Enum memchr::memmem::Prefilter

#[non_exhaustive]
pub enum Prefilter {
    None,
    Auto,
}

Prefilter controls whether heuristics are used to accelerate searching.

A prefilter refers to the idea of detecting candidate matches very quickly, and then confirming whether those candidates are full matches. This idea can be quite effective since it’s often the case that looking for candidates can be a lot faster than running a complete substring search over the entire input. Namely, looking for candidates can be done with extremely fast vectorized code.

The downside of a prefilter is that it assumes false positives (which are candidates generated by a prefilter that aren’t matches) are somewhat rare relative to the frequency of full matches. That is, if a lot of false positives are generated, then it’s possible for search time to be worse than if the prefilter wasn’t enabled in the first place.

Another downside of a prefilter is that it can result in highly variable performance, where some cases are extraordinarily fast and others aren’t. Typically, variable performance isn’t a problem, but it may be for your use case.

The use of prefilters in this implementation does use a heuristic to detect when a prefilter might not be carrying its weight, and will dynamically disable its use. Nevertheless, this configuration option gives callers the ability to disable prefilters if you have knowledge that they won’t be useful.

Variants (Non-exhaustive)

Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.

None

Never used a prefilter in substring search.

Auto

Automatically detect whether a heuristic prefilter should be used. If it is used, then heuristics will be used to dynamically disable the prefilter if it is believed to not be carrying its weight.

Trait Implementations

impl Clone for Prefilter

fn clone(&self) -> Prefilter

Returns a copy of the value.

const fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Prefilter

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for Prefilter

fn default() -> Prefilter

Returns the “default value” for a type.

impl Copy for Prefilter