// import 'dart:convert';

import 'dart:io';

main() async {

　　var src = File('./lib/convert/source.txt');

　　var output = File('./lib/convert/output.txt');

　　var ins = src.openRead();

　　var encoded = convert(ins, 128);

　　var outs = output.openWrite();

　　await encoded.pipe(outs);

　　outs.close();

　　// the second transform.

　　var output2 = File('./lib/convert/output2.txt');

　　var outs2 = output2.openWrite();

　　await convert(output.openRead(), -128).pipe(outs2);

　　outs2.close();

}

Stream<List<int>> convert(Stream<List<int>> source,

　　　　[int key = 128, int base = 256]) async* {

　　await for (var chunk in source) {

　　　　yield encode(chunk, key, base);

　　}

}

List<int> encode(List<int> original, [int key = 128, int base = 256]) {

var encoded = List<int>(original.length);

for (var i = 0; i < original.length; i++) {

　　encoded[i] = (original[i] + key) % base;

}

return encoded;

}