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How I reduce the size of library with genetic algorithm

TL; DR I've decreased the size of nanoid by 1 byte using a genetic algorithm.

UPDATE I've tried to run this algorithm on another files of this project and it reduced the size of main script by 2 bytes! PR

Nanoid is a tiny (139 bytes) string ID generator for JavaScript.

The server sends to browsers gzipped files, so if we can optimize the library's code for gzip algorithm then the amount of transferred data would be lower.

The size of this library contains the code itself of course and the alphabet to get the symbols.

If we look in git history of nanoid library we can see that the first commit has this string:

module.exports =
    '_~0123456789' +
    'abcdefghijklmnopqrstuvwxyz' +

If we check the size of the library using size-limit than we get that this simple alphabet weight is 73 bytes.

Size of simple alphabet string is 73 bytes

The lates (2.1.6) version of nanoid has alphabet looking like this:

module.exports =

You can see that this string contains a word like Module, Symb, has, Own. Because the code contains these words and gzip can pack url.js in more efficient way (only 64 bytes).

In one of the issues on github repository of nanoid, I've read that genetic algoritm can help to find the best string that can be packed as much as possible. And I will try to do it.

I've used the library geneticalgorithm. This library needs to define 3 functions: function to mutate chromosome, function to crossover chromosomes and function to check how good chromosome is.

I've started with a fitness function. This function has one input parameter and returns the number:

function fitnessFunction (phenotype) {
    const file = js.replace(/[A-Za-z0-9-_]{30,}/, phenotype.alphabet)
    const size = gzipSize.sync(file)

    return -1 * size

To check the size I've used gzip-size library.

After that I've defined a function to mutate chromosome:

function mutationFunction (phenotype) {
    const i = Math.floor(Math.random() * phenotype.alphabet)
    const j = Math.floor(Math.random() * phenotype.alphabet)

    return {
        alphabet: swapChars(alphabetTest, i, j)

function swapChars (str, index1, index2) {
    let l = index1 < index2 ? index1 : index2
    let h = index1 > index2 ? index1 : index2
    return str.substring(0, l) +
        str[h] +
        str.substring(l + 1, h) +
        str[l] +
        str.substring(h + 1, str.length)

And also the crossover function:

function crossoverFunction (phenotypeA, phenotypeB) {
    const alphabetA = phenotypeA.alphabet
    const alphabetB = phenotypeB.alphabet
    const indexA =
        Math.floor(Math.random() * alphabetA.length / 2 + alphabetA.length / 2)
    const indexB =
        Math.floor(Math.random() + alphabetA.length / 2)
    const newStrA = alphabetA.substring(indexA, alphabetA.length)
    const newStrB = alphabetB.substring(0, indexB)

    return [
        { alphabet: addMissingCharacter(newStrA, alphabetB) },
        { alphabet: addMissingCharacter(newStrB, alphabetA) }

function addMissingCharacter (str, proto) {
    let newStr = str
    for (let i = 0; i < proto.length; i++) {
        if (str.indexOf(proto[i]) === -1) {
            newStr += proto[i]
    return newStr

I've started from the population size of 1000 and the 500 generations. And I get another alphabet string but the size was the same. After that I've increased the population size to 10000 and 1000 generations and after I wait a while I get this string:


How you can see this string also contains some words but lighter on 1 byte.

Size limit show that url.js is only 63 bytes.

Size of generated string is only 63 bytes

After I get this result I was trying to normalize this string a little. I've moved all words to the start of the string and trying symbol by symbol moved all characters in alphabetic order. And here what I got:


I know that ain't much but with 3 simple functions and a half an hour I managed to find a better solution to decrease the size.

All code you can find in my pull request. Actually, you can run this code and maybe you will find a better string that I've found.

Thanks for reading.

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