Thoughts on the stability of PHP sorting!
PHP sorting stability problem
I recently encountered a very interesting problem at work. The online input is a sequence of sorted associative arrays. After a The array output after series processing is out of order, and cannot be reproduced when running locally. After checking the relevant code, the most concerning part is this paragraph:
$categories = Arr::sort($categories, function ($node) {
return $node['default'];
}, true); is used to bring elements to the front according to default priority. First, confirm the illuminate on the bottom line. /supportThe version is consistent with the local one, check Arr::sort()source code:
...
$descending ? arsort($results, $options)
: asort($results, $options);In the end, php's asort is called, and online is php5. Both local and testing were replaced by php7 due to recent upgrade considerations. Finally, it was successfully reproduced in the php5 environment and it was determined to be the sort problem.
The relative position of equal elements before and after sorting does not change, which means that this algorithm is stable.
If you have a certain understanding of quick sort, you can know that quick sort is unstable, so the output of this code will not be when the elements default have the same priority. The previous order, but why do the test results retain the original order in the php7 environment. Is it wrong that I understood that all default sorting in the world is quick sort?
Okay, let’s take a quick look at how the PHP source code solves this problem of. Go to the official php-src of github and directly search asort in this repository, find the c file, and find this function defined in arr.c:814
PHP_FUNCTION(asort)
{
zval *array;
zend_long sort_type = PHP_SORT_REGULAR;
bucket_compare_func_t cmp;
ZEND_PARSE_PARAMETERS_START(1, 2)
Z_PARAM_ARRAY_EX(array, 0, 1)
Z_PARAM_OPTIONAL
Z_PARAM_LONG(sort_type)
ZEND_PARSE_PARAMETERS_END();
// 设置比较函数
cmp = php_get_data_compare_func(sort_type, 0);
zend_hash_sort(Z_ARRVAL_P(array), cmp, 0);
RETURN_TRUE;
}You can see that the final call is zend_hash_sort(), we continue to search:
and find that this function is the nesting doll of zend_hash_sort_ex(), and finally find zend_hash.c: 2497
ZEND_API void ZEND_FASTCALL zend_hash_sort_ex(HashTable *ht, sort_func_t sort, bucket_compare_func_t compar, zend_bool renumber)
{
Bucket *p;
uint32_t i, j;
IS_CONSISTENT(ht);
HT_ASSERT_RC1(ht);
if (!(ht->nNumOfElements>1) && !(renumber && ht->nNumOfElements>0)) {
/* Doesn't require sorting */
return;
}
// 这里的hole指数组元素被unset掉产生的洞
if (HT_IS_WITHOUT_HOLES(ht)) {
/* Store original order of elements in extra space to allow stable sorting. */
for (i = 0; i < ht->nNumUsed; i++) {
Z_EXTRA(ht->arData[i].val) = i;
}
} else {
/* Remove holes and store original order. */
for (j = 0, i = 0; j < ht->nNumUsed; j++) {
p = ht->arData + j;
if (UNEXPECTED(Z_TYPE(p->val) == IS_UNDEF)) continue;
if (i != j) {
ht->arData[i] = *p;
}
Z_EXTRA(ht->arData[i].val) = i;
i++;
}
ht->nNumUsed = i;
}
sort((void *)ht->arData, ht->nNumUsed, sizeof(Bucket), (compare_func_t) compar,
(swap_func_t)(renumber? zend_hash_bucket_renum_swap :
((HT_FLAGS(ht) & HASH_FLAG_PACKED) ? zend_hash_bucket_packed_swap : zend_hash_bucket_swap)));
...Okay! , the official comments explain how to achieve the stability of sorting. Before sorting, use this Z_EXTRA to retain the mapping of original array elements to subscripts.
But when I searched for the submission information of this code, I found a problem: the rfc related to stable sorting is at https://wiki.php.net/rfc/stable_sorting. You can see that it happened in May this year. And for php8.0.
?? What happened to the previous php7 sorting stability?
Immediately construct an example:
$count = 10;
$cc = [];
for ($i=0; $i<$count; $i++) {
$cc[] = [
'id' => $i,
'default' => rand(0, 10),
];
}
$cc = Arr::sort($cc, function ($i) {
return $i['default'];
});
dd($cc);After many tests under php7, it was found that when $count is relatively small, the sorting is stable, but$countThe sorting becomes unstable again in larger cases. That is to say, online sorting is unstable and cannot be reproduced locally because the array length of the use case is too short.
Seeing this, it can be determined that it is a problem of quick sort length threshold optimization, and finally checked the relevant information. sort in php7 is based on the std::sort implementation of libc in LLVM. There is special optimization when the number of elements is less than or equal to 16. If the number of elements is greater than 16, quick sorting will be performed, while php5 will directly use quick sorting.
<?php
$count = 100;
$cc = [];
for ($i=0; $i<$count; $i++) {
$cc[] = [
'id' => $i,
'default' => rand(0, 10),
];
}
usort($cc, function($a, $b){
if ($a['default'] == $b['default']) return 0;
return ($a['default'] < $b['default']) ? 1 : -1;
});
print_r($cc);Finally tried it in php8 environment, the sorting is absolutely stable
The above is the detailed content of Thoughts on the stability of PHP sorting!. For more information, please follow other related articles on the PHP Chinese website!
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
1670
14
1428
52
1329
25
1273
29
1256
24
PHP: A Key Language for Web Development
Apr 13, 2025 am 12:08 AM
PHP is a scripting language widely used on the server side, especially suitable for web development. 1.PHP can embed HTML, process HTTP requests and responses, and supports a variety of databases. 2.PHP is used to generate dynamic web content, process form data, access databases, etc., with strong community support and open source resources. 3. PHP is an interpreted language, and the execution process includes lexical analysis, grammatical analysis, compilation and execution. 4.PHP can be combined with MySQL for advanced applications such as user registration systems. 5. When debugging PHP, you can use functions such as error_reporting() and var_dump(). 6. Optimize PHP code to use caching mechanisms, optimize database queries and use built-in functions. 7
PHP vs. Python: Understanding the Differences
Apr 11, 2025 am 12:15 AM
PHP and Python each have their own advantages, and the choice should be based on project requirements. 1.PHP is suitable for web development, with simple syntax and high execution efficiency. 2. Python is suitable for data science and machine learning, with concise syntax and rich libraries.
PHP and Python: Comparing Two Popular Programming Languages
Apr 14, 2025 am 12:13 AM
PHP and Python each have their own advantages, and choose according to project requirements. 1.PHP is suitable for web development, especially for rapid development and maintenance of websites. 2. Python is suitable for data science, machine learning and artificial intelligence, with concise syntax and suitable for beginners.
PHP in Action: Real-World Examples and Applications
Apr 14, 2025 am 12:19 AM
PHP is widely used in e-commerce, content management systems and API development. 1) E-commerce: used for shopping cart function and payment processing. 2) Content management system: used for dynamic content generation and user management. 3) API development: used for RESTful API development and API security. Through performance optimization and best practices, the efficiency and maintainability of PHP applications are improved.
The Enduring Relevance of PHP: Is It Still Alive?
Apr 14, 2025 am 12:12 AM
PHP is still dynamic and still occupies an important position in the field of modern programming. 1) PHP's simplicity and powerful community support make it widely used in web development; 2) Its flexibility and stability make it outstanding in handling web forms, database operations and file processing; 3) PHP is constantly evolving and optimizing, suitable for beginners and experienced developers.
PHP and Python: Different Paradigms Explained
Apr 18, 2025 am 12:26 AM
PHP is mainly procedural programming, but also supports object-oriented programming (OOP); Python supports a variety of paradigms, including OOP, functional and procedural programming. PHP is suitable for web development, and Python is suitable for a variety of applications such as data analysis and machine learning.
PHP vs. Other Languages: A Comparison
Apr 13, 2025 am 12:19 AM
PHP is suitable for web development, especially in rapid development and processing dynamic content, but is not good at data science and enterprise-level applications. Compared with Python, PHP has more advantages in web development, but is not as good as Python in the field of data science; compared with Java, PHP performs worse in enterprise-level applications, but is more flexible in web development; compared with JavaScript, PHP is more concise in back-end development, but is not as good as JavaScript in front-end development.
PHP and Python: Code Examples and Comparison
Apr 15, 2025 am 12:07 AM
PHP and Python have their own advantages and disadvantages, and the choice depends on project needs and personal preferences. 1.PHP is suitable for rapid development and maintenance of large-scale web applications. 2. Python dominates the field of data science and machine learning.
