PEGO
R130: Document Snapshot Record Sharing
Retaining several immutable Document snapshots need not duplicate records that are exactly equal. Reuse lowers retained version storage while preserving snapshot ownership and mutable Node independence.
Mechanism and Invariants
Capture freezes a new graph, then compares each record with the previous live snapshot at the corresponding traversal index. Kind, absolute positions, text, flags, nil/empty state, child IDs and ordered fields must all match exactly; a hash alone is insufficient. Fewer than 128 records or less than one-quarter reuse keeps a contiguous copy fallback. A weak previous-graph reference avoids retaining an old snapshot solely through the Document. Text is still copied for owned snapshot storage; shifted positions can make records ineligible.
Scope and State
Default Document snapshot capture in closure and both VMs. Generated parsers and TypeScript do not expose the Document snapshot API. Public mutable Nodes are not shared by this immutable-record policy.
Reference Control
Build tag pego_snapshot_sharing_reference selects copied records at the same
source. It controls record sharing, not parsing, text ownership or
materialization behavior.
Evidence and Implementation
Record 097 measures retained per-version graph storage from about 455 KB to 85 KB at 4 KiB and 7.79 MB to 1.54 MB at 64 KiB. Capture costs roughly 12%/6% more in its closure CodePoints samples, and prefix insertion takes the low-reuse fallback. This trades capture work for version retention; it is not an ordinary parse speedup.
Evidence records: 097.
Source owners: snapshot graph freezing and exact sharing, default policy, copy comparator.
The comparison policy distinguishes same-source controls, historical changes and full checkpoints.