import React, { useState, useMemo } from 'react';
import './Calendar.css';

const WEEKS = ['日', '一', '二', '三', '四', '五', '六'];
function fmt(dt) {
  const p = x => (x < 10 ? '0' + x : '' + x);
  return dt.getFullYear() + '-' + p(dt.getMonth() + 1) + '-' + p(dt.getDate());
}

export default function Calendar({ value = '', onChange }) {
  const now = new Date();
  const [viewY, setViewY] = useState(now.getFullYear());
  const [viewM, setViewM] = useState(now.getMonth());
  const [selected, setSelected] = useState(value);
  const today = fmt(now);

  const cells = useMemo(() => {
    const first = new Date(viewY, viewM, 1).getDay();
    const days = new Date(viewY, viewM + 1, 0).getDate();
    const prevDays = new Date(viewY, viewM, 0).getDate();
    const arr = [];
    for (let i = 0; i < first; i++) {
      let pd = prevDays - first + 1 + i, pm = viewM - 1, py = viewY;
      if (pm < 0) { pm = 11; py--; }
      arr.push({ day: pd, out: true, key: fmt(new Date(py, pm, pd)) });
    }
    for (let d = 1; d <= days; d++) {
      const k = fmt(new Date(viewY, viewM, d));
      arr.push({ day: d, out: false, today: k === today, key: k });
    }
    const tail = (7 - (arr.length % 7)) % 7;
    for (let t = 1; t <= tail; t++) {
      let nm = viewM + 1, ny = viewY; if (nm > 11) { nm = 0; ny++; }
      arr.push({ day: t, out: true, key: fmt(new Date(ny, nm, t)) });
    }
    return arr;
  }, [viewY, viewM, today]);

  function prev() { let m = viewM - 1, y = viewY; if (m < 0) { m = 11; y--; } setViewM(m); setViewY(y); }
  function next() { let m = viewM + 1, y = viewY; if (m > 11) { m = 0; y++; } setViewM(m); setViewY(y); }
  function pick(c) {
    setSelected(c.key);
    setViewY(parseInt(c.key.slice(0, 4), 10));
    setViewM(parseInt(c.key.slice(5, 7), 10) - 1);
    onChange && onChange(c.key);
  }

  return (
    <div className="aa-calendar">
      <div className="aa-calendar-head">
        <button className="aa-calendar-nav" onClick={prev}>‹</button>
        <span className="aa-calendar-title">{viewY} 年 {viewM + 1} 月</span>
        <button className="aa-calendar-nav" onClick={next}>›</button>
      </div>
      <div className="aa-calendar-week">
        {WEEKS.map(w => <span className="aa-calendar-weekday" key={w}>{w}</span>)}
      </div>
      <div className="aa-calendar-grid">
        {cells.map((c, i) => (
          <div key={i} className={'aa-calendar-cell' + (c.out ? ' is-out' : '') + (c.today ? ' is-today' : '') + (c.key === selected ? ' is-selected' : '')}
               onClick={() => pick(c)}>
            {c.day}
          </div>
        ))}
      </div>
    </div>
  );
}