Module regex_automata::meta::wrappers

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This module contains a boat load of wrappers around each of our internal regex engines. They encapsulate a few things:

  1. The wrappers manage the conditional existence of the regex engine. Namely, the PikeVM is the only required regex engine. The rest are optional. These wrappers present a uniform API regardless of which engines are available. And availability might be determined by compile time features or by dynamic configuration via meta::Config. Encapsulating the conditional compilation features is in particular a huge simplification for the higher level code that composes these engines.
  2. The wrappers manage construction of each engine, including skipping it if the engine is unavailable or configured to not be used.
  3. The wrappers manage whether an engine can be used for a particular search configuration. For example, BoundedBacktracker::get only returns a backtracking engine when the haystack is bigger than the maximum supported length. The wrappers also sometimes take a position on when an engine ought to be used, but only in cases where the logic is extremely local to the engine itself. Otherwise, things like “choose between the backtracker and the one-pass DFA” are managed by the higher level meta strategy code.

There are also corresponding wrappers for the various Cache types for each regex engine that needs them. If an engine is unavailable or not used, then a cache for it will not actually be allocated.

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