// One readable line per parsed lesson, the form both snapshots compare: what a student sees
// (day, time, weeks or dates, subject, kind, room, teacher, groups), not ids or source colours.
const DAYS = ["", "пн", "вт", "ср", "чт", "пт", "сб", "вс"];

const weeks = (ranges) =>
  ranges?.map(([a, b]) => (b === null ? `с ${a}` : a === b ? `${a}` : `${a}-${b}`)).join(",") || "";

export const lessonLine = (l) =>
  [
    `${l.date || DAYS[l.day]} ${l.start}${l.end ? `–${l.end}` : ""}`,
    (l.date ? "один раз" : l.dates?.length ? `даты ${l.dates.join(",")}` : l.weekRanges ? `нед. ${weeks(l.weekRanges)}` : "каждую неделю") +
      (l.variant === "upper" ? ", верхняя строка" : l.variant === "lower" ? ", нижняя строка" : ""),
    l.subject,
    l.type,
    // The town only when the cell names one or the day's note puts the lesson there.
    [l.room, l.place].filter(Boolean).join(", "),
    l.teacher,
    [...l.groups, ...l.subgroups.map((s) => `${s.group}/${s.label.replace("Подгруппа ", "п")}`)].join(",") +
      // "Немецкий язык" or "Французский язык": a part of the group takes one of them.
      (l.option ? " · на выбор" : "") +
      // Russian for foreign students while the others have physical education.
      (l.audience === "foreign" ? " · иностранцам" : l.audience === "local" ? " · кроме иностранцев" : ""),
  ]
    .map((part) => String(part ?? "").replace(/\s+/g, " ").trim())
    .join(" | ");

// "Занятия в Полоцке": a day of a group or subgroup in another town, not a lesson.
const placeLine = (p) =>
  [
    `@ ${p.date || DAYS[p.day]}${p.from ? ` с ${p.from}` : ""}`,
    p.text,
    [...p.groups, ...p.subgroups.map((s) => `${s.group}/${s.label.replace("Подгруппа ", "п")}`)].join(","),
  ].join(" | ");

/** Sorted lesson lines of a parsed document, with its groups and period on top. */
export const snapshotOf = (schedule) => [
  `# ${schedule.period} · ${schedule.groups.join(", ")}`,
  ...(schedule.places || []).map(placeLine).sort(),
  ...schedule.lessons.map(lessonLine).sort(),
];

/** Lines only in `before` (removed) and only in `after` (added), duplicates counted. */
export function diffLines(before, after) {
  const count = (lines) => lines.reduce((m, x) => m.set(x, (m.get(x) || 0) + 1), new Map());
  const a = count(before), b = count(after);
  const removed = [], added = [];
  for (const [line, n] of a) for (let i = b.get(line) || 0; i < n; i++) removed.push(line);
  for (const [line, n] of b) for (let i = a.get(line) || 0; i < n; i++) added.push(line);
  return { removed, added };
}