secret-santa.ts

const randomInteger = (max: number) => {
  return Math.floor(Math.random() * max);
};

const secretSanta = (people: string[]): Array<[string, string]> => {
  const result: Array<[string, string]> = [];
  const available = people.slice(1);
  let person1 = people[0];

  while (available.length > 0) {
    const index = randomInteger(available.length);
    const person2 = available[index];

    result.push([person1, person2]);
    person1 = person2;

    available.splice(index, 1);
  }

  result.push([person1, people[0]]);

  return result;
};

describe('secret santa distribution', () => {
  const count = 10;

  it('secretSanta', () => {
    const people = Array(count)
      .fill(null)
      .map((_, index) => `person${String(index).padStart(4, '0')}`);

    const result = secretSanta(people);

    expect(result.map(([person1]) => person1).sort()).toEqual(people);
    expect(result.map(([, person2]) => person2).sort()).toEqual(people);

    for (const [person1, person2] of result) {
      expect(person1).not.toBe(person2);
    }
  });
});