projectedBranchBudgetCut function

int? projectedBranchBudgetCut(List<SessionRecord> branch, int budget)

The index of the OLDEST record of branch (root-first) to keep so the projected estimate of the kept suffix stays within budget — walking from the newest end backward, the record that would push the tally PAST the budget is dropped together with everything older — or null when the whole branch already fits. The newest record is always kept (a 0 budget floors at keeping the tail: never an empty window).

Record-granular budget stop for the resume walk (gh-968, AC-R4): the old stop fired after a whole doubling block was paged, so one giant record in the last block read the tally tens of thousands of tokens past the budget. Exact mirror of estimateProjectedBranchTokens walked in reverse: hidden-range/checkpoint markers sit NEWER than the records they hide, so the backward pass already knows every record's hidden/covered state when it reaches it, and the classic compaction transform makes everything below the last compaction record's kept id price zero — those records are never the crossing.

Implementation

int? projectedBranchBudgetCut(List<SessionRecord> branch, int budget) {
  final hidden = <String>{};
  final covered = <String>{};
  var belowTransform = false; // passed the last compaction record
  var tally = 0;
  for (var i = branch.length - 1; i >= 0; i--) {
    final record = branch[i];
    if (belowTransform) continue; // dropped by the classic transform
    final contribution = covered.contains(record.id)
        ? 0
        : hidden.contains(record.id)
        ? _hiddenMarkerTokens
        : switch (record) {
            MessageRecord(:final message) => estimateTokens(message),
            CustomMessageRecord(:final content, :final timestamp) =>
              estimateTokens(
                UserMessage(content: content, timestamp: timestamp),
              ),
            CompactionRecord(:final summary) => estimateStringTokens(summary),
            BranchSummaryRecord(:final summary) => estimateStringTokens(
              summary,
            ),
            CompactCheckpointRecord(:final text) => estimateStringTokens(text),
            _ => 0,
          };
    if (tally + contribution > budget && i < branch.length - 1) {
      return i + 1; // stop BEFORE the crossing record
    }
    tally += contribution;
    // Marker sets grow only from records that project (mirror: the
    // forward estimator collects markers over the KEPT region only).
    switch (record) {
      case HiddenRangeRecord(:final recordIds):
        hidden.addAll(recordIds);
      case CompactCheckpointRecord(
        :final coversRecordIds,
        :final firstRecordId,
        :final lastRecordId,
      ):
        covered
          ..addAll(coversRecordIds)
          ..add(firstRecordId)
          ..add(lastRecordId);
      case CompactionRecord():
        // The last (newest) compaction record on the branch: its summary
        // contributed above; everything OLDER is dropped by the classic
        // transform and projects zero from here on.
        belowTransform = true;
      default:
        break;
    }
  }
  return null; // whole branch fits
}