// CSV roster import parsing — Phase 5A §29. Deliberately hand-rolled, not a CSV library: the
// expected input is simple (no quoted fields containing commas expected in practice for
// names/identifiers), and pulling in a dependency for a three-column parser isn't justified
// (ponytail: stdlib/hand-rolled first). Validates the WHOLE file and returns every row-level
// error at once — the caller decides whether to import (transactional, all-or-nothing).

const REQUIRED_HEADERS = ['display_name'];
const KNOWN_HEADERS = ['display_name', 'email', 'login_identifier', 'cohort_id'];
const FORBIDDEN_HEADERS = ['password', 'password_hash', 'credential_token_hash']; // never accepted from a file — see admin-routes.js's import route comment

function splitCsvLine(line) {
  // No quoted-comma support needed for this column set (names/identifiers/cohort slugs don't
  // contain commas in practice) — a plain split covers the real input shape.
  return line.split(',').map((cell) => cell.trim());
}

function parseStudentCsv(csv) {
  const lines = csv.split(/\r?\n/).map((l) => l.trim()).filter((l) => l.length > 0);
  const errors = [];
  if (!lines.length) return { rows: [], errors: [{ row: 0, message: 'CSV is empty' }] };

  const headers = splitCsvLine(lines[0]).map((h) => h.toLowerCase());
  for (const required of REQUIRED_HEADERS) {
    if (!headers.includes(required)) errors.push({ row: 0, message: `missing required column: ${required}` });
  }
  for (const forbidden of FORBIDDEN_HEADERS) {
    if (headers.includes(forbidden)) errors.push({ row: 0, message: `column '${forbidden}' is never accepted from a file — credentials are always generated server-side` });
  }
  const unknown = headers.filter((h) => !KNOWN_HEADERS.includes(h));
  if (unknown.length) errors.push({ row: 0, message: `unknown column(s): ${unknown.join(', ')}` });
  if (errors.length) return { rows: [], errors };

  const seenEmails = new Set();
  const seenIdentifiers = new Set();
  const rows = [];
  for (let i = 1; i < lines.length; i++) {
    const rowNum = i + 1; // 1-indexed with header as row 1, matching what a spreadsheet user sees
    const cells = splitCsvLine(lines[i]);
    if (cells.length !== headers.length) {
      errors.push({ row: rowNum, message: `expected ${headers.length} columns, got ${cells.length}` });
      continue;
    }
    const raw = Object.fromEntries(headers.map((h, idx) => [h, cells[idx]]));
    if (!raw.display_name) {
      errors.push({ row: rowNum, message: 'display_name is required' });
      continue;
    }
    const email = raw.email ? raw.email.toLowerCase() : null;
    const loginIdentifier = raw.login_identifier || null;
    if (email && !/^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email)) {
      errors.push({ row: rowNum, message: `invalid email: ${raw.email}` });
      continue;
    }
    if (email && seenEmails.has(email)) {
      errors.push({ row: rowNum, message: `duplicate email within this file: ${email}` });
      continue;
    }
    if (loginIdentifier && seenIdentifiers.has(loginIdentifier)) {
      errors.push({ row: rowNum, message: `duplicate login_identifier within this file: ${loginIdentifier}` });
      continue;
    }
    if (email) seenEmails.add(email);
    if (loginIdentifier) seenIdentifiers.add(loginIdentifier);
    rows.push({ display_name: raw.display_name, email, login_identifier: loginIdentifier, cohort_id: raw.cohort_id || null });
  }
  if (errors.length) return { rows: [], errors };
  return { rows, errors: [] };
}

module.exports = { parseStudentCsv };