PEGO
R160: Relative Snapshot Position Overlays
Shifted immutable records can share payload storage when version-specific positions live in a separate overlay.
Mechanism and Invariants
Capture freezes the mutable graph first, then compares corresponding traversal
indices. Payload equality excludes start/end/generation; field scalars remain
exact, including action-derived positions. Overflowing int32 deltas reject
reuse. A 12-byte entry per node stores start/end deltas and absolute generation;
handles cache its pointer. Lookup is bounded and replays no edit history.
Relative sharing requires at least 128 nodes, 25% matching records and a reuse allocation heuristic. That heuristic is not a retained-heap guarantee: reused records can retain older backing arrays.
Scope and State
Private, experimental and default-off for engine Document snapshots across closure and both VMs. Generated parsers are outside scope. The empty default state preserves the 64-byte graph and 24-byte handle on measured arm64; enabled handles are 32 bytes.
Reference Control
Build tag pego_snapshot_position_overlays enables the candidate. The same-source
no-tag build is the unchanged default comparator.
Evidence and Implementation
Three serialized ABBA blocks (500 ms per benchmark invocation) used Go 1.27.1,
Apple M3 Max, GOMAXPROCS=16 and GOGC=100. Closure/CodePoints prefix capture
medians cost 6.5%/9.1% more at
4/64 KiB; indexed reads cost 25–38% more after middle edits and 4–9% after
prefix edits. Materialization ranges include parity.
Closure/Bytes all-history retention falls about 64%/72%, but retaining one
version costs 4.4%/7.6% more; three-version windows improve. Heap totals include
the live Document/cache. Other backends/workloads remain unmeasured; no adoption is
established.
See record 100,
snapshot design,
shareSnapshotGraph, and the
enabled/
default representations.