function doPost(e) {
  var lock = LockService.getScriptLock();
  try {
    lock.waitLock(10000); // Tunggu antrean maksimal 10 detik agar tidak ada bentrok antar pengiriman
    var ss = SpreadsheetApp.getActiveSpreadsheet();
    var data = JSON.parse(e.postData.contents);

    // 1. Jika sinkronisasi otomatis dari Dashboard Guru
    if (data.type === 'dashboard_sync') {
      var sheetGuru = ss.getSheetByName("Pengaturan_Guru");
      if (!sheetGuru) {
        sheetGuru = ss.insertSheet("Pengaturan_Guru");
      }
      if (sheetGuru.getLastRow() == 0) {
        sheetGuru.appendRow(["ID Sync", "Waktu Sync", "Judul Ujian", "Subjudul", "Token", "Durasi (Menit)", "Jumlah Soal", "Acak Soal", "Acak Opsi", "Batas Pelanggaran", "Nilai Max", "KKM", "Total Bank Soal", "Data Lengkap (JSON)"]);
      }
      var syncId = String(data.submissionId || data.timestamp || "");
      if (syncId && sheetGuru.getLastRow() > 1) {
        var existingSyncIds = sheetGuru.getRange(2, 1, sheetGuru.getLastRow() - 1, 1).getValues();
        for (var s = 0; s < existingSyncIds.length; s++) {
          if (String(existingSyncIds[s][0]) === syncId) {
            return ContentService.createTextOutput(JSON.stringify({result: "already_exists", type: "dashboard_sync"})).setMimeType(ContentService.MimeType.JSON);
          }
        }
      }
      sheetGuru.appendRow([
        syncId,
        data.waktuSync || new Date().toLocaleString(),
        data.judul || "",
        data.subJudul || "",
        data.token || "",
        data.durasi || 0,
        data.jumlahSoal || 0,
        data.acak ? "Ya" : "Tidak",
        data.acakOpsi ? "Ya" : "Tidak",
        data.maxPelanggaran || 0,
        data.nilaiMaksimal || 100,
        data.kkm || 75,
        data.totalBankSoal || 0,
        JSON.stringify(data)
      ]);
      SpreadsheetApp.flush();
      return ContentService.createTextOutput(JSON.stringify({result: "success", type: "dashboard_sync"})).setMimeType(ContentService.MimeType.JSON);
    }

    // 2. Pengiriman nilai ujian siswa (DENGAN PROTEKSI ANTI-DUPLIKAT)
    var sheetSiswa = ss.getSheetByName("Hasil_Ujian") || ss.getActiveSheet();
    if (sheetSiswa.getLastRow() == 0) {
      sheetSiswa.appendRow(["ID Pengiriman", "Timestamp", "Nama", "Nomor Absen", "Kelas", "Nilai", "Benar", "Total Soal", "Pelanggaran", "Auto Submit", "Waktu Selesai"]);
    }

    // Periksa apakah ID Pengiriman sudah ada untuk mencegah data dobel
    var subId = String(data.submissionId || "");
    if (subId && sheetSiswa.getLastRow() > 1) {
      var existingIds = sheetSiswa.getRange(2, 1, sheetSiswa.getLastRow() - 1, 1).getValues();
      for (var i = 0; i < existingIds.length; i++) {
        if (String(existingIds[i][0]) === subId) {
          return ContentService.createTextOutput(JSON.stringify({result: "already_exists", type: "hasil_ujian"})).setMimeType(ContentService.MimeType.JSON);
        }
      }
    }

    sheetSiswa.appendRow([
      subId,
      data.waktuSelesai || new Date(),
      data.nama || "",
      data.nomor || "",
      data.kelas || "",
      data.nilai || 0,
      data.benar || 0,
      data.totalSoal || 0,
      data.pelanggaran || 0,
      data.autoSubmit ? "Ya" : "Tidak",
      data.waktuSelesai || ""
    ]);
    SpreadsheetApp.flush();
    return ContentService.createTextOutput(JSON.stringify({result: "success", type: "hasil_ujian"})).setMimeType(ContentService.MimeType.JSON);
  } catch(error) {
    return ContentService.createTextOutput(JSON.stringify({result: "error", error: error.toString()})).setMimeType(ContentService.MimeType.JSON);
  } finally {
    lock.releaseLock();
  }
}