similarity.php

<?php
// 7 kyu - Simple Fun #138: Similarity
// Task
// Your task is to find the similarity of given sorted arrays a and b, which is defined as follows:
//
// you take the number of elements which are present in both arrays and divide it by the number of elements which are present in at least one array.
//
// It also can be written as a formula similarity(A, B) = #(A ∩ B) / #(A ∪ B), where #(C) is the number of elements in C, ∩ is intersection of arrays, ∪ is union of arrays.
//
// This is known as Jaccard similarity.
//
// The result is guaranteed to fit any floating-point type without rounding.
//
// Example
// For a = [1, 2, 4, 6, 7] and b = [2, 3, 4, 7]:
//
// elements [2, 4, 7] are present in both arrays;
// elements [1, 2, 3, 4, 6, 7] are present in at least one of the arrays.
// So the similarity equals to 3 / 6 = 0.5.

function similarity(array $a, array $b): float {
  if (!empty($b)){
    return count(array_intersect($a, $b)) / count(array_unique(array_merge($a, $b)));
  }
  return null;
}

?>