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. /support
The 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 $count
is relatively small, the sorting is stable, but$count
The 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!

What’s still popular is the ease of use, flexibility and a strong ecosystem. 1) Ease of use and simple syntax make it the first choice for beginners. 2) Closely integrated with web development, excellent interaction with HTTP requests and database. 3) The huge ecosystem provides a wealth of tools and libraries. 4) Active community and open source nature adapts them to new needs and technology trends.

PHP and Python are both high-level programming languages that are widely used in web development, data processing and automation tasks. 1.PHP is often used to build dynamic websites and content management systems, while Python is often used to build web frameworks and data science. 2.PHP uses echo to output content, Python uses print. 3. Both support object-oriented programming, but the syntax and keywords are different. 4. PHP supports weak type conversion, while Python is more stringent. 5. PHP performance optimization includes using OPcache and asynchronous programming, while Python uses cProfile and asynchronous programming.

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 originated in 1994 and was developed by RasmusLerdorf. It was originally used to track website visitors and gradually evolved into a server-side scripting language and was widely used in web development. Python was developed by Guidovan Rossum in the late 1980s and was first released in 1991. It emphasizes code readability and simplicity, and is suitable for scientific computing, data analysis and other fields.

PHP is suitable for web development and rapid prototyping, and Python is suitable for data science and machine learning. 1.PHP is used for dynamic web development, with simple syntax and suitable for rapid development. 2. Python has concise syntax, is suitable for multiple fields, and has a strong library ecosystem.

PHP remains important in the modernization process because it supports a large number of websites and applications and adapts to development needs through frameworks. 1.PHP7 improves performance and introduces new features. 2. Modern frameworks such as Laravel, Symfony and CodeIgniter simplify development and improve code quality. 3. Performance optimization and best practices further improve application efficiency.

PHPhassignificantlyimpactedwebdevelopmentandextendsbeyondit.1)ItpowersmajorplatformslikeWordPressandexcelsindatabaseinteractions.2)PHP'sadaptabilityallowsittoscaleforlargeapplicationsusingframeworkslikeLaravel.3)Beyondweb,PHPisusedincommand-linescrip

PHP type prompts to improve code quality and readability. 1) Scalar type tips: Since PHP7.0, basic data types are allowed to be specified in function parameters, such as int, float, etc. 2) Return type prompt: Ensure the consistency of the function return value type. 3) Union type prompt: Since PHP8.0, multiple types are allowed to be specified in function parameters or return values. 4) Nullable type prompt: Allows to include null values and handle functions that may return null values.


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

SublimeText3 English version
Recommended: Win version, supports code prompts!

VSCode Windows 64-bit Download
A free and powerful IDE editor launched by Microsoft

SAP NetWeaver Server Adapter for Eclipse
Integrate Eclipse with SAP NetWeaver application server.

SublimeText3 Linux new version
SublimeText3 Linux latest version

Dreamweaver CS6
Visual web development tools