branchAt method

DataSourceTrace branchAt(int step, int throughStep, Map<String, Object?> continuation(int step))

A new trace: this one's own history up to and including step, then whatever continuation resolves for every step after it and up to throughStep — a "what if", branched at step without touching this trace at all. continuation is a plain function rather than a second DataSourceRegistry because branching is a property of the trace, not of who is asking for one: the caller already knows which source it swapped and for what.

Implementation

DataSourceTrace branchAt(
  int step,
  int throughStep,
  Map<String, Object?> Function(int step) continuation,
) {
  final branch = DataSourceTrace();
  for (var i = 0; i < _steps.length; i++) {
    if (_steps[i] > step) break;
    branch.record(_steps[i], _values[i]);
  }
  for (var s = step + 1; s <= throughStep; s++) {
    branch.record(s, continuation(s));
  }
  return branch;
}