NonGNU ELPA - flx


fuzzy matching with good sorting
flx-0.6.2.tar (.sig), 2024-Mar-31, 60.0 KiB
Le Wang
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To install this package from Emacs, use package-install or list-packages.

Full description

Build Status MELPA MELPA Stable


This project is more than a year old now. Lots of bugs have been worked out.

It appears some people use it on a regular basis.


Screencast showing rationale and ido workflow



Just drop all .el files somewhere on your load-path. Here's an example using the folder ~/.emacs.d/vendor:

lisp (add-to-list 'load-path "~/emacs.d/vendor")

Package Repositories

Available packages:

  • flx - matching engine
  • flx-ido - ido interface for flx

Install flx-ido will pull in flx as a dependency.


If you're an Emacs 24 user or you have a recent version of package.el you can install flx-ido from MELPA.

This version will always be up-to-date.


flx-ido is also available on the Marmalade package.el repository.

Emacs Prelude

flx-ido is part of the Emacs Prelude. If you're a Prelude user - flx-ido is already properly configured and ready for action.

Debian and Ubuntu

Users of Debian 9 or Ubuntu 16.04 or later may simply apt-get install elpa-flx.


The sorting algorithm is a balance between word beginnings (abbreviation) and contiguous matches (substring).

The longer the substring match, the higher it scores. This maps well to how we think about matching.

In general, it's better form queries with only lowercase characters so the sorting algorithm can do something smart.

For example, if you have these files:


If the search term was pre-mode, you might expect "prelude-mode.el" to rank higher. However because the substring match "re-mode" is so long, "clojure-mode.el" actually scores higher.

Here, using premode would give the expected order. Notice that the "-" actually prevents the algorithm from helping you.

uppercase letters

Flx always folds lowercase letters to match uppercase. However, you can use uppercase letters for force flx to only match uppercase.

This is similar to Emacs' case-folding. The difference is mixing in uppercase letters does not disable folding.

completing file names

Matches within the basepath score higher.

ido support

Add this to your init file and flx match will be enabled for ido.

lisp (require 'flx-ido) (ido-mode 1) (ido-everywhere 1) (flx-ido-mode 1) ;; disable ido faces to see flx highlights. (setq ido-enable-flex-matching t) (setq ido-use-faces nil)

If you don't want to use the flx's highlights you can turn them off like this:

lisp (setq flx-ido-use-faces nil)

Flx uses a complex matching heuristics which can be slow for large collections

Customize flx-ido-threshold to change the collection size above which flx will revert to flex matching.

As soon as the collection is narrowed below flx-ido-threshold, flx will kick in again.

As a point of reference for a 2.3 GHz quad-core i7 processor, a value of 10000 still provides a reasonable completion experience.

Helm support

Helm is not supported yet. There is a demo showing how it could work, but I'm still working through how to integrate it into helm.

The Helm demo shows the score of the top 20 matches.

Memory Usage

The flx algorithm willingly sacrifices memory usage for speed.

For 10k file names, about 10 MB of memory will be used to speed up future matching. This memory is never released to keep the match speed fast.

So far with modern computers, this feels like a reasonable design decision.

It may change in future.

GC Optimization

Emacs's garbage collector is fairly primitive stop the world type. GC time can contribute significantly to the run-time of computation that allocates and frees a lot of memory.

Consider the following example:

```lisp (defun uuid () (format "%08x-%08x-%08x-%08x" (random (expt 16 4)) (random (expt 16 4)) (random (expt 16 4)) (random (expt 16 4))))

(benchmark-run 1 (let ((cache (flx-make-filename-cache))) (dolist (i (number-sequence 0 10000)) (flx-process-cache (uuid) cache)))) ;;; ⇒ (0.899678 9 0.33650300000000044) ```

This means that roughly 30% of time is spent just doing garbage-collection.

flx can benefit significantly from garbage collection tuning.

By default Emacs will initiate GC every 0.76 MB allocated (gc-cons-threshold == 800000). If we increase this to 20 MB (gc-cons-threshold == 20000000) we get:

lisp (benchmark-run 1 (setq gc-cons-threshold 20000000) (let ((cache (flx-make-filename-cache))) (dolist (i (number-sequence 0 10000)) (flx-process-cache (uuid) cache)))) ;;; ⇒ (0.62035 1 0.05461100000000041)

So if you have a modern machine, I encourage you to add the following:

lisp (setq gc-cons-threshold 20000000)

to your init file.