initiative/packages/domain/src/set-temp-hp.ts

import type { DomainEvent } from "./events.js";
import type { CombatantId, DomainError, Encounter } from "./types.js";

export interface SetTempHpSuccess {
	readonly encounter: Encounter;
	readonly events: DomainEvent[];
}

/**
 * Pure function that sets or clears a combatant's temporary HP.
 *
 * - Setting tempHp when the combatant already has tempHp keeps the higher value.
 * - Clearing tempHp (undefined) removes temp HP entirely.
 * - Requires HP tracking to be enabled (maxHp must be set).
 */
export function setTempHp(
	encounter: Encounter,
	combatantId: CombatantId,
	tempHp: number | undefined,
): SetTempHpSuccess | 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 (tempHp !== undefined && (!Number.isInteger(tempHp) || tempHp < 1)) {
		return {
			kind: "domain-error",
			code: "invalid-temp-hp",
			message: `Temp HP must be a positive integer, got ${tempHp}`,
		};
	}

	const previousTempHp = target.tempHp;

	// Higher value wins when both are defined
	let newTempHp: number | undefined;
	if (tempHp === undefined) {
		newTempHp = undefined;
	} else if (previousTempHp === undefined) {
		newTempHp = tempHp;
	} else {
		newTempHp = Math.max(previousTempHp, tempHp);
	}

	return {
		encounter: {
			combatants: encounter.combatants.map((c) =>
				c.id === combatantId ? { ...c, tempHp: newTempHp } : c,
			),
			activeIndex: encounter.activeIndex,
			roundNumber: encounter.roundNumber,
		},
		events: [
			{
				type: "TempHpSet",
				combatantId,
				previousTempHp,
				newTempHp,
			},
		],
	};
}