export const MONTH_NAMES = [
  "January",
  "February",
  "March",
  "April",
  "May",
  "June",
  "July",
  "August",
  "September",
  "October",
  "November",
  "December",
];

export function pad2(n: number): string {
  return String(n).padStart(2, "0");
}

export function todayIso(): string {
  const d = new Date();
  return `${d.getFullYear()}-${pad2(d.getMonth() + 1)}-${pad2(d.getDate())}`;
}

export function fmtDate(iso: string): string {
  if (!/^\d{4}-\d{2}-\d{2}$/.test(iso)) {
    return "";
  }
  const [y, m, d] = iso.split("-").map(Number);
  return `${MONTH_NAMES[m - 1].slice(0, 3)} ${d}, ${y}`;
}

export function addDaysIso(iso: string, days: number): string {
  const d = new Date(`${iso}T12:00:00`);
  d.setDate(d.getDate() + days);
  return `${d.getFullYear()}-${pad2(d.getMonth() + 1)}-${pad2(d.getDate())}`;
}

export function mondayOf(iso: string): string {
  const d = new Date(`${iso}T12:00:00`);
  const dow = d.getDay();
  d.setDate(d.getDate() + (dow === 0 ? -6 : 1 - dow));
  return `${d.getFullYear()}-${pad2(d.getMonth() + 1)}-${pad2(d.getDate())}`;
}

export function weeksOfMonth(y: number, m: number): string[] {
  const firstIso = `${y}-${pad2(m)}-01`;
  const lastIso = `${y}-${pad2(m)}-${pad2(new Date(y, m, 0).getDate())}`;
  const out: string[] = [];
  for (let mon = mondayOf(firstIso); mon <= lastIso; mon = addDaysIso(mon, 7)) {
    if (addDaysIso(mon, 4) >= firstIso) {
      out.push(mon);
    }
  }
  return out;
}

export function weekRangeLabel(mondayIso: string): string {
  if (!/^\d{4}-\d{2}-\d{2}$/.test(mondayIso)) {
    return "";
  }
  const fri = addDaysIso(mondayIso, 4);
  const [, sm, sd] = mondayIso.split("-").map(Number);
  const [, em, ed] = fri.split("-").map(Number);
  return `${MONTH_NAMES[sm - 1].slice(0, 3)} ${sd} – ${MONTH_NAMES[em - 1].slice(0, 3)} ${ed}`;
}

export function fmtWeekRange(startIso: string): string {
  if (!/^\d{4}-\d{2}-\d{2}$/.test(startIso)) {
    return fmtDate(startIso);
  }
  return weekRangeLabel(startIso);
}

/** Short date for ranges: "Jun 8". */
export function fmtShortDate(iso: string): string {
  if (!iso || !/^\d{4}-\d{2}-\d{2}$/.test(iso)) {
    return "—";
  }
  const [, m, d] = iso.split("-").map(Number);
  return `${MONTH_NAMES[m - 1].slice(0, 3)} ${d}`;
}

/** "Jun 8 – Jun 12" for a Schedule On range. */
export function fmtDateRange(start: string, end: string): string {
  return `${fmtShortDate(start)} – ${fmtShortDate(end)}`;
}

/** Inclusive day count between two ISO dates (order-independent). */
export function rangeDayCount(a: string, b: string): number {
  const d1 = new Date(`${a}T12:00:00`).getTime();
  const d2 = new Date(`${b}T12:00:00`).getTime();
  return Math.abs(Math.round((d2 - d1) / 86400000)) + 1;
}

export function fmt12(t: string): string {
  const [h, m] = t.split(":").map(Number);
  const ampm = h < 12 ? "AM" : "PM";
  const h12 = h === 0 ? 12 : h > 12 ? h - 12 : h;
  return `${h12}:${String(m).padStart(2, "0")} ${ampm}`;
}

export function calcDuration(s: string, e: string): string {
  const [sh, sm] = s.split(":").map(Number);
  const [eh, em] = e.split(":").map(Number);
  const mins = eh * 60 + em - (sh * 60 + sm);
  if (mins <= 0) {
    return "";
  }
  const h = Math.floor(mins / 60);
  const m = mins % 60;
  if (h && m) {
    return `${h}h ${m}m`;
  }
  if (h) {
    return `${h}h`;
  }
  return `${m}m`;
}

export function formatApptRange(start: string, end: string): string {
  if (!start) {
    return "—";
  }
  return end ? `${fmt12(start)} – ${fmt12(end)}` : fmt12(start);
}

export const TIME_SLOTS: string[] = (() => {
  const out: string[] = [];
  for (let m = 5 * 60; m <= 22 * 60; m += 30) {
    const h = Math.floor(m / 60);
    const mm = m % 60;
    out.push(`${String(h).padStart(2, "0")}:${String(mm).padStart(2, "0")}`);
  }
  return out;
})();

export function parseApptTime(appt: string): { start: string; end: string } {
  const m = appt.match(/(\d{1,2}):(\d{2})\s*[–-]\s*(\d{1,2}):(\d{2})/);
  if (m) {
    return {
      start: `${m[1].padStart(2, "0")}:${m[2]}`,
      end: `${m[3].padStart(2, "0")}:${m[4]}`,
    };
  }
  const single = appt.match(/(\d{1,2}):(\d{2})\s*(AM|PM)/i);
  if (single) {
    let h = Number(single[1]) % 12;
    if (/pm/i.test(single[3])) {
      h += 12;
    }
    return { start: `${pad2(h)}:${single[2]}`, end: "" };
  }
  const startOnly = appt.trim().match(/^(\d{1,2}):(\d{2})$/);
  if (startOnly) {
    return {
      start: `${startOnly[1].padStart(2, "0")}:${startOnly[2]}`,
      end: "",
    };
  }
  return { start: "", end: "" };
}

export function formatApptTimeValue(start: string, end: string): string {
  if (!start) {
    return "";
  }
  return end ? `${start} – ${end}` : start;
}

export function fmtWeekOf(mondayIso: string): string {
  if (!mondayIso) {
    return "";
  }
  return `Week of ${weekRangeLabel(mondayIso)}`;
}