PHP finds difference set and large array memory overflows

In PHP development, it is easy to encounter memory problems when dealing with large arrays. This article will discuss how to use the array_diff algorithm to solve the difference of huge arrays. Additionally, you'll learn how to use different memory management techniques to optimize performance when working with large arrays.

1. Problem description

Consider a scenario: there are two arrays, both of them are very large, each array has 100,000 elements. Now we want to find the difference between these two arrays. Simply put, it is to find elements that only exist in an array. The following is the code implementation:

<?php
$array1 = array();
$array2 = array();

// 初始化数组1，2，每个数组都有 10 万个元素
for($i=0;$i<1000000;$i++){
    $array1[$i] = $i;
    $array2[$i] = $i+1;
}

// 计算差集
$result = array_diff($array1, $array2);

print_r($result);
?>

When we run the above code, we will find that the page quickly becomes unresponsive, and then an error is reported saying that our PHP script has run out of allocable memory. This is because PHP's default memory limit is 128MB, which is not large enough to handle large arrays. Therefore, optimization algorithms or other memory management techniques need to be considered to solve this problem.

2. Optimization Algorithm

If the elements in the array are already arranged in order, you can use a cursor to speed up the search, which can reduce running time and memory usage. The following is the code implementation:

<?php
$array1 = array();
$array2 = array();

// 初始化数组1，2，每个数组都有 10 万个元素
for($i=0;$i<1000000;$i++){
    $array1[$i] = $i;
    $array2[$i] = $i+1;
}

// 排序数组1、2
sort($array1);
sort($array2);

// 初始化游标
$cursor1 = $cursor2 = 0;

// 计算差集
$result = array();
while($cursor1 < count($array1) && $cursor2 < count($array2)){
    if($array1[$cursor1] < $array2[$cursor2]){
        $result[] = $array1[$cursor1];
        $cursor1++;
    }
    elseif($array1[$cursor1] > $array2[$cursor2]){
        $cursor2++;
    }
    else{
        $cursor1++;
        $cursor2++;
    }
}

// 将数组1中剩余的元素添加入结果数组
while($cursor1 < count($array1)){
    $result[] = $array1[$cursor1];
    $cursor1++;
}

print_r($result);
?>

The above code will optimize execution time and make memory usage more efficient. However, if the array is not in order, then this algorithm will not work.

3. Use segmented processing technology

In PHP, the memory overhead used by array_diff when processing large arrays is very large. However, PHP's memory manager maintains a memory allocation table for each memory allocation. This table detects the size and location of each memory allocation. Therefore, you can use segmentation processing technology to divide a large array into many small sub-arrays, and process each sub-array separately to avoid taking up too much memory space. The following is the code implementation:

<?php
$array1 = array();
$array2 = array();

// 初始化数组1，2，每个数组都有 10 万个元素
for($i=0;$i<1000000;$i++){
    $array1[$i] = $i;
    $array2[$i] = $i+1;
}

// 分段，每段 10000 个元素
$chunkSize = 10000;
$chunks1 = array_chunk($array1, $chunkSize);
$chunks2 = array_chunk($array2, $chunkSize);

// 计算差集
$result = array();
foreach($chunks1 as $chunk1){
    $temp = array_diff($chunk1, array_merge(...$chunks2));
    $result = array_merge($result,$temp);
}

print_r($result);
?>

In the above code, we divide the array into many sub-arrays of size 10000 and store them in the chunks1 and chunks2 arrays. We then loop over chunks1, use array_diff to calculate the difference between each subarray and chunks2, and append the results to the $result results array. Finally, we merge $result into the final result.

4. Use generators to simulate traversal algorithms

Another way to solve the memory problem of large arrays is to use PHP's generator to simulate the traversal of finding the difference between two arrays. PHP's generators allow you to generate values ​​from a sequence one by one, rather than building the entire sequence in memory. The following is the code implementation:

<?php
$array1 = array();
$array2 = array();

// 初始化数组1，2，每个数组都有 10 万个元素
for($i=0;$i<1000000;$i++){
    $array1[$i] = $i;
    $array2[$i] = $i+1;
}

// 计算差集
$result = array();
function diff($arr1, $arr2) {
    sort($arr1);
    sort($arr2);
    $i = $j = 0;
    while($i < count($arr1) && $j < count($arr2)) {
        if($arr1[$i] < $arr2[$j]) {
            yield $arr1[$i];
            $i++;
        }
        elseif($arr1[$i] > $arr2[$j]){
            $j++;
        }
        else{
            $i++;
            $j++;
        }
    }
    while($i < count($arr1)) {
        yield $arr1[$i];
        $i++;
    }
}

// 遍历 generator
foreach (diff($array1, $array2) as $value) {
    $result[] = $value;
}

print_r($result);
?>

In the above code, we define a diff function and use a generator to simulate the traversal of calculating the array difference set. This algorithm uses less memory and CPU time by sorting the subarrays sequentially and then using cursor comparison to find the difference between the two arrays.

5. Summary

In PHP development, you need to be particularly careful when dealing with large arrays, because they may take up too much memory and cause memory overflow. In this article, we introduced techniques such as algorithm optimization, piecewise processing techniques, and generator simulated traversal algorithms that can be used to process large arrays. Which method you choose depends on your requirements and environment. Depending on your needs, you can use different techniques to optimize your code to improve code performance and maintainability when dealing with large arrays.

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