Certified Split Points for Parallel Lexing: Exact and Modulo Discarded Tokens

Abstract

Table-driven DFA lexing is sequential: each transition depends on the previous byte's state. Scanning one input in parallel needs each chunk's entry state, which existing methods recover by simulation, speculation, prescanning, or overlap. We give two conditions under which none is needed. For a longest-match scanner restarting from q0 at every token boundary, a byte b is a certified split symbol when no reachable state other than q0 has a b-transition whose target can reach acceptance, and q0 is not re-entrant if it has one. Every occurrence of such a byte in completely tokenizable input begins a token, so chunks starting there reproduce the serial sequence of kinds and lengths by ordered concatenation. The condition is necessary as well as sufficient, and fragile: one string, comment, or whitespace run can eliminate every useful certificate, and comments and whitespace are usually discarded. We therefore weaken the guarantee to equality after deleting a declared discarded set, and give a second condition, sound, no less permissive, but conservative rather than exact; the same tables answer it with a second constant-time one-bit query. It recovers newline for a conventional C-like tokenization and tab, newline and carriage return for JSON, without altering their token definitions, and refuses it where block comments are unrestricted. It ships as a query only: the library's planner and every measurement here use the exact condition, so a caller must plan boundaries itself. Splitting at exact certificates in the munch library reaches 92.6-95.3% parallel efficiency at eight threads on a restricted CPU set, on a 512 MiB dense corpus beyond last-level cache, and a 3.46-3.94x end-to-end speedup at four threads, across two benchmark revisions on one machine. It turns delimiter-based parallel lexing from a language-specific assumption into a property a compiler checks.