PEGO
M090: Diagnostic Line Index
Repeated errors should not rescan the whole source to compute line and column. A lazily built line-start index makes location lookup a binary search plus bounded column work.
Mechanism and Invariants
Whole-input parsers build the table at the first location request. Line-break interpretation and the selected position unit remain exact. Streams, whose prefix is discarded, scan from committed positions instead. Recovered errors use the same lookup, avoiding quadratic prefix scans on error-heavy input.
Scope and State
Default diagnostic location indexing in closure, both VMs, generated Go Node/typed Go and TypeScript. Stream diagnostics retain their separate committed-prefix route.
Reference Control
No maintained same-source scan-from-start comparator exists.
Evidence and Implementation
Historical record 039 measures the Recovery workload 15–32% faster for full parsing and 17–30% for recognition. Valid inputs that never ask for a location do not build the table. These are error-heavy workload results, not a general valid-parse gain.
Evidence records: 039.
Source owners: input.buildLines and lineCol, generated line lookup, TypeScript line index.
The comparison policy distinguishes same-source controls, historical changes and full checkpoints.