import Papa, { ParseConfig, Parser } from 'papaparse';

export type CsvAsyncIteratorOptions = Exclude<
  ParseConfig,
  'step' | 'chunk' | 'complete' | 'error'
>;

/**
 * Helper to allow async iteration over the contents of
 * a CSV input.
 *
 * This allows the caller to walk through the results
 * using a `for await` loop and deal with rows asyncronously
 * one at a time.
 *
 * When process a large input this reduces the memory
 * use of the process because we don't have to store
 * the parsed representation of all the rows in memory
 * at once; just the last returned row is represented
 * as JavaScript objects.
 *
 * Also, if the input is coming from a file or URL this
 * will avoid loading the entire file into memory.
 *
 * The combination of these two factors allows processing
 * of extremely large files without using tons of memory.
 */
const csvAsyncIterator = <RowType = any>(
  input: Buffer | string | NodeJS.ReadableStream,
  config?: CsvAsyncIteratorOptions,
): AsyncIterable<RowType[]> => {
  return {
    [Symbol.asyncIterator]: () => {
      let parser: Parser | null = null;
      let done: boolean = false;
      let error: any = null;
      let resolve: (r: IteratorResult<RowType[]>) => void = () => undefined;
      let reject: (err: any) => void = () => undefined;
      return {
        return: () => {
          done = true;
          if (parser) parser.abort();
          return Promise.resolve({
            done,
            value: (undefined as any) as RowType[],
          });
        },
        throw: err => {
          done = true;
          error = err;
          if (parser) parser.abort();
          return Promise.reject(err);
        },
        next: () =>
          error
            ? Promise.reject(error)
            : done
            ? Promise.resolve({
                done: true,
                value: (undefined as any) as RowType[],
              })
            : new Promise<IteratorResult<RowType[]>>((res, rej) => {
                try {
                  resolve = res;
                  reject = rej;
                  if (parser === null) {
                    Papa.parse(
                      (Buffer.isBuffer(input)
                        ? input.toString('utf8').replace('\r', '')
                        : input) as any,
                      {
                        ...config,
                        error: (ioError, f) => {
                          done = true;
                          error = ioError;
                          return reject(ioError);
                        },
                        step: (results, p) => {
                          parser = p;
                          parser.pause();
                          if (results.errors && results.errors.length) {
                            done = true;
                            return reject(results.errors[0]);
                          } else {
                            return resolve({
                              value: (results.data as any) as RowType[],
                              done: false,
                            });
                          }
                        },
                        complete: () => {
                          done = true;
                          if (error) return reject(error);
                          else
                            return resolve({
                              done: true,
                              value: (undefined as any) as RowType[],
                            });
                        },
                      },
                    );
                  } else {
                    parser.resume();
                  }
                } catch (err) {
                  error = err;
                  return reject(err);
                }
              }),
      };
    },
  };
};

export default csvAsyncIterator;