Discussing the problem of handling array hash conflicts in PHP7

When writing PHP7 programs, arrays are a commonly used data structure. Arrays can store large amounts of data, and are very convenient to search and operate. However, when a large amount of data needs to be stored in the array, hash conflicts may occur, which will affect the performance and efficiency of the array. This article will explore how to deal with array hash collisions in PHP7.

Basic principle of hash table

Hash table is a data structure based on hash function. Hash functions map data into fixed-size buckets. A hash collision occurs when two data are mapped into the same bucket. To resolve hash collisions, a common approach is to use chain hashing or open address hashing algorithms.

Using hash tables to store arrays in PHP7

PHP7 uses hash tables as the internal implementation of arrays. Each element in the array has a hash value, and the function zend_inline_hash_func() is used when calculating the hash value. This function is a fast hash algorithm, and its core idea is to convert the value of the element into a hash code. In PHP7, the number of buckets in the hash table is fixed and is a power of 2, usually 8, 16, 32, 64, etc.

Elements in the array are stored in buckets, which in turn are stored in hash tables. Each bucket is a linked list structure. When a hash conflict occurs, elements will be added to the end of the linked list of the corresponding bucket. The hash table also expands dynamically when the number of elements in the array increases. When the number of elements in the array decreases, the hash table also shrinks and all elements are rehashed.

Methods to deal with hash conflicts

Due to the way the hash table stores elements in the array, hash conflicts may occur. To solve this problem, you can use the following method:

1. Open address hashing

Open address hashing is a common method to resolve hash conflicts. When an element is inserted, if a hash conflict occurs, a series of probing algorithms are used to find the next suitable bucket until a suitable free bucket is found. Open address hashing can also use different probing algorithms, such as linear probing, square probing, double hashing, etc.

2. Chained Hashing

Chained hashing is another common method of resolving hash collisions. When a hash collision occurs, the elements in the array will be added to the linked list of the corresponding bucket. If you need to find or remove an element, you need to traverse the entire linked list to find the target element.

The hash table implementation used internally in PHP7 uses chain hashing. When there are multiple elements in the same bucket, they form a linked list. When an element needs to be found or manipulated, PHP7 will traverse the entire linked list to find the target element.

3. Change the number of buckets

The number of buckets is related to the performance of the hash table. If the number of buckets is too small, hash conflicts will increase and reduce the performance of the hash table. If the number of buckets is too large, it will waste space in the hash table. The performance and space usage of the hash table can be controlled by changing the number of buckets.

In PHP7, the number of buckets is fixed and cannot be changed. When the number of elements in the array increases, PHP7 will control the number of hash conflicts by adjusting the number of buckets in the hash table. This adjustment is dynamic and can be achieved by adjusting the size of the hash table, re-hashing, etc.

Conclusion

PHP7 uses a hash table to store array elements. In order to solve the problem of hash conflicts, PHP7 uses a chain hash algorithm internally. When there are multiple elements in a bucket, they form a linked list. If you need to find or operate an element, you need to traverse the entire linked list to find the target element. The performance and space usage of the hash table can be controlled by changing the number of buckets. In addition, PHP7 will also dynamically adjust the size of the hash table and re-hash to control the number of hash conflicts.

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