Custom Similarity Tables

Substitutions are scored by a similarity table: for each ordered pair of symbols it gives a similarity in 0.0..=1.0, and the substitution penalty is substitution_cost * (1 - sim). Identical symbols are 1.0 (no penalty); unrelated symbols default to 0.0 (full penalty).

The default table

Out of the box the engine ships a general-purpose table that gives a reduced penalty to substitutions between related symbols:

• vowel ↔ vowel (e.g. a↔e) — moderately similar,
• consonant ↔ consonant — mildly similar,
• common OCR/typo confusions such as 0↔o, 1↔l, 1↔i, 5↔s.

This is why, with the default configuration, hell0 fuzzily matches hello and w0rld matches world.

Providing your own

Supply a &'static Similarity built from a map of (char, char) → similarity. A LazyLock is the usual way to get a 'static:

use fuzzy_aho_corasick::{FuzzyAhoCorasickBuilder, structs::Similarity};
use std::sync::LazyLock;

static SIMILARITY: LazyLock<Similarity> = LazyLock::new(|| {
    Similarity::from_map([
        (('@', 'a'), 0.9),
        (('a', '@'), 0.9),
    ])
});

let engine = FuzzyAhoCorasickBuilder::new()
    .similarity(&SIMILARITY)
    .build(["cat"]);

Similarity::from_map sets the diagonal (identical pairs) to 1.0 for you and precomputes a fast lookup table for ASCII pairs, falling back to the map for non-ASCII.

Notes

• Directionality. Entries are ordered pairs. If you want @↔a to behave symmetrically, insert both ('@','a') and ('a','@'), as above.
• Replacing vs. extending. Providing a table replaces the default entirely — you get exactly the pairs you insert (plus the identity diagonal). If you want the default confusions too, reproduce them in your map.
• Single symbols only. The table maps one symbol to one symbol. For equivalences spanning several graphemes (ligatures, transliterations), use multi-character mappings.
• Interaction with the floor. A high similarity makes a substitution cheap; the weakest-link floor can still reject substitutions whose similarity is below a threshold regardless of how the overall score comes out.