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

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 && (!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,
							tempHp: newMaxHp === undefined ? undefined : c.tempHp,
						}
					: c,
			),
			activeIndex: encounter.activeIndex,
			roundNumber: encounter.roundNumber,
		},
		events: [
			{
				type: "MaxHpSet",
				combatantId,
				previousMaxHp,
				newMaxHp,
				previousCurrentHp,
				newCurrentHp,
			},
		],
	};
}