Deterministic Replay
THotPDF.DeterministicReplayLog captures external decisions that can make an operation behave differently across runs, then substitutes the recorded results during replay
Captured decisions
- Cancellation checkpoints record the operation name and whether cancellation was requested
- Progressive random-access probes record the exact byte offset, byte count, and availability result
- Boolean and 64-bit integer callback helpers record a stable callback name, two caller-defined context values, and the returned result
- Annotation filtering and tile-render cancellation use the replay log automatically when recording or replaying through
THotPDF
Record and replay
var
ReplayJSON: AnsiString;
begin
PDF.DeterministicReplayLog.BeginRecording;
PDF.ReadProgressiveLinearizedFirstPageSection(Source, Output, Info);
ReplayJSON := PDF.DeterministicReplayLog.ExportJSON;
PDF.DeterministicReplayLog.ImportJSON(ReplayJSON);
PDF.ReadProgressiveLinearizedFirstPageSection(Source, Output, Info);
if not PDF.DeterministicReplayLog.CompleteReplay then
raise Exception.Create(PDF.DeterministicReplayLog.LastDiagnostic);
end;Ordering and validation
Every event carries a monotonic sequence number, event kind, name, two signed 64-bit arguments, and a typed result
Replay consumes events in order and validates their complete signature before returning the recorded result; strict mode raises EHPDFDeterministicReplayMismatch on exhaustion or divergence, while non-strict mode retains a mismatch count and diagnostic and uses the observed result
CompleteReplay succeeds only after every retained event has been consumed without mismatches or dropped recording events
Bounds and performance
EventLimit defaults to 100,000 events, DroppedEventCount reports overflow, and event storage grows geometrically so append cost remains amortised constant time
The JSON schema is hotpdf-deterministic-replay-v1; 64-bit arguments and integer results are encoded as decimal strings to preserve exact values across JSON implementations