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/adjust-hp.ts

@@ -0,0 +1,77 @@
+import type { DomainEvent } from "./events.js";
+import type { CombatantId, DomainError, Encounter } from "./types.js";
+
+export interface AdjustHpSuccess {
+	readonly encounter: Encounter;
+	readonly events: DomainEvent[];
+}
+
+/**
+ * Pure function that adjusts a combatant's current HP by a delta.
+ *
+ * The result is clamped to [0, maxHp]. Requires the combatant to have
+ * HP tracking enabled (maxHp must be set).
+ */
+export function adjustHp(
+	encounter: Encounter,
+	combatantId: CombatantId,
+	delta: number,
+): AdjustHpSuccess | 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 "${combatantId}"`,
+		};
+	}
+
+	const target = encounter.combatants[targetIdx];
+
+	if (target.maxHp === undefined || target.currentHp === undefined) {
+		return {
+			kind: "domain-error",
+			code: "no-hp-tracking",
+			message: `Combatant "${combatantId}" does not have HP tracking enabled`,
+		};
+	}
+
+	if (delta === 0) {
+		return {
+			kind: "domain-error",
+			code: "zero-delta",
+			message: "Delta must not be zero",
+		};
+	}
+
+	if (!Number.isInteger(delta)) {
+		return {
+			kind: "domain-error",
+			code: "invalid-delta",
+			message: `Delta must be an integer, got ${delta}`,
+		};
+	}
+
+	const previousHp = target.currentHp;
+	const newHp = Math.max(0, Math.min(target.maxHp, previousHp + delta));
+
+	return {
+		encounter: {
+			combatants: encounter.combatants.map((c) =>
+				c.id === combatantId ? { ...c, currentHp: newHp } : c,
+			),
+			activeIndex: encounter.activeIndex,
+			roundNumber: encounter.roundNumber,
+		},
+		events: [
+			{
+				type: "CurrentHpAdjusted",
+				combatantId,
+				previousHp,
+				newHp,
+				delta,
+			},
+		],
+	};
+}