Debug functions for PHP arrays to calculate things like hash distribution and other internal information
Arrays in PHP (when they are not using linear keys) are stored as a hashmap. The hash used is either a user-provided integer (unmodified) or a rolling string hash when string keys are used. Depending on the number of keys, the table size used for the hash table varies.
The table is indexed by taking a certain number of the least significant bits of the key hash (or the naked key value, in the case of integers). This means that if your keys have poor distribution in the least significant bits, many hash collisions will occur.
Hash collisions in PHP arrays can be a major performance bottleneck which isn't obvious to the eye.
When two different keys produce the same hash, they are stored in a linked-list under that hash. When the hash is looked up, PHP then has to traverse every element of this linked-list one by one to find the actual matching key. When the array has a high number of hash collisions, these linked lists can get very long, resulting in a long brute-force search every time an array key is accessed.
The average length of these lists is known as the load factor, and gives us an idea of just how much the performance will suffer because of this. Ideally we want our load factor to be as close to 1 as possible, to avoid the bruteforce searches we just mentioned.
For example, the following code:
$array = [];
for($i = 0; $i < 10000; $i++){
$array[$i << 1] = $i;
}produces an array with a load factor of 1, while this code:
$array = [];
for($i = 0; $i < 10000; $i++){
$array[$i << 10] = $i;
}has a load factor of 312.5!
This means that, despite having the same number of keys, accesses to the second array will be significantly slower than the first.