function onOpen() {
  const ui = SpreadsheetApp.getUi();
  ui.createMenu('CMS Document Generation')
    .addItem('Generate document for current row', 'generateForCurrentRow')
    .addToUi();
}

type DocSection =
  | GoogleAppsScript.Document.Body
  | GoogleAppsScript.Document.HeaderSection
  | GoogleAppsScript.Document.FooterSection;

function copyElement(
  source_element: GoogleAppsScript.Document.Element,
  dest: DocSection,
  index?: number
) {
  const element = source_element.copy();
  // based on https://stackoverflow.com/questions/6783819/google-app-script-copy-document-page
  if (element.getType() == DocumentApp.ElementType.PARAGRAPH)
    if (index !== undefined) dest.insertParagraph(index, element.asParagraph());
    else dest.appendParagraph(element.asParagraph());
  else if (element.getType() == DocumentApp.ElementType.TABLE)
    if (index !== undefined) dest.insertTable(index, element.asTable());
    else dest.appendTable(element.asTable());
  else if (element.getType() == DocumentApp.ElementType.LIST_ITEM)
    if (index !== undefined) dest.insertListItem(index, element.asListItem());
    else dest.appendListItem(element.asListItem());
  else if (element.getType() == DocumentApp.ElementType.INLINE_IMAGE)
    if (index !== undefined) dest.insertImage(index, element.asInlineImage());
    else dest.appendImage(element.asInlineImage());
  else
    throw new Error(
      "According to the doc this type couldn't appear in the body: " +
        element.getType()
    );
}

function copySection(source: DocSection, dest: DocSection, index?: number) {
  const totalElements = source.getNumChildren();
  for (let j = 0; j < totalElements; ++j) {
    if (index) copyElement(source.getChild(j), dest, index + j);
    else copyElement(source.getChild(j), dest);
  }
}

function spreadsheetRowToObject(
  sheet: GoogleAppsScript.Spreadsheet.Sheet,
  rownum: number
): { [key: string]: any } {
  // TODO: could be more efficient
  const values = sheet.getDataRange().getValues();
  const headers = values[0];
  const row_data = values[rownum - 1];

  return headers.reduce((acc, header, index) => {
    acc[String(header)] = row_data[index];
    return acc;
  }, {});
}

function generateForCurrentRow() {
  const spreadsheet = SpreadsheetApp.getActive();
  const cell = spreadsheet.getCurrentCell();
  if (!cell) throw new Error('No Cell selected for operation on row');

  const row = spreadsheetRowToObject(
    spreadsheet.getActiveSheet(),
    cell.getRow()
  );

  const template_doc = DocumentApp.openById(row['Template ID']);

  const source_file = DriveApp.getFileById(row['Document ID']);
  const out_folder = DriveApp.getFolderById(
    '1ROyJXk-QANTHM6Jiw0ne3EQiR1f2UsLr'
  );

  const out_file = source_file.makeCopy(
    row['Document Name'] + row['Version'],
    out_folder
  );
  const out_doc = DocumentApp.openById(out_file.getId());

  // Copy header
  out_doc.getHeader().clear();
  copySection(template_doc.getHeader(), out_doc.getHeader());

  // Copy footer
  out_doc.getFooter().clear();
  copySection(template_doc.getFooter(), out_doc.getFooter());

  const insert_point = template_doc
    .getBody()
    .findText('{{body}}')
    ?.getElement();

  if (!insert_point) {
    throw new Error("Could not find insert point '{{body}}'");
  }

  // find the parent element that is a direct descendant of the body
  let parent = insert_point;
  while (parent.getParent().getType() != DocumentApp.ElementType.BODY_SECTION) {
    parent = parent.getParent();
  }
  const index = template_doc.getBody().getChildIndex(parent);

  // Copy body contents above and below {{body}} tag
  for (let j = 0; j < template_doc.getBody().getNumChildren(); j++) {
    if (j < index)
      copyElement(template_doc.getBody().getChild(j), out_doc.getBody(), j);
    // don't copy {{body}} tag
    else if (j == index) continue;
    else copyElement(template_doc.getBody().getChild(j), out_doc.getBody());
  }

  // do text replacement
  Object.entries(row).forEach(([header, data]) => {
    out_doc.getBody().replaceText(`{{${header}}}`, String(data));
    out_doc.getHeader().replaceText(`{{${header}}}`, String(data));
    out_doc.getFooter().replaceText(`{{${header}}}`, String(data));
  });

  out_doc.saveAndClose();

  // create PDF file
  out_folder.createFile(out_file.getAs('application/pdf'));
}