Functions for identifying duplicates within arrays.

count-duplicates-both-ways.php

<?php
/**
 * Count duplicates in an array before and after each element.
 * 
 * <code>
 * $counts = count_duplicates_both_ways([0, 1, 1, 2, 2, 1]);
 * print_r($counts);
 * // [
 * //   'before' => [0, 0, 1, 0, 1, 2],
 * //   'after' => [0, 2, 1, 1, 0, 0]
 * // ]
 * </code>
 */
function count_duplicates_both_ways(array $arr)
{
    // Store results of duplicate checks
    $results_before = $results_after = array();

    // Count occurrences of elements before and after current element
    $counter_before = $counter_after = array();

    // Initialise counters. By default, $counter_before should be full of
    // zeroes, and $counter_after should contain the number of occurences of
    // each element.
    foreach ($arr as $v) {
        if (!isset($counter_before[$v])) {
            $counter_before[$v] = 0;
        }

        if (!isset($counter_after[$v])) {
            $counter_after[$v] = 0;
        }

        $counter_before[$v] = 0;
        $counter_after[$v]++;
    }

    // Calculate before/after duplicates
    foreach ($arr as $v) {
        // Remove the current element from $counter_after
        $counter_after[$v]--;

        // If the counters for this element are > 0, then that indicates that
        // there exists an element equal to this one before/after this one in
        // $arr.
        $results_before[] = $counter_before[$v] > 0;
        $results_after[] = $counter_after[$v] > 0;

        // Add this element to $counter_before
        $counter_before[$v]++;
    }
    
    return array('before' => $results_before, 'after' => $results_after);
}

has-duplicates.php

<?php
/**
 * @param array $arr The array to check for duplicates
 * @return bool True if the array contains any duplicates
 */
function has_duplicate(array $arr)
{
    $head = array();
    
    foreach ($arr as $e) {
        if (in_array($e, $head)) return true;
        $head[] = $e;
    }
}

/**
 * @param array $arr The array to check for duplicates
 * @return bool True if the array contains any duplicates
 */
function has_duplicates(array $arr)
{
    for ($i = 0; $i < count($arr) ; $i++) {
        for ($j = 0; $j < $i; $j++) {
            if ($arr[$j] === $arr[$i]) return true;
        }   
    }
}

/**
 * Determine which elements of an array have backward duplicates.
 * 
 * <code>
 * $arr = [0,0,1,1,2,3,4,3,2,1];
 * $dupes = has_backward_duplicates($arr);
 * 
 * print_r($dupes); // [0, 1, 0, 1, 0, 0, 0, 1, 1, 1]
 * print_r($arr);   // [0, 0, 1, 1, 2, 3, 4, 3, 2, 1]
 * </code>
 * 
 * @param array $arr The array to check for duplicates
 * @return array An array of booleans, same length as $arr. Values are true
 * where $arr contains backwards duplicates
 */
function has_backward_duplicates(array $arr)
{
    $results = array();
    
    for ($i = 0; $i < count($arr) ; $i++, $duped = false) {
        
        // Loop backward through previous elements to look for dupes
        for ($j = 0; $j < $i; $j++) {
            if ($arr[$j] === $arr[$i]) $duped = true;
        }
        
        $results[] = $duped;
    }
    
    return $results;
}