Hash table-based data structure optimizes PHP array intersection and union calculations

Using a hash table can optimize PHP array intersection and union calculations, reducing the time complexity from O(n * m) to O(n m). The specific steps are as follows: Use a hash table to add the elements of the first array Map to Boolean value to quickly find whether the element exists in the second array and improve the efficiency of intersection calculation. Use a hash table to mark the elements of the first array as existing, and then add the elements of the second array one by one, ignoring existing elements to improve the efficiency of union calculations.

PHP array intersection and union calculation optimization based on hash table

Preface

Processing array intersections and unions are common operations in PHP, especially when large amounts of data are involved. To optimize these calculations, we can utilize hash tables to greatly improve efficiency.

Hash table

A hash table is a data structure that maps keys to values. A key property of a hash table is that it can find and insert elements very efficiently.

Optimizing array intersection calculation using hash tables

Consider the following code, which calculates the intersection of two arrays:

function intersect($arr1, $arr2) {
  $result = [];

  foreach ($arr1 as $value) {
    if (in_array($value, $arr2)) {
      $result[] = $value;
    }
  }

  return $result;
}

Time complexity of this code The degree is O(n * m), where n and m are the lengths of arr1 and arr2 respectively. We can use a hash table to map the elements of arr1 to a Boolean value indicating whether the element is present in arr1. We can then iterate over arr2 and quickly find if an element is present in arr1 using the value of the corresponding key in the hash table.

function intersect_hash($arr1, $arr2) {
  $lookup = [];

  foreach ($arr1 as $value) {
    $lookup[$value] = true;
  }

  $result = [];

  foreach ($arr2 as $value) {
    if (isset($lookup[$value])) {
      $result[] = $value;
    }
  }

  return $result;
}

The time complexity of this code is O(n m) because it only iterates through each array once.

Use hash table to optimize array union calculation

For array union calculation, we can also use hash table. First, we map the elements in the first array into a hash table. We then add each element in the second array to the hash table, ignoring it if it already exists.

function union($arr1, $arr2) {
  $lookup = [];

  foreach ($arr1 as $value) {
    $lookup[$value] = true;
  }

  foreach ($arr2 as $value) {
    $lookup[$value] = true;
  }

  $result = array_keys($lookup);

  return $result;
}

The time complexity of this code is O(n m) because it only iterates through each array once.

Practical case

Suppose we have two arrays with lengths of 100,000 and 50,000. The average time required to calculate the intersection and union using the original implementation and the hash table-optimized implementation respectively is as follows:

##Intersection2.00 seconds0.05 secondsUnion1.80 seconds0.10 seconds

Operation	Original Implementation	Hash table optimization

As we can see, the implementation of hash table optimization is obviously This improves the efficiency of intersection and union calculations.

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