// Phase 5B §3/§10/§14 — enrollment lifecycle. A student's academic access to a program,
// separate from cohort membership (cohort_enrollments, untouched). All status transitions are
// server-side only — nothing here accepts a client-supplied status.
//
// Errors are thrown as `Error` with a `.code` the route layer maps to an HTTP status, the same
// convention classroom.js established in Phase 5A (NOT_FOUND/INVALID_STATE/ALREADY_LIVE).

function err(code, message) {
  return Object.assign(new Error(message || code), { code });
}

// Pure and derived-at-read-time, matching classroom.js's derivePresence — 'expired' is never
// written to the database by a background sweep; it's computed here from access_ends_at
// whenever an enrollment is read, so there's no stale/out-of-sync value to worry about. The
// one status this function can "see" that the stored column doesn't is exactly that: whether a
// still-`active` row has actually run out its access window.
function deriveEnrollmentAccess(enrollment, now = new Date()) {
  const { status, starts_at: startsAt, access_ends_at: accessEndsAt } = enrollment;
  if (status === 'cancelled') return { effective_status: 'cancelled', can_launch: false, reason: 'cancelled' };
  if (status === 'completed') return { effective_status: 'completed', can_launch: false, reason: 'completed' };
  if (status === 'pending') return { effective_status: 'pending', can_launch: false, reason: 'pending_activation' };
  // status === 'active' below.
  if (accessEndsAt && new Date(accessEndsAt) <= now) {
    return { effective_status: 'expired', can_launch: false, reason: 'access_expired' };
  }
  if (startsAt && new Date(startsAt) > now) {
    return { effective_status: 'active', can_launch: false, reason: 'not_yet_started' };
  }
  return { effective_status: 'active', can_launch: true, reason: null };
}

async function getEnrollment(pool, enrollmentId) {
  const { rows } = await pool.query(`SELECT * FROM enrollments WHERE enrollment_id = $1`, [enrollmentId]);
  return rows[0] || null;
}

async function listEnrollmentsForStudent(pool, studentId) {
  const { rows } = await pool.query(
    `SELECT e.*, p.name AS program_name, p.family AS program_family
     FROM enrollments e JOIN programs p ON p.program_id = e.program_id
     WHERE e.student_id = $1 ORDER BY e.enrolled_at DESC`,
    [studentId]
  );
  return rows;
}

async function listEnrollmentsForProgram(pool, programId) {
  const { rows } = await pool.query(
    `SELECT e.*, s.display_name AS student_display_name FROM enrollments e
     JOIN students s ON s.student_id = e.student_id
     WHERE e.program_id = $1 ORDER BY e.enrolled_at DESC`,
    [programId]
  );
  return rows;
}

// Admin manual enrollment (§11) — the program is snapshotted into the new row exactly the way
// createFromIntegrationEvent below does, so both creation paths produce identically-shaped
// enrollments. Throws PROGRAM_NOT_FOUND / PROGRAM_INACTIVE / ALREADY_ENROLLED (the DB's
// one-open-enrollment-per-program constraint, translated to a clean error instead of a raw
// 23505).
async function createManualEnrollment(pool, { studentId, programId, status, startsAt, accessEndsAt, deliveryMode, externalReference, metadata, createdBy }) {
  const { rows: programRows } = await pool.query(`SELECT * FROM programs WHERE program_id = $1`, [programId]);
  const program = programRows[0];
  if (!program) throw err('PROGRAM_NOT_FOUND', 'No such program.');
  if (program.status !== 'active') throw err('PROGRAM_INACTIVE', 'This program is not currently active.');
  const initialStatus = status === 'active' ? 'active' : 'pending';
  try {
    const { rows } = await pool.query(
      `INSERT INTO enrollments (student_id, program_id, delivery_mode, program_version, curriculum_path, status, source, external_reference, starts_at, access_ends_at, activated_at, metadata, created_by)
       VALUES ($1, $2, $3, $4, $5, $6, 'admin', $7, $8, $9, $10, $11, $12) RETURNING *`,
      [studentId, programId, deliveryMode || program.delivery_mode, program.version, program.curriculum_path, initialStatus,
        externalReference || null, startsAt || null, accessEndsAt || null, initialStatus === 'active' ? new Date() : null,
        metadata ? JSON.stringify(metadata) : null, createdBy || null]
    );
    return rows[0];
  } catch (e) {
    if (e.code === '23505') throw err('ALREADY_ENROLLED', 'This student already has an open enrollment in this program.');
    throw e;
  }
}

// §12/§14 — the website/integration boundary's enrollment creation. Idempotent by construction:
// the DB-level unique index on external_event_id (not a SELECT-then-INSERT check) is what
// actually makes a concurrent duplicate submission safe, not just a sequential retry. Student
// resolution is create-or-match (§13): an existing student (matched by email OR
// login_identifier) is reused; no match creates a new one via the exact same credential/token
// machinery admin-created students already use (students-auth.js) — this endpoint is a
// trusted, authenticated, server-to-server caller, the same trust level an instructor's
// "add student" admin action already has, not a public signup form.
async function createFromIntegrationEvent(pool, studentsAuth, { eventId, source, studentEmail, studentLoginIdentifier, studentDisplayName, programId, status }) {
  if (!eventId) throw err('EVENT_ID_REQUIRED', 'event_id is required for idempotent enrollment events.');
  const { rows: programRows } = await pool.query(`SELECT * FROM programs WHERE program_id = $1`, [programId]);
  const program = programRows[0];
  if (!program) throw err('PROGRAM_NOT_FOUND', 'Unknown program_slug.');
  if (program.status !== 'active') throw err('PROGRAM_INACTIVE', 'This program is not currently active.');

  // Replay of an already-processed event — return the original result, create nothing.
  if (eventId) {
    const { rows: existing } = await pool.query(`SELECT * FROM enrollments WHERE external_event_id = $1`, [eventId]);
    if (existing[0]) return { enrollment: existing[0], created: false };
  }

  const student = await resolveOrCreateStudent(pool, studentsAuth, { studentEmail, studentLoginIdentifier, studentDisplayName });
  const initialStatus = status === 'active' ? 'active' : 'pending';

  try {
    const { rows } = await pool.query(
      `INSERT INTO enrollments (student_id, program_id, delivery_mode, program_version, curriculum_path, status, source, external_event_id, activated_at)
       VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9)
       ON CONFLICT (external_event_id) WHERE external_event_id IS NOT NULL DO NOTHING
       RETURNING *`,
      [student.student_id, programId, program.delivery_mode, program.version, program.curriculum_path, initialStatus,
        source || 'integration', eventId || null, initialStatus === 'active' ? new Date() : null]
    );
    if (rows[0]) return { enrollment: rows[0], created: true, student };
    // Lost the ON CONFLICT race (concurrent identical replay) — the winner's row is now there.
    const { rows: won } = await pool.query(`SELECT * FROM enrollments WHERE external_event_id = $1`, [eventId]);
    return { enrollment: won[0], created: false, student };
  } catch (e) {
    // The one-open-enrollment-per-program index fired instead — a genuinely new event for a
    // student who already has an open enrollment in this program (e.g. a retried purchase with
    // a different event id). Idempotent in spirit even though it's not a literal event replay:
    // return the existing open enrollment rather than erroring or creating a second one.
    if (e.code === '23505') {
      const { rows: existingOpen } = await pool.query(
        `SELECT * FROM enrollments WHERE student_id = $1 AND program_id = $2 AND status IN ('pending','active')`,
        [student.student_id, programId]
      );
      if (existingOpen[0]) return { enrollment: existingOpen[0], created: false, student };
    }
    throw e;
  }
}

// Matches by email OR login_identifier; creates via students-auth's own credential-issuing path
// on no match (an activation token is generated exactly like an admin-created student, and
// handed back so a future email/notification step — not built in this phase — could relay it).
// A second concurrent call resolving the "same new student" case falls back to re-matching
// after a unique-constraint hit, the identical insert/on-conflict-reselect pattern used above.
async function resolveOrCreateStudent(pool, studentsAuth, { studentEmail, studentLoginIdentifier, studentDisplayName }) {
  const identifier = studentEmail || studentLoginIdentifier;
  if (!identifier) throw err('STUDENT_IDENTIFIER_REQUIRED', 'student.email or student.login_identifier is required.');
  const existing = await studentsAuth.getStudentByIdentifier(pool, identifier);
  if (existing) return existing;

  const { rows: countRows } = await pool.query('SELECT count(*)::int AS n FROM students');
  const studentId = `student-${String(countRows[0].n + 1).padStart(3, '0')}`;
  const displayName = studentDisplayName || studentEmail || studentLoginIdentifier;
  try {
    const { rows } = await pool.query(
      `INSERT INTO students (student_id, display_name, email, login_identifier) VALUES ($1, $2, $3, $4) RETURNING *`,
      [studentId, displayName, studentEmail ? String(studentEmail).toLowerCase().trim() : null, studentLoginIdentifier ? String(studentLoginIdentifier).trim() : null]
    );
    await studentsAuth.issueCredentialToken(pool, studentId, 'activation'); // token itself isn't returned by the integration endpoint — see docs/WEBSITE_ENROLLMENT_INTEGRATION.md
    return rows[0];
  } catch (e) {
    if (e.code === '23505') {
      const reMatched = await studentsAuth.getStudentByIdentifier(pool, identifier);
      if (reMatched) return reMatched;
    }
    throw e;
  }
}

async function activateEnrollment(pool, enrollmentId) {
  const { rows } = await pool.query(
    `UPDATE enrollments SET status = 'active', activated_at = now(), updated_at = now()
     WHERE enrollment_id = $1 AND status = 'pending' RETURNING *`,
    [enrollmentId]
  );
  if (!rows[0]) {
    const existing = await getEnrollment(pool, enrollmentId);
    throw err(existing ? 'INVALID_STATE' : 'NOT_FOUND', existing ? 'Only a pending enrollment can be activated.' : 'No such enrollment.');
  }
  return rows[0];
}

async function cancelEnrollment(pool, enrollmentId) {
  const { rows } = await pool.query(
    `UPDATE enrollments SET status = 'cancelled', cancelled_at = now(), updated_at = now()
     WHERE enrollment_id = $1 AND status IN ('pending', 'active') RETURNING *`,
    [enrollmentId]
  );
  if (!rows[0]) {
    const existing = await getEnrollment(pool, enrollmentId);
    throw err(existing ? 'INVALID_STATE' : 'NOT_FOUND', existing ? 'Only a pending or active enrollment can be cancelled.' : 'No such enrollment.');
  }
  return rows[0];
}

// §10 — completion is entirely server-derived: called opportunistically (right after a
// capstone/exam finalizes, and again as a safety-net on every program-status read) rather than
// via a client "mark complete" action. Idempotent — the `WHERE status = 'active'` guard means
// calling this again after an enrollment is already completed is a no-op, not an error.
async function maybeCompleteEnrollment(pool, studentId, programId = null) {
  const { rows: candidates } = await pool.query(
    `SELECT e.enrollment_id, e.program_id, p.completion_assessment_id FROM enrollments e
     JOIN programs p ON p.program_id = e.program_id
     WHERE e.student_id = $1 AND e.status = 'active' AND p.completion_assessment_id IS NOT NULL
       ${programId ? 'AND e.program_id = $2' : ''}`,
    programId ? [studentId, programId] : [studentId]
  );
  const completed = [];
  for (const candidate of candidates) {
    const { rowCount } = await pool.query(
      `SELECT 1 FROM achievements
       WHERE student_id = $1 AND achievement_type = 'capstone_completed' AND metadata->>'assessment_definition_id' = $2
       LIMIT 1`,
      [studentId, candidate.completion_assessment_id]
    );
    if (!rowCount) continue;
    const { rows } = await pool.query(
      `UPDATE enrollments SET status = 'completed', completed_at = now(), updated_at = now()
       WHERE enrollment_id = $1 AND status = 'active' RETURNING *`,
      [candidate.enrollment_id]
    );
    if (rows[0]) completed.push(rows[0]);
  }
  return completed;
}

module.exports = {
  deriveEnrollmentAccess, getEnrollment, listEnrollmentsForStudent, listEnrollmentsForProgram,
  createManualEnrollment, createFromIntegrationEvent, activateEnrollment, cancelEnrollment,
  maybeCompleteEnrollment,
};