/**
 * Read a spreadsheet without any library.
 *
 * .xlsx is a ZIP of XML, so this walks the ZIP central directory, inflates the
 * two parts that matter with the browser's own DecompressionStream, and pulls
 * the cells out of the XML. CSV is handled too, for anyone who would rather
 * save as CSV or is on an older browser.
 */

const dec = new TextDecoder();

// ------------------------------------------------------------------- zip

function findEOCD(buf) {
  // End-of-central-directory: 'PK\5\6', within the last 64KB.
  for (let i = buf.length - 22; i >= Math.max(0, buf.length - 65558); i--) {
    if (buf[i] === 0x50 && buf[i + 1] === 0x4b && buf[i + 2] === 0x05 && buf[i + 3] === 0x06) return i;
  }
  return -1;
}

function listEntries(buf) {
  const eocd = findEOCD(buf);
  if (eocd < 0) throw new Error('not a zip file');
  const dv = new DataView(buf.buffer, buf.byteOffset, buf.byteLength);
  const count = dv.getUint16(eocd + 10, true);
  let p = dv.getUint32(eocd + 16, true);

  const entries = new Map();
  for (let i = 0; i < count; i++) {
    if (dv.getUint32(p, true) !== 0x02014b50) break;
    const method = dv.getUint16(p + 10, true);
    const compressedSize = dv.getUint32(p + 20, true);
    const nameLen = dv.getUint16(p + 28, true);
    const extraLen = dv.getUint16(p + 30, true);
    const commentLen = dv.getUint16(p + 32, true);
    const localOff = dv.getUint32(p + 42, true);
    const name = dec.decode(buf.subarray(p + 46, p + 46 + nameLen));
    entries.set(name, { method, compressedSize, localOff });
    p += 46 + nameLen + extraLen + commentLen;
  }
  return entries;
}

async function readEntry(buf, entry) {
  const dv = new DataView(buf.buffer, buf.byteOffset, buf.byteLength);
  const { localOff } = entry;
  if (dv.getUint32(localOff, true) !== 0x04034b50) throw new Error('bad zip entry');
  const nameLen = dv.getUint16(localOff + 26, true);
  const extraLen = dv.getUint16(localOff + 28, true);
  const start = localOff + 30 + nameLen + extraLen;
  const data = buf.subarray(start, start + entry.compressedSize);

  if (entry.method === 0) return dec.decode(data);
  if (entry.method !== 8) throw new Error(`unsupported compression (${entry.method})`);
  if (typeof DecompressionStream !== 'function') {
    throw new Error('this browser cannot open .xlsx files — save the sheet as CSV instead');
  }
  const stream = new Blob([data]).stream().pipeThrough(new DecompressionStream('deflate-raw'));
  return dec.decode(new Uint8Array(await new Response(stream).arrayBuffer()));
}

// ----------------------------------------------------------------- xlsx

const colIndex = (ref) => {
  let n = 0;
  for (const ch of ref) {
    const c = ch.charCodeAt(0);
    if (c < 65 || c > 90) break;
    n = n * 26 + (c - 64);
  }
  return n - 1;
};

function parseXml(text) {
  const doc = new DOMParser().parseFromString(text, 'application/xml');
  if (doc.querySelector('parsererror')) throw new Error('could not read the spreadsheet XML');
  return doc;
}

/** @returns {Promise<string[][]>} rows of plain strings from the first sheet */
export async function readXlsx(bytes) {
  const entries = listEntries(bytes);
  const sheetName = entries.has('xl/worksheets/sheet1.xml')
    ? 'xl/worksheets/sheet1.xml'
    : [...entries.keys()].find((n) => /^xl\/worksheets\/.*\.xml$/.test(n));
  if (!sheetName) throw new Error('no worksheet found in that file');

  let shared = [];
  if (entries.has('xl/sharedStrings.xml')) {
    const doc = parseXml(await readEntry(bytes, entries.get('xl/sharedStrings.xml')));
    shared = [...doc.getElementsByTagName('si')].map((si) =>
      [...si.getElementsByTagName('t')].map((t) => t.textContent).join(''));
  }

  const doc = parseXml(await readEntry(bytes, entries.get(sheetName)));
  const rows = [];
  for (const row of doc.getElementsByTagName('row')) {
    const cells = [];
    for (const c of row.getElementsByTagName('c')) {
      const type = c.getAttribute('t');
      const ref = c.getAttribute('r') || '';
      const at = ref ? colIndex(ref) : cells.length;
      let value = '';
      if (type === 'inlineStr') {
        value = [...c.getElementsByTagName('t')].map((t) => t.textContent).join('');
      } else {
        const v = c.getElementsByTagName('v')[0];
        const raw = v ? v.textContent : '';
        value = type === 's' ? (shared[Number(raw)] ?? '') : raw;
      }
      while (cells.length < at) cells.push('');
      cells[at] = (value ?? '').trim();
    }
    rows.push(cells);
  }
  return rows;
}

// ------------------------------------------------------------------ csv

export function readCsv(text) {
  const rows = [];
  let row = [], field = '', quoted = false;
  for (let i = 0; i < text.length; i++) {
    const ch = text[i];
    if (quoted) {
      if (ch === '"') {
        if (text[i + 1] === '"') { field += '"'; i++; } else quoted = false;
      } else field += ch;
      continue;
    }
    if (ch === '"') quoted = true;
    else if (ch === ',') { row.push(field.trim()); field = ''; }
    else if (ch === '\n') { row.push(field.trim()); rows.push(row); row = []; field = ''; }
    else if (ch !== '\r') field += ch;
  }
  if (field || row.length) { row.push(field.trim()); rows.push(row); }
  return rows;
}

// -------------------------------------------------------------- mapping

const norm = (s) => String(s ?? '').toLowerCase().replace(/[^a-z]/g, '');

/**
 * Work out which columns hold the controller name and the sign text.
 * The template that ships with these buses uses
 * `Line | Line Name | Description | Content (Display)`.
 */
export function mapRows(rows) {
  let company = '';
  for (const r of rows.slice(0, 5)) {
    const i = r.findIndex((c) => norm(c).startsWith('companyname'));
    if (i >= 0) { company = (r[i + 1] || '').trim(); break; }
  }

  const headerAt = rows.findIndex((r) => r.some((c) => {
    const n = norm(c);
    return n === 'linename' || n === 'content' || n.startsWith('contentdisplay') || n === 'destination';
  }));

  let nameCol = -1, textCol = -1, start = 0;
  if (headerAt >= 0) {
    const head = rows[headerAt].map(norm);
    // Search by priority, not by column order: the shipped template has both a
    // "Line" (a row number) and a "Line Name", and the name must win.
    const pick = (...wanted) => {
      for (const w of wanted) { const i = head.indexOf(w); if (i >= 0) return i; }
      return -1;
    };
    nameCol = pick('linename', 'name', 'destinationname', 'route', 'line');
    textCol = pick('contentdisplay', 'content', 'display', 'destination', 'text', 'message');
    start = headerAt + 1;
  }

  const out = [];
  for (const r of rows.slice(start)) {
    if (!r.length || r.every((c) => !c)) continue;
    const cells = r.filter((c) => c !== '');
    let name = nameCol >= 0 ? (r[nameCol] || '') : '';
    let text = textCol >= 0 ? (r[textCol] || '') : '';
    if (!name && !text) {                      // no recognisable header — take what is there
      text = cells[cells.length - 1] || '';
      name = cells.length > 1 ? cells[0] : text;
    }
    if (!text) text = name;
    // A bare row number is a line code, not something worth showing a driver.
    if (/^\d+$/.test(name) && text && text !== name) name = text;
    if (!name) name = text;
    if (!text.trim()) continue;
    if (norm(name) === 'linename' || norm(text).startsWith('contentdisplay')) continue;
    out.push({ name: name.trim().slice(0, 16), text: text.trim() });
  }
  return { company, destinations: out };
}