* perf(parsing): memoize the markup-producing entity check (billion-laughs)
splitAtMarkupEntity / producesMarkup recursed into every reference of
every replacement with no memoization, so a nested entity chain was
traversed O(references^depth) times: a classic eight-level billion-laughs
took ~18s of CPU to reject. The check is now memoized per entity name (the
property depends only on the fixed entity graph), so it is O(entities). The
same document now rejects in ~0.6s, bounded by the expansion budget. No
change to which documents are accepted; cyclic entities still rejected.
Found by adversarial probing; closes the no-markup case of #339.
* perf(parsing): three-colour DFS for the No-Recursion check
checkRecursion validated the entity reference graph with only an
in-progress (grey) set and no record of fully-verified entities, so a
billion-laughs graph was re-walked O(references^depth) times: a
markup-producing eight-level document took ~31s to reject. It is now a
three-colour DFS: an entity is added to a black set only after its whole
reachable subgraph returns acyclic, so it can never reach a chain node
that reaches it, and skipping it on revisit is sound. The walk becomes one
pass over the distinct edges (~31s -> ~11s for that document). Cyclic
entities are still rejected; no accepted document changes. Part of #339.
Mihaela Mihaljevic
b05b428e30