ttrpg_manager/src/lib/mechanics/deathSaves.ts

import type { Character } from '@/lib/schemas/character';

export interface DeathSaveOutcome {
  patch: Partial<Character>;
  /** 'pending' = still rolling; 'stable' = 3 successes; 'dead' = 3 failures; 'revived' = nat 20 */
  outcome: 'pending' | 'stable' | 'dead' | 'revived';
  log: string;
}

/**
 * Apply one 5e death saving throw given a raw d20 face. Pure:
 *  - nat 20 → regain 1 HP, clear the death-save clock (back in the fight)
 *  - nat 1  → two failures
 *  - ≥10    → one success (3 → stable)
 *  - <10    → one failure (3 → dead)
 */
export function rollDeathSave(c: Character, d20: number): DeathSaveOutcome {
  const ds = c.defenses.deathSaves;

  if (d20 === 20) {
    return {
      patch: {
        hp: { ...c.hp, current: 1 },
        defenses: { ...c.defenses, deathSaves: { successes: 0, failures: 0 } },
      },
      outcome: 'revived',
      log: `${c.name} rolls a natural 20 and regains 1 HP!`,
    };
  }

  let successes = ds.successes;
  let failures = ds.failures;
  if (d20 === 1) failures = Math.min(3, failures + 2);
  else if (d20 >= 10) successes = Math.min(3, successes + 1);
  else failures = Math.min(3, failures + 1);

  const patch: Partial<Character> = { defenses: { ...c.defenses, deathSaves: { successes, failures } } };

  if (failures >= 3) return { patch, outcome: 'dead', log: `${c.name} fails a third death save and dies.` };
  if (successes >= 3) return { patch, outcome: 'stable', log: `${c.name} stabilizes.` };
  return {
    patch,
    outcome: 'pending',
    log: d20 >= 10
      ? `${c.name} succeeds a death save (${successes}/3).`
      : `${c.name} fails a death save (${failures}/3).`,
  };
}