// A source of the bytes of a capsule read in pieces, for the tests of the
// inspection and of the opening from a ByteSource. It reads no file.
library;

import 'dart:typed_data';

import 'package:datekeys/datekeys.dart';

/// A source that gives at most [step] bytes per read, so that every range
/// is read in several pieces, and counts what it reads.
final class ChunkySource implements ByteSource {
  ChunkySource(this._b, [this.step = 4093]);

  final Uint8List _b;
  final int step;
  int reads = 0;
  int bytesRead = 0;
  int largest = 0;

  /// The end of the farthest range read.
  int farthest = 0;

  @override
  int get length => _b.length;

  @override
  Future<Uint8List> read(int offset, int n) async {
    reads++;
    final k = n < step ? n : step;
    final end = offset + k < _b.length ? offset + k : _b.length;
    bytesRead += end - offset;
    if (end - offset > largest) largest = end - offset;
    if (end > farthest) farthest = end;
    // A copy: the opening owns nothing that the source keeps.
    return Uint8List.fromList(Uint8List.sublistView(_b, offset, end));
  }
}