The Set class

    (PECL ds >= 1.0.0)

    Giriş

    A Set is a sequence of unique values. This implementation uses the same hash table as Ds\Map, where values are used as keys and the mapped value is ignored.

    Strengths

    • Values can be any type, including objects.
    • Supports array syntax (square brackets).
    • Insertion order is preserved.
    • Automatically frees allocated memory when its size drops low enough.
    • add(), remove() and contains() are all O(1).

    Weaknesses

    • Doesn’t support push(), pop(), insert(), shift(), or unshift().
    • get() is O(n) if there are deleted values in the buffer before the accessed index, O(1) otherwise.

    Sınıf Sözdizimi

    class Ds\Set implements Ds\Collection, ArrayAccess {
    /* Sabitler */
    const int MIN_CAPACITY = 8;
    /* Yöntemler */
    public function add(mixed ...$values): void
    public function allocate(int $capacity): void
    public function capacity(): int
    public function clear(): void
    public function contains(mixed ...$values): bool
    public function copy(): Ds\Set
    public function diff(Ds\Set $set): Ds\Set
    public function filter(callable $callback = ?): Ds\Set
    public function first(): mixed
    public function get(int $index): mixed
    public function intersect(Ds\Set $set): Ds\Set
    public function isEmpty(): bool
    public function join(string $glue = ?): string
    public function last(): mixed
    public function map(callable $callback): Ds\Set
    public function merge(mixed $values): Ds\Set
    public function reduce(callable $callback, mixed $initial = ?): mixed
    public function remove(mixed ...$values): void
    public function reverse(): void
    public function reversed(): Ds\Set
    public function slice(int $index, int $length = ?): Ds\Set
    public function sort(callable $comparator = ?): void
    public function sorted(callable $comparator = ?): Ds\Set
    public function sum(): int|float
    public function toArray(): array
    public function union(Ds\Set $set): Ds\Set
    public function xor(Ds\Set $set): Ds\Set
    }

    Öntanımlı Sabitler

    Ds\Set::MIN_CAPACITY

    Sürüm Bilgisi

    Sürüm: Açıklama
    PECL ds 1.3.0 The class now implements ArrayAccess.
    PECL ds 1.2.7 Added the Ds\Set::map() method.
    PECL ds 1.2.0 Ds\Set::MIN_CAPACITY changed from 16 to 8.

    İçindekiler

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    User Contributed Notes 2 notes

    up
    19
    Anonymous
    8 years ago
    Lookup for a Set should be O(1). This is true for sets and hashtables (eg maps) in any language.

The way this is possible is that sets store values differently than arrays.

In an array values are stored sequentially based on what their place is in the array and where that array is in memory, so to find your item you need to scan through the array sequentially to find your item (unless it's a sorted array, then you can use binary search at O(logn)).

Sets declare a block of memory, like an array, but instead of putting items in memory in sequence, like an array, they determine the index of the item to add by running the item through a hash function (essentially a function that takes in an object and returns an evenly distributed, very large random number), and then modulousing the result of that hash function by the size of the memory block they have.

So, when you call contains($needle, $mySetHaystack), php will take $needle, and feed it into a hashfunction, which will return a big number like 9283472378, then it takes the length of $mySetHaystack (let's say 31), and does 9283472378 % 31 = 28, so it checks the 28th index of $mySetHaystack to see if $needle is there. Everything in this list of operations is independent of the size of $mySetHaystack, hence the perf being O(1).

If a hash function returns the same value for two different items (a hash collision, which totally happens), or if the modulo of that value is the same, then an array of values is stored in the set at that index. Since sets don't allow duplicate values, this happens rarely and is negligible from a perf perspective.

You should check out the wikipedia page on hash tables (similar to sets), as there are lots of pictures that will make this concept easier to understand.
    up
    7
    Sbastien
    3 years ago
    One nice thing is the type-awarness. array_unique() lose values (whatever flag), while Ds\Set keep them.

<?php

$array = [true, false, null, '', 0, '0', 123, '123'];
var_dump(array_unique($array));
var_dump(new \Ds\Set($array));

/*

Gives :

array(4) {
  [0]=> bool(true)
  [1]=> bool(false)
  [4]=> int(0)
  [6]=> int(123)
}
object(Ds\Set)#1 (8) {
  [0]=> bool(true)
  [1]=> bool(false)
  [2]=> NULL
  [3]=> string(0) ""
  [4]=> int(0)
  [5]=> string(1) "0"
  [6]=> int(123)
  [7]=> string(3) "123"
}

*/
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