant list order.
+ */
+export function editCombatant(
+	encounter: Encounter,
+	id: CombatantId,
+	newName: string,
+): EditCombatantSuccess | DomainError {
+	const trimmed = newName.trim();
+
+	if (trimmed === "") {
+		return {
+			kind: "domain-error",
+			code: "invalid-name",
+			message: "Combatant name must not be empty",
+		};
+	}
+
+	const index = encounter.combatants.findIndex((c) => c.id === id);
+
+	if (index === -1) {
+		return {
+			kind: "domain-error",
+			code: "combatant-not-found",
+			message: `No combatant found with ID "${id}"`,
+		};
+	}
+
+	const oldName = encounter.combatants[index].name;
+
+	return {
+		encounter: {
+			combatants: encounter.combatants.map((c) =>
+				c.id === id ? { ...c, name: trimmed } : c,
+			),
+			activeIndex: encounter.activeIndex,
+			roundNumber: encounter.roundNumber,
+		},
+		events: [
+			{
+				type: "CombatantUpdated",
+				combatantId: id,
+				oldName,
+				newName: trimmed,
+			},
+		],
+	};
+}