/**
 * Global configuration object for script settings.
 * @type {Object}
 * @property {string} label - The Gmail label used to filter emails.
 * @property {string} spreadsheetId - The ID of the target Google Spreadsheet.
 * @property {string} sheetName - The name of the sheet in the spreadsheet where data will be appended.
 * @property {string} emailQuery - The Gmail search query used to find relevant emails, update as needed.
 * @property {Object} emailFields - Configuration for parsing fields from email body.
 * @property {Object} emailFields.field1 - Configuration for the first field, with start and end delimiters.
 * @property {string} emailFields.field1.start - The start delimiter for field1 in the email body.
 * @property {string} emailFields.field1.end - The end delimiter for field1 in the email body.
 */
const config = {
  label: "LabelName",
  spreadsheetId: "YOUR_SPREADSHEET_ID",
  sheetName: "Sheet1",
  emailQuery: "label:LabelName is:unread",
  emailFields: {
    field: {
      start: "Start:",
      end: "End:"
    }
  }
};

/**
 * Main function that processes emails and appends parsed data to the spreadsheet.
 */
function processEmailsToSpreadsheet() {
  try {
    const emails = getEmails_();
    if (emails.length === 0) {
      console.info("No emails found.");
      return;
    }
    const parsedData = emails.map(parseEmail_).filter(Boolean);
    if (parsedData.length > 0) {
      appendDataToSheet_(parsedData);
    } else {
      console.info("No valid data to append to the sheet.");
    }
  } catch (error) {
    console.error(`Error in processing emails: ${error.message}`);
  }
}

/**
 * Retrieves emails based on the search query defined in the config.
 * @returns {GmailMessage[]} An array of Gmail messages that match the query.
 * @private
 */
function getEmails_() {
  try {
    const threads = GmailApp.search(config.emailQuery);
    return threads.flatMap((thread) => thread.getMessages());
  } catch (error) {
    console.error(`Error fetching emails: ${error.message}`);
    return [];
  }
}

/**
 * Parses the body of an email to extract fields defined in the config.
 * @param {GmailMessage} email - The email object to parse.
 * @returns {Object|null} An object containing the extracted data fields, keyed by field names defined in the config, or null if parsing fails.
 * @private
 */
function parseEmail_(email) {
  try {
    const body = email.getBody();
    const { start, end } = config.emailFields.field;
    const value = extractField_(body, start, end);
    return value
      ? {
          field: value
        }
      : null;
  } catch (error) {
    console.error(`Error parsing email: ${error.message}`);
    return null;
  }
}

/**
 * Extracts a specific field from the email body using start and end delimiters.
 * @param {string} body - The body of the email.
 * @param {string} start - The start delimiter for the field.
 * @param {string} end - The end delimiter for the field.
 * @returns {string|null} The extracted field value trimmed of leading and trailing whitespace, or null if extraction fails.
 * @private
 */
function extractField_(body, start, end) {
  try {
    const startIndex = body.indexOf(start);
    if (startIndex === -1) return null;
    const endIndex = body.indexOf(end, startIndex + start.length);
    if (endIndex === -1) return null;
    return body.substring(startIndex + start.length, endIndex).trim();
  } catch (error) {
    console.error(`Error extracting field: ${error.message}`);
    return null;
  }
}

/**
 * Appends an array of parsed data objects to the target spreadsheet.
 * @param {Object[]} data - An array of objects, each representing parsed email data, where each object’s values are appended as rows in the spreadsheet.
 * @private
 */
function appendDataToSheet_(data) {
  try {
    const sheet = SpreadsheetApp.openById(config.spreadsheetId).getSheetByName(
      config.sheetName
    );
    const values = data.map((item) => Object.values(item));
    if (values.length > 0) {
      const range = sheet.getRange(
        sheet.getLastRow() + 1,
        1,
        values.length,
        values[0].length
      );
      range.setValues(values);
    }
  } catch (error) {
    console.error(`Error appending data to sheet: ${error.message}`);
  }
}