// The month grid behind the date button. Off-by-one week starts and month
// boundaries are how a hand-rolled calendar goes wrong, and none of it shows
// until somebody opens the month that breaks.
import { load } from "./extract.mjs";
import { suite, eq, report } from "./assert.mjs";

const app = load({ names: ["ymd", "calendarGridStart"] });

const MONDAY = 1, SUNDAY = 0;
const cells = (year, monthIdx, start) => {
  const gs = app.calendarGridStart(new Date(year, monthIdx, 1), start);
  return Array.from({ length: 42 }, (_, i) => {
    const d = new Date(gs);
    d.setDate(gs.getDate() + i);
    return d;
  });
};

suite("the grid starts on the locale's first weekday");
for (const [name, start] of [["Monday", MONDAY], ["Sunday", SUNDAY]]) {
  const everyMonth = Array.from({ length: 12 }, (_, m) => cells(2026, m, start)[0].getDay());
  eq(everyMonth.every(day => day === start), true,
     `${name} start: all twelve months of 2026 open on a ${name}`);
}

suite("six rows, covering the month exactly once");
{
  let alwaysSix = true, coversAll = true;
  for (let m = 0; m < 12; m++) {
    const cs = cells(2026, m, MONDAY);
    if (cs.length !== 42) alwaysSix = false;
    const inMonth = cs.filter(d => d.getMonth() === m).length;
    if (inMonth !== new Date(2026, m + 1, 0).getDate()) coversAll = false;
  }
  eq(alwaysSix, true, "42 cells every month, so the panel never changes height");
  eq(coversAll, true, "every day of every month appears exactly once");
}

suite("the months that catch people out");
{
  // 1 Feb 2026 is a Sunday: under Monday weeks that needs six leading days from
  // January, which is precisely what a naive `1 - getDay()` gets wrong.
  const feb = cells(2026, 1, MONDAY);
  eq(app.ymd(feb[0]), "2026-01-26", "Feb 2026 starts Sunday: the grid opens on 26 Jan");
  eq(app.ymd(feb[6]), "2026-02-01", "…putting the 1st in the last column of row one");
  eq(app.ymd(cells(2026, 1, SUNDAY)[0]), "2026-02-01",
     "the same month under Sunday weeks needs no leading days at all");
  eq(cells(2026, 7, MONDAY).filter(d => d.getMonth() === 7).length, 31,
     "a 31-day month starting late in the week still fits");
  eq(cells(2024, 1, MONDAY).filter(d => d.getMonth() === 1).length, 29, "Feb 2024 shows 29 days");
  eq(cells(2026, 1, MONDAY).filter(d => d.getMonth() === 1).length, 28, "Feb 2026 shows 28");
}

suite("year boundaries");
{
  eq(app.ymd(cells(2026, 0, MONDAY)[0]).startsWith("2025"), true,
     "January's leading cells come from the previous December");
  eq(app.ymd(cells(2026, 11, MONDAY)[41]).startsWith("2027"), true,
     "December's trailing cells run into January");
}

export default report("calendar");