import { REVIEW_RATING_MAX, REVIEW_RATING_MIN } from '@sport/types';

/**
 * The rating aggregate, extracted so it can be pinned without a database.
 *
 * These four lines decide the number under every product name in the store. A
 * mistake here is not a crash — it is a product quietly displaying 4.5 stars it
 * did not earn, on every page, until somebody notices by eye.
 */
export function summarise(ratings: readonly number[]): {
  average: number;
  count: number;
  distribution: { rating: number; count: number }[];
} {
  const count = ratings.length;
  const sum = ratings.reduce((total, rating) => total + rating, 0);

  const distribution: { rating: number; count: number }[] = [];
  for (let rating = REVIEW_RATING_MAX; rating >= REVIEW_RATING_MIN; rating -= 1) {
    distribution.push({ rating, count: ratings.filter((value) => value === rating).length });
  }

  return {
    average: count === 0 ? 0 : Math.round((sum / count) * 10) / 10,
    count,
    distribution,
  };
}

describe('review summary', () => {
  it('averages to one decimal place', () => {
    expect(summarise([5, 4, 4]).average).toBe(4.3);
  });

  it('does not divide by zero on an unreviewed product', () => {
    const summary = summarise([]);

    expect(summary.average).toBe(0);
    expect(summary.count).toBe(0);
  });

  it('reports every star level, including the empty ones', () => {
    // A distribution with gaps renders as a bar chart with missing bars rather
    // than bars at zero, which reads as a broken chart.
    const summary = summarise([5, 5, 1]);

    expect(summary.distribution).toEqual([
      { rating: 5, count: 2 },
      { rating: 4, count: 0 },
      { rating: 3, count: 0 },
      { rating: 2, count: 0 },
      { rating: 1, count: 1 },
    ]);
  });

  it('orders the distribution from best to worst', () => {
    expect(summarise([3]).distribution.map((bucket) => bucket.rating)).toEqual([5, 4, 3, 2, 1]);
  });

  it('distinguishes a polarised product from a mediocre one', () => {
    // Both average 3.0. The distribution is the only thing that tells a shopper
    // which one has a sizing problem, which is why it is sent alongside.
    const mediocre = summarise([3, 3, 3, 3]);
    const polarised = summarise([5, 5, 1, 1]);

    expect(mediocre.average).toBe(polarised.average);
    expect(mediocre.distribution).not.toEqual(polarised.distribution);
  });

  it('rounds half up so 4.25 does not read as 4.2', () => {
    expect(summarise([5, 4, 4, 4]).average).toBe(4.3);
  });

  it('keeps a whole number whole', () => {
    // 4.0 rather than 4 matters for display, but the value must still equal 4.
    expect(summarise([4, 4, 4])).toMatchObject({ average: 4, count: 3 });
  });
});