// Opens a connection to an IndexedDB database.
//
// @param [String] name the IDB database name
// @return [Promise<IDBDatabase>] a connection to the opened database
function OpenDatabase(name) {
  return new Promise((resolve, reject) => {
    const request = indexedDB.open(name);
    request.onsuccess = () => { resolve(request.result); };
    request.onerror = () => { reject(request.error); };
  });
}

// Internal function for reading and/or deleting data from IndexedDB.
//
// Must be called in a readwrite transaction whose scope includes the object
// store(s) owning the given source. Guarantees that the transaction will still
// be alive when the callback is called.
//
// @param [IDBObjectStore | IDBIndex] source the IDB object store or index whose
//     values are getting deleted
// @param [Object] options
// @option options [Number]  limit        stop after reading this many values
// @option options [Boolean] deleteValues delete the read values if true
// @param [function(?Error, ?Number)] callback called when done; in case of
//     success, the callback is passed the number of items that were read (and
//     potentially deleted)
// @return undefined
function DoIterate(source, options, callback) {
  let items_read = 0;
  let items_left =
      ('limit' in options) ? options.limit : Number.MAX_SAFE_INTEGER;

  const request =
      options.deleteValues ? source.openCursor() : source.openKeyCursor();
  request.onsuccess = () => {
    const cursor = request.result;
    if (!cursor) {
      callback(null, items_read);
      return;
    }

    const delete_request = options.deleteValues ? cursor.delete() : {};
    delete_request.onsuccess = () => {
      items_left -= 1; items_read += 1;
      if (items_left === 0) {
        callback(null, items_read);
        return;
      }
      cursor.continue();
    };
    delete_request.onerror = () => { callback(request.error); };

    if (!options.deleteValues)
      delete_request.onsuccess();
  };
  request.onerror = () => { callback(request.error); };
}

// Deletes the first N items in an IDB data source and iterates over the rest.
//
// Must be called in a readwrite transaction whose scope includes the object
// store(s) owning the given source. Guarantees that the transaction will still
// be alive when the callback is called.
//
// @param [IDBObjectStore | IDBIndex] source the IDB object store or index whose
//     values are getting deleted
// @param [Object] options
// @option options [Number]  limit        stop after reading this many values
// @option options [Boolean] deleteValues delete the read values if true
// @param [function(?Error, ?Boolean)] callback called when done; in case of
//     success, the callback is passed "true" if at least one item was deleted,
//     or "false" if no item was deleted
// @return undefined
function IterateAndDelete(source, options, callback) {
  DoIterate(source, { limit: options.limit, deleteValues: true },
            (error, deleted_count) => {
    if (error) {
      callback(error);
      return;
    }
    const has_deleted_items = (deleted_count !== 0);
    DoIterate(source, { deleteValues: false }, (error, _) => {
      if (error) {
        callback(error);
        return;
      }
      callback(null, has_deleted_items);
    });
  });
}

// Deletes the first N keys in each index and iterates over all the indexes.
//
// Must be called in a readwrite transaction whose scope includes the object
// store(s) owning the given indexes. Guarantees that the transaction will still
// be alive when the callback is called.
//
// @param [Array<IDBIndex>] indexes the indexes to iterate over; to ensure
//     tombstone cleanup, this set must contain all the indexes of a object
//     store
// @param [Object] options
// @option options [Number] limit the number of keys to delete from each index
// @param [function(Error, Boolean)] callback called when done; in case of
//     success,
function DeleteIndexKeys(indexes, options, callback) {
  let current_index = -1;
  let has_deleted_items = false;
  const Callback = (error, has_deleted_items_in_index) => {
    has_deleted_items = has_deleted_items || has_deleted_items_in_index;
    current_index += 1;
    if (current_index === indexes.length) {
      callback(null, has_deleted_items);
      return;
    }
    IterateAndDelete(indexes[current_index], options, Callback);
  };
  Callback(null, false);
}

// Deletes N items from an object store.
//
// This is intended to be called repeatedly when deleting an object store, to
// avoid generating a single IndexedDB transaction that's too large to fit in
// memory.
//
// @param [IDBDatabase] database the database owning the object store
// @param [string] object_store_name the name of the target object store
// @param [Object] options
// @option options [number] limit          the maximum number of items to delete
// @option options [Boolean] drain_indexes if true, deletes items via indexes;
//     otherwise, deletes item by iterating over the object store
// @return [Promise<Boolean>] resolves with true if at least one item was
//     deleted, and false otherwise
function DrainObjectStore(database, object_store_name, options) {
  return new Promise((resolve, reject) => {
    let has_deleted_items = false;
    const transaction = database.transaction([object_store_name], 'readwrite');
    transaction.oncomplete = () => { resolve(has_deleted_items); };
    transaction.onerror = () => { reject(transaction.error); };

    const object_store = transaction.objectStore(object_store_name);
    const indexes = Array.from(object_store.indexNames).map(
        (name) => object_store.index(name));

    const per_index_limit =
        (options.drain_indexes && indexes.length !== 0) ?
        options.limit / indexes.length : 0;

    DeleteIndexKeys(
        indexes, { limit: per_index_limit }, (error, call_deleted_items) => {
      if (error) {
        reject(error);
        return;
      }
      has_deleted_items = call_deleted_items;
      if (has_deleted_items) {
        // Let the transaction commit. Enough work was done.
        return;
      }

      DoIterate(
          object_store, { limit: options.limit }, (error, deleted_count) => {
        if (error) {
          reject(error);
          return;
        }
        has_deleted_items = (deleted_count !== 0);
        // Let the transaction commit.
      });
    });
  });
}

// Deletes the contents of an IndexedDB object store.
//
// The deletion is done in many small transactions, to avoid generating a single
// IndexedDB transaction that's too large to fit in memory. Does not delete the
// object store itself.
//
// @param [IDBDatabase] database the database owning the object store
// @param [string] object_store_name the name of the target object store
// @param [Object] options
// @option options [number] step_limit maximum number of items to delete in a
//     single transaction
// @return [Promise<>] resolves when the data inside the object store is deleted
async function DeleteObjectStore(database, object_store_name, options) {
  while (true) {
    const seen_items = await DrainObjectStore(
        database, object_store_name,
        { limit: options.step_limit, drain_indexes: true });
    if (!seen_items)
      break;
  }

  // Delete any items that may not be listed in indexes.
  while (true) {
    const seen_items = await DrainObjectStore(
        database, object_store_name,
        { limit: options.step_limit, drain_indexes: false });
    if (!seen_items)
      break;
  }
}

// Deletes the contents of an IndexedDB database.
//
// The deletion is done in many small transactions, to avoid generating a single
// IndexedDB transaction that's too large to fit in memory. Does not delete the
// database's object stores or indexes.
//
// @param [IDBDatabase] database the database to be deleted
// @return [Promise<>] resolves when the data inside the database is deleted
async function DeleteDatabase(database, options) {
  for (let object_store_name of database.objectStoreNames)
    await DeleteObjectStore(database, object_store_name, options);
}

// Driver for testing the code above.
(async () => {
  const database = await OpenDatabase('messages0');
  await DeleteDatabase(database, { step_limit: 10 });
  database.close();
  console.log('Database drained');
})();