const RsvCalendarPicker = (() => {

  function get_first_day_of_month(date) {
    const day = new Date(date.getFullYear(), date.getMonth(), 1).getDay();
    return day === 0 ? 6 : day - 1; // Mon=0 … Sun=6
  }

  function is_same_day(a, b) {
    return a.getUTCFullYear() === b.getUTCFullYear()
        && a.getUTCMonth()    === b.getUTCMonth()
        && a.getUTCDate()     === b.getUTCDate();
  }

  function is_same_month(a, b) {
    return a.getFullYear() === b.getFullYear()
        && a.getMonth()    === b.getMonth();
  }

  function clear_class(root, cls) {
    root.querySelectorAll('.' + cls).forEach(el => el.classList.remove(cls));
  }

  function set_cell(cell, date, outside) {
    cell.classList.toggle('dimm', outside);
    const iso = date.toISOString();
    cell.setAttribute('datetime', iso);
    cell.children[0].id = iso;
    cell.children[0].setAttribute('datetime', iso);
    cell.children[1].textContent = date.getUTCDate();
    cell.children[1].setAttribute('for', iso);
  }

  function render(state, date) {
    const year   = date.getFullYear();
    const month  = date.getMonth();
    const first  = get_first_day_of_month(date);
    const in_cur = new Date(year, month + 1, 0).getDate();
    const in_prev = new Date(year, month, 0).getDate();
    const today  = new Date();
    const rows   = state.body.querySelectorAll('tr');

    clear_class(state.body, 'rsv-cal-cell-current');
    clear_class(state.body, 'rsv-cal-cell-today');

    let idx = 0;
    for (let d = in_prev - first + 1; d <= in_prev; d++, idx++) {
      const dt   = new Date(Date.UTC(year, month - 1, d));
      const cell = rows[0].children[idx];
      set_cell(cell, dt, true);
      if (is_same_day(dt, today)) cell.classList.add('rsv-cal-cell-today');
    }
    for (let i = 1; i <= in_cur; i++, idx++) {
      const dt   = new Date(Date.UTC(year, month, i));
      const cell = rows[Math.floor(idx / 7)].children[idx % 7];
      set_cell(cell, dt, false);
      if (is_same_day(dt, date)) cell.querySelector('input').checked = true;
      if (is_same_day(dt, today)) cell.classList.add('rsv-cal-cell-today');
    }
    for (let i = 1; idx < 42; i++, idx++) {
      const dt   = new Date(Date.UTC(year, month + 1, i));
      const cell = rows[Math.floor(idx / 7)].children[idx % 7];
      set_cell(cell, dt, true);
      if (is_same_day(dt, today)) cell.classList.add('rsv-cal-cell-today');
    }

    state.month_el.textContent =
      new Date(year, month).toLocaleString(navigator.language, { month: 'long' }) + ' ' + year;
  }

  function day_names() {
    // Generate short weekday names starting on Monday using the browser locale.
    return Array.from({ length: 7 }, (_, i) =>
      new Date(2024, 0, 1 + i) // 2024-01-01 is a Monday
        .toLocaleDateString(navigator.language, { weekday: 'short' })
    );
  }

  const ARROW_L = `<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor" viewBox="0 0 16 16"><path fill-rule="evenodd" d="M11.354 1.646a.5.5 0 0 1 0 .708L5.707 8l5.647 5.646a.5.5 0 0 1-.708.708l-6-6a.5.5 0 0 1 0-.708l6-6a.5.5 0 0 1 .708 0"/></svg>`;
  const ARROW_R = `<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor" viewBox="0 0 16 16"><path fill-rule="evenodd" d="M4.646 1.646a.5.5 0 0 1 .708 0l6 6a.5.5 0 0 1 0 .708l-6 6a.5.5 0 0 1-.708-.708L10.293 8 4.646 2.354a.5.5 0 0 1 0-.708"/></svg>`;

  function nav_btn(icon, handler) {
    const btn = document.createElement('button');
    btn.type      = 'button';
    btn.innerHTML = icon;
    btn.classList.add('rsv-cal-btn-nav');
    btn.addEventListener('click', handler);
    const wrap = document.createElement('div');
    wrap.appendChild(btn);
    return wrap;
  }

  function build_header(state) {
    const month_el = document.createElement('span');
    month_el.classList.add('rsv-cal-month');
    state.month_el = month_el;

    const controls = document.createElement('div');
    controls.classList.add('rsv-cal-controls');
    controls.append(
      nav_btn(ARROW_L, () => state.set_date(new Date(state.date.getFullYear(), state.date.getMonth() - 1, state.date.getDate()))),
      month_el,
      nav_btn(ARROW_R, () => state.set_date(new Date(state.date.getFullYear(), state.date.getMonth() + 1, state.date.getDate())))
    );

    const ctrl_td = document.createElement('td');
    ctrl_td.colSpan = 7;
    ctrl_td.appendChild(controls);

    const ctrl_row = document.createElement('tr');
    ctrl_row.appendChild(ctrl_td);

    const names_row = document.createElement('tr');
    day_names().forEach(name => {
      const th = document.createElement('th');
      th.textContent = name;
      names_row.appendChild(th);
    });

    const header = document.createElement('thead');
    header.classList.add('rsv-cal-header');
    header.append(ctrl_row, names_row);
    return header;
  }

  function build_body(name, on_select) {
    const tbody = document.createElement('tbody');
    tbody.classList.add('rsv-cal-grid');

    for (let y = 0; y < 6; y++) {
      const row = document.createElement('tr');
      for (let x = 0; x < 7; x++) {
        const radio = document.createElement('input');
        radio.type  = 'radio';
        radio.name  = name + '.date';
        radio.hidden = true;
        radio.addEventListener('change', on_select);

        const label = document.createElement('label');
        label.setAttribute('unselectable', 'true');

        const cell = document.createElement('td');
        cell.classList.add('rsv-cal-cell');
        cell.append(radio, label);
        row.appendChild(cell);
      }
      tbody.appendChild(row);
    }
    return tbody;
  }

  return {
    create(container, name) {
      const state = {
        date: null,
        month_el: null,
        body: null,
        container,

        set_date(date) {
          if (this.date !== null && is_same_day(date, this.date)) return;

          const month_changed = this.date === null || !is_same_month(date, this.date);
          if (month_changed) {
            const prev = this.body.querySelector('input[type="radio"]:checked');
            if (prev) prev.checked = false;
            render(this, date);
            const next = this.body.querySelector(`input[id="${date.toISOString()}"]`);
            if (next) next.checked = true;
          }

          const first    = get_first_day_of_month(date);
          const cell_idx = first + date.getDate() - 1;
          const rows     = this.body.querySelectorAll('tr');
          rows[Math.floor(cell_idx / 7)].children[cell_idx % 7].querySelector('input').checked = true;

          this.date = date;
          this.container.value = date;
          this.container.dispatchEvent(
            new InputEvent('change', { bubbles: true, cancelable: true, composed: true })
          );
        },
      };

      container.classList.add('rsv-calendar');

      const table = document.createElement('table');
      table.appendChild(build_header(state));
      table.appendChild(build_body(name, e => state.set_date(new Date(e.target.getAttribute('datetime')))));

      state.body = table.querySelector('tbody');
      container.appendChild(table);
      state.set_date(new Date());
      return state;
    },
  };
})();