String combinations

A simple, low-memory footprint function to generate all string combinations from a series of characters.

Installation

PHP 7.4+ is required.

composer require bentools/string-combinations

Usage

I want to get all combinations with the letters a, b, c.

require_once __DIR__ . '/vendor/autoload.php';
use function BenTools\StringCombinations\string_combinations;

foreach (string_combinations('abc') as $combination) { // Can also be string_combinations(['a', 'b', 'c'])
    echo $combination . PHP_EOL;
}

Output:

a
b
c
aa
ab
ac
ba
bb
bc
ca
cb
cc
aaa
aab
aac
aba
abb
abc
aca
acb
acc
baa
bab
bac
bba
bbb
bbc
bca
bcb
bcc
caa
cab
cac
cba
cbb
cbc
cca
ccb
ccc

Array output

It will dump all combinations into an array.

var_dump(string_combinations('abc')->asArray());

Count combinations

It will return the number of possible combinations.

var_dump(count(string_combinations('abc'))); // 39

Specifying min length, max length

foreach (string_combinations('abc', $min = 2, $max = 2) as $combination) {
    echo $combination . PHP_EOL;
}

Output:

aa
ab
ac
ba
bb
bc
ca
cb
cc

Using an array as first argument, and a separator

foreach (string_combinations(['woof', 'meow', 'roar'], 2, 3, '-') as $combination) {
    echo $combination . PHP_EOL;
}

Output:

woof-woof
woof-meow
woof-roar
meow-woof
meow-meow
meow-roar
roar-woof
roar-meow
roar-roar
woof-woof-woof
woof-woof-meow
woof-woof-roar
woof-meow-woof
woof-meow-meow
woof-meow-roar
woof-roar-woof
woof-roar-meow
woof-roar-roar
meow-woof-woof
meow-woof-meow
meow-woof-roar
meow-meow-woof
meow-meow-meow
meow-meow-roar
meow-roar-woof
meow-roar-meow
meow-roar-roar
roar-woof-woof
roar-woof-meow
roar-woof-roar
roar-meow-woof
roar-meow-meow
roar-meow-roar
roar-roar-woof
roar-roar-meow
roar-roar-roar

No duplicates

You can avoid generating stings having the same letter more than once with the withoutDuplicates() method:

$combinations = string_combinations('abc');
var_dump(count($combinations)); // 39
print_r($combinations->asArray());

Array ( [0] => a [1] => b [2] => c [3] => aa [4] => ab [5] => ac [6] => ba [7] => bb [8] => bc [9] => ca [10] => cb [11] => cc [12] => aaa [13] => aab [14] => aac [15] => aba [16] => abb [17] => abc [18] => aca [19] => acb [20] => acc [21] => baa [22] => bab [23] => bac [24] => bba [25] => bbb [26] => bbc [27] => bca [28] => bcb [29] => bcc [30] => caa [31] => cab [32] => cac [33] => cba [34] => cbb [35] => cbc [36] => cca [37] => ccb [38] => ccc )

$combinations = $combinations->withoutDuplicates();
var_dump(count($combinations)); // 15
print_r($combinations->asArray());

Array ( [0] => a [1] => b [2] => c [3] => ab [4] => ac [5] => ba [6] => bc [7] => ca [8] => cb [9] => abc [10] => acb [11] => bac [12] => bca [13] => cab [14] => cba )

Performance considerations

Because it uses Generators to generate all possible combinations, you can iterate over thousands of possibilities without losing a single MB.

This has been executed on my Core i7 personnal computer with 8GB RAM:

require_once __DIR__ . '/vendor/autoload.php';
use function BenTools\StringCombinations\string_combinations;

$start = microtime(true);
$combinations = string_combinations('abcd1234');
foreach ($combinations as $c => $combination) {
    continue;
}
$end = microtime(true);

printf(
    'Generated %d combinations in %ss - Memory usage: %sMB / Peak usage: %sMB' . PHP_EOL,
    ++$c,
    round($end - $start, 3),
    round(memory_get_usage(true) / 1024 / 1024),
    round(memory_get_peak_usage(true) / 1024 / 1024)
);

Output:

Generated 19173960 combinations in 5.579s - Memory usage: 2MB / Peak usage: 2MB

Tests

./vendor/bin/phpunit

See also

• bentools/cartesian-product
• bentools/iterable-functions