initiative/packages/domain/src/adjust-hp.ts

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 previousTempHp = target.tempHp ?? 0;
	let newTempHp = previousTempHp;
	let effectiveDelta = delta;

	if (delta < 0 && previousTempHp > 0) {
		const absorbed = Math.min(previousTempHp, Math.abs(delta));
		newTempHp = previousTempHp - absorbed;
		effectiveDelta = delta + absorbed;
	}

	const newHp = Math.max(
		0,
		Math.min(target.maxHp, previousHp + effectiveDelta),
	);

	const events: DomainEvent[] = [];

	if (newTempHp !== previousTempHp) {
		events.push({
			type: "TempHpSet",
			combatantId,
			previousTempHp: previousTempHp || undefined,
			newTempHp: newTempHp || undefined,
		});
	}

	if (newHp !== previousHp) {
		events.push({
			type: "CurrentHpAdjusted",
			combatantId,
			previousHp,
			newHp,
			delta,
		});
	}

	return {
		encounter: {
			combatants: encounter.combatants.map((c) =>
				c.id === combatantId
					? { ...c, currentHp: newHp, tempHp: newTempHp || undefined }
					: c,
			),
			activeIndex: encounter.activeIndex,
			roundNumber: encounter.roundNumber,
		},
		events,
	};
}