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Similarity Algorithms

Local Files search compares the cleaned ROM name with every image filename in your image folder. Three algorithms are available, selectable under Set Similarity Algorithm.

The algorithms

Jaro-Winkler Distance (default)

Measures how similar two strings are, with extra weight for a common prefix. It is forgiving of small differences and transpositions, which makes it a good default for game names that share prefixes such as series titles.

  • Best for: most collections
  • Strengths: handles typos, punctuation differences, and short suffixes
  • Weaknesses: can over-match names that share a long prefix

Jaccard Similarity

Compares the sets of character n-grams (trigrams) in both names: the size of the intersection divided by the size of the union.

  • Best for: filenames with reordered words, for example Mario Bros, The vs The Mario Bros
  • Strengths: order-insensitive, fast with the trigram index
  • Weaknesses: less sensitive to small edits than Levenshtein

Levenshtein Distance

Counts the minimum number of insertions, deletions, and substitutions needed to turn one name into the other, then converts that distance into a similarity percentage.

  • Best for: strict edit-distance matching
  • Strengths: intuitive, well suited to near-identical names
  • Weaknesses: longer names can produce lower scores even when they look similar; an early-exit optimization keeps large collections fast

The similarity threshold

Set Similarity Threshold sets the minimum score (10%–90%, default 70%) required for an image to appear in the results.

Threshold Effect
90% Only near-identical names
70% (default) Balanced precision and recall
50% Tolerant of abbreviations and missing words
10–30% Very permissive; expect noise

Lower thresholds produce longer result lists. The same threshold pre-filters AI candidates in the main window; AI Settings has a separate Candidate similarity value.

Filename cleaning

Before comparison, FindRomCover cleans the search name:

  • removes file extensions;
  • strips region and revision tags such as (USA), (Europe), [!];
  • replaces separators (_, -, .) with spaces;
  • collapses repeated whitespace;
  • sanitizes characters that are unsafe in filenames.

The result is used for similarity scoring, web searches, and AI prompts. When Use MAME Descriptions is enabled, the MAME description replaces the cleaned filename.

Performance

For image folders with 50 or more files, a trigram index pre-filters candidates before the full similarity comparison. The index is built per search and keeps large collections responsive.

Additional optimizations:

  • Levenshtein distance stops early once the minimum possible distance exceeds the threshold.
  • Jaccard n-grams are computed once per query.
  • Jaro-Winkler reuses pooled buffers to reduce garbage collection.

Choosing an algorithm

Situation Recommendation
You do not know where to start Jaro-Winkler at 70%
Filenames use different word order Jaccard
Covers differ by one or two characters Levenshtein
AI picks are enabled Jaro-Winkler with a lower AI Candidate similarity