PEGO
R140: Document Snapshot Child Run Sharing
A changed wide snapshot record can have nearly all of its child IDs unchanged. Sharing those immutable runs avoids copying a full child-ID slice into every retained version.
Mechanism and Invariants
After exact-record sharing, changed records with at least 1,024 children compare exact 256-ID runs with the previous live graph. At least 75% of runs must match; otherwise the list stays flat. Matching runs reuse immutable backing and changed runs copy into private exact-size slices. A sidecar descriptor exists only for chunked records. Materialize expands an independent mutable Children slice. Weak previous ownership, exact ID checks and bounded copy charging prevent stale or partially published snapshots.
Scope and State
Default eligible closure/recursive-VM/iterative-VM Document snapshot graphs. Generated Go/TypeScript and ordinary Parse/Recognize do not expose this snapshot path. Small graphs, low record reuse and length-changing shifts often retain the flat fallback.
Reference Control
Private build tag pego_snapshot_child_chunks_reference keeps flat child
slices. Cached handles remain in both variants, so the control isolates child
storage rather than descriptor caching.
Evidence and Implementation
Record 099 measures about 34%/32% lower additional retained bytes per version on 4/64 KiB wide-child fixtures. Capture ratios have inconsistent signs; eligible captures add 4/5 allocations and materialization costs up to about 7% more in same-source samples. Other grammars/backends/units/length-changing edits are unmeasured. The 24-byte handle and read results belong to the separate cached-descriptor mechanism.
Evidence records: 099.
Source owners: shareSnapshotChildren and snapshotChildChunks, flat child comparator.
The comparison policy distinguishes same-source controls, historical changes and full checkpoints.