Implement the 009-combatant-hp feature that adds optional max HP and current HP tracking per combatant with +/- controls, direct entry, and persistence

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/domain/src/set-hp.ts

@@ -0,0 +1,88 @@
+import type { DomainEvent } from "./events.js";
+import type { CombatantId, DomainError, Encounter } from "./types.js";
+
+export interface SetHpSuccess {
+	readonly encounter: Encounter;
+	readonly events: DomainEvent[];
+}
+
+/**
+ * Pure function that sets, updates, or clears a combatant's max HP.
+ *
+ * - Setting maxHp initializes currentHp to maxHp (full health).
+ * - Updating maxHp clamps currentHp to the new maxHp if needed.
+ * - Clearing maxHp (undefined) also clears currentHp.
+ */
+export function setHp(
+	encounter: Encounter,
+	combatantId: CombatantId,
+	maxHp: number | undefined,
+): SetHpSuccess | DomainError {
+	const targetIdx = encounter.combatants.findIndex((c) => c.id === combatantId);
+
+	if (targetIdx === -1) {
+		return {
+			kind: "domain-error",
+			code: "combatant-not-found",
+			message: `No combatant found with ID "${combatantId}"`,
+		};
+	}
+
+	if (maxHp !== undefined) {
+		if (!Number.isInteger(maxHp) || maxHp < 1) {
+			return {
+				kind: "domain-error",
+				code: "invalid-max-hp",
+				message: `Max HP must be a positive integer, got ${maxHp}`,
+			};
+		}
+	}
+
+	const target = encounter.combatants[targetIdx];
+	const previousMaxHp = target.maxHp;
+	const previousCurrentHp = target.currentHp;
+
+	let newMaxHp: number | undefined;
+	let newCurrentHp: number | undefined;
+
+	if (maxHp === undefined) {
+		newMaxHp = undefined;
+		newCurrentHp = undefined;
+	} else if (previousMaxHp === undefined) {
+		// First time setting HP — full health
+		newMaxHp = maxHp;
+		newCurrentHp = maxHp;
+	} else {
+		// Updating existing maxHp
+		newMaxHp = maxHp;
+		if (previousCurrentHp === previousMaxHp) {
+			// At full health — stay at full health
+			newCurrentHp = maxHp;
+		} else {
+			// Clamp currentHp to new max
+			newCurrentHp = Math.min(previousCurrentHp ?? maxHp, maxHp);
+		}
+	}
+
+	return {
+		encounter: {
+			combatants: encounter.combatants.map((c) =>
+				c.id === combatantId
+					? { ...c, maxHp: newMaxHp, currentHp: newCurrentHp }
+					: c,
+			),
+			activeIndex: encounter.activeIndex,
+			roundNumber: encounter.roundNumber,
+		},
+		events: [
+			{
+				type: "MaxHpSet",
+				combatantId,
+				previousMaxHp,
+				newMaxHp,
+				previousCurrentHp,
+				newCurrentHp,
+			},
+		],
+	};
+}