Frank Mitchell

Algorithms matter on the mobile web

Leo Fabrikant’s article on optimizing the performance of a React autocomplete form is worth reading. It covers performance profiling, async rendering, and multi-threading with Web Workers. I loved that he outlined the conditions that pushed him toward focusing on optimizing the rendering pipeline. What we know creates the set of spaces where we look for solutions.

The search algorithm library had painfully long search times as the length of the search term got longer…I don’t know if the library I chose for the search algorithm was bad or if this was an inevitability of any “fuzzy” search algorithm. But thankfully, I didn’t bother trying to find alternatives.

When I built my word search game, I spent a fair bit of time learning how to store and search strings efficiently. So when I read Leo’s article, I thought about data structures like BK-trees and algorithms for finding the Levenshtein distance. My experience says long search terms shouldn’t mean long search times. So let’s see if we can build a better search engine.

Sometimes you need to DIY

We’ll need some strings to work with that satisfy the original requirements.

Here is a data set retrieved from a backend. It contains 13,000 items with very long, wordy names (Scientific Organizations). Make a search bar with an auto-suggest using this data.

I don’t have a list of very long, wordy scientific organizations handy. However, the Free Music Archive has 19,212 tracks in it with names of five or more words. I figure that’s a pretty equivalent data set.

We’ll also want some JavaScript to benchamrk different fuzzy search algorithms. Let’s assumes a worst case scenario, where the user’s typed out the entire song name, and the song they’re looking for isn’t in the database.

const query = 'Where the Streets Have No Name';

const data = [...new Set(require('./tracks.json'))];
const tracks = data.filter(track => {
  return track.split(/\s+/).length >= 5;
});

const start = Date.now();

const matches = tracks.map(track => {
  const score = compare(query, track);

  return {score, track};
}).sort((a, b) => {
  return a.score - b.score;
}).slice(0, 10)
.map(info => info.track);

const elapsed = Date.now() - start;

console.log(`
${matches.join('\n')}

Searched ${tracks.length} tracks in ${elapsed} milliseconds
`);

We need a comparison function that scores two strings based on how similar they are. To make it easy to sort results, we’ll say that a smaller score means the strings are more similar. The Levenshtein distance metric is the reference measurement for string similarity. It counts the number of edits it would take to make two strings identical.

Here’s a memoized recursive implementation.

function compare(s, t, memo = {}) {
  const args = [s, t];

  if (args in memo) {
    return memo[args];
  }

  if (!s) {
    memo[args] = t.length;
    return t.length;
  }

  if (!t) {
    memo[args] = s.length;
    return s.length;
  }

  const snext = s.slice(1);
  const tnext = t.slice(1);

  const cost = s[0] !== t[0];
  const delCost = compare(snext, t, memo) + 1;
  const insCost = compare(s, tnext, memo) + 1;
  const subCost = compare(snext, tnext, memo) + cost;
  const minCost = Math.min(delCost, insCost, subCost);

  memo[args] = minCost;
  return minCost;
}

I ended up needing to memoize it, because the non-memoized version took too long. As it is, even the memoized version takes about two minutes to find matches.

Where Childrens Have a Place
Where the Walls Have a Soul
When the Guests Have Left
The Extra Party Has No Name
So, What If I Have No Name?
The One With No Name
Where The Land Meets The Sea
When the Lights Came On
This Game Has No Name
Down the Streets (Life Beyond)

Searched 19212 tracks in 122963 milliseconds

That’s not something I’d want to use in an autocomplete form. My general rule of thumb for UI responsiveness is that anything more than a third of a second (about 300 milliseconds) is too long. Fortunately, the Levenshtein distance metric has an iterative implementation that avoids the recursion and memoization.

function compare(s, t) {
  let v0 = [];
  let v1 = [];

  for (let i = 0; i <= t.length; i += 1) {
    v0[i] = i;
  }

  for (let i = 0; i < s.length; i += 1) {
    v1[0] = i + 1;

    for (let j = 0; j < t.length; j += 1) {
      const delCost = v0[j + 1] + 1;
      const insCost = v1[j] + 1;
      const subCost = v0[j] + (s[i] !== t[j]);
      v1[j + 1] = Math.min(delCost, insCost, subCost);
    }

    [v0, v1] = [v1, v0];
  }

  return v0[t.length];
}

That takes about a third of a second and finds the same matches.

Where Childrens Have a Place
Where the Walls Have a Soul
When the Guests Have Left
The Extra Party Has No Name
So, What If I Have No Name?
The One With No Name
Where The Land Meets The Sea
When the Lights Came On
This Game Has No Name
Down the Streets (Life Beyond)

Searched 19212 tracks in 322 milliseconds

Can we do any better? Sure! I turns out the Sorensen-Dice coefficient of the sets of bigrams in two strings makes a pretty good fuzzy match. We have to negate the score though, because a larger value means the strings are more similar.

function bigrams(string) {
  const result = [];

  for (let i = 0; i < string.length - 1; i += 1) {
    result.push(string.slice(i, i + 2));
  }

  return result;
}

function compare(s, t) {
  const sGrams = bigrams(s);
  const tGrams = bigrams(t);

  const hits = sGrams.filter(n => {
    return tGrams.includes(n);
  }).length;

  const total = sGrams.length + tGrams.length;
  const score = (hits * 2) / total;

  return -score;
}

We get a different set of songs, which makes sense, because we changed the algorithm. I’m not sure if the 64 millisecond speed improvement is noise or not. I’d need to plug it into a more robust benchmarking tool (like Benchmark.js) to measure that.

No Secrets In the H4C
The Streets of New York
Where the Walls Have a Soul
Liberty Is In the Street
The Waves Call Her Name
The Extra Party Has No Name
Nameless: the Hackers Title Screen
There is Nothing to Fear
Down the Streets (Life Beyond)
So, What If I Have No Name?

Searched 19212 tracks in 258 milliseconds

What about the quality of the results? The Sorensen-Dice coefficient feels like it does a better job of finding relevant songs when the words in the query are out of oder. The Levenshtein distance feels like it does a better job when you know the name of the song you want.

Where do we go from here?

The more experiences we have, the better a chance we give ourselves of finding solutions to problems and answers to questions. Leo’s article helped me learn how to profile and fix UI blocking issues. Writing this helped me learn that fuzzy search algorithms aren’t just about speed. Normalization and measuring similarity vs. edit distance changes the quality of the results. You need to understand the your use cases first, and pick an algorithm that supports them.

I’ll probably just use Kiro Risk’s excellent Fuse.js library if I need a fuzzy search engine in the future. It uses the bitap algorithm, so it’ll probably be fast enough. Plus, that’s the same algorithm used in agrep, so it’s probably a good fit for most text.

Probably.

Addendum

For completeness, here’s the Ruby code I used to turn the raw_tracks.csv file from the fma_metadata.zip archive into a JSON list of track names.

require 'csv'
require 'json'

$stdout.sync = true

csv = CSV.read(ARGV[0], headers: true)
tracks = csv.map { |row| row['track_title'].strip }
puts tracks.to_json

Projects

Bowling pins and the pencil from Spare Me on a wood background.Spare Me is a game about bowling, a fun way to learn math, and a love letter to the world’s second best solitaire card game. It won 9th place in the mobile category for the 2014 js13kGames competition.

The copter from Hard Vacuum: Recon flys over a field of ice.Hard Vacuum: Recon pits your memory skills against the wasteland of a frozen planet in an iPhone game for the 2013 js13kGames competition.

Prolix’s icon spells “lightheartedness”Prolix is a word search game for the iPhone and iPod touch which lets you tweet your scores so your friends can play with you.