PEGO
R090: Moving Document Trees
After an edit shifts reused results, copying every descendant defeats incremental reuse. Generation-aware in-place movement updates positions of reusable Nodes.
Mechanism and Invariants
Each Node tracks the edit generation represented by its positions. moveResult
applies accumulated shifts, replaying edits only when a shared descendant is
already current through another result. Empty nodes at insertion points are
ambiguous and use a copy path that preserves descendant shifts.
Length-preserving edits need no position movement. Earlier mutable trees can
change on later Document parses; Clone or owned snapshots preserve older
versions.
Scope and State
Default engine Document result reuse in closure and both VMs. Generated parsers/TypeScript have no Document move path.
Reference Control
No maintained current-source copy-every-tree control exists. Snapshot sharing compares immutable record storage and does not disable mutable tree movement.
Evidence and Implementation
Historical record 045 measures Minilang one-character edit/reparse from 1.34 to 0.90 ms in closure, with 2.4 to 0.34 MB and 151 to 49 allocations. Record 029 previously reduced tree-copy allocation using chunks. These revisions do not isolate current snapshot capture or every edit schedule; earlier mutable trees can change as positions move.
Source owners: moveResult and moved Node handling, Node generation and Clone, edit log and result shift.
The comparison policy distinguishes same-source controls, historical changes and full checkpoints.