Counting Sort Algo

Author: lerabela

Sorting/Counting Sort/countingSort_1.php

@@ -0,0 +1,101 @@
+<?php
+
+$randomSeq = array(5,14,3,2,23,3,40,38,3,4,41,8,18);
+
+/**
+ * Counting Sort algorithm for positve intergers
+ * @param $rand Array
+ * @return Array sorted array
+ */
+function countingSort(array &$rand): array{
+
+    // output array
+    $output = array(count($rand));
+
+    /**
+     * unsorted array size [$rand] - for [n] elements
+     * Total size of [n] elements in the unsorted array 
+     */
+    $array_size = count($rand);
+
+    /**
+     * find the max value in the range of [n] elements
+     */
+    $max_val = max($rand);
+
+    /**
+     * create counting slots - for [k] elements in range
+     * In this instance [k] will equal the max value in 
+     * range.
+     */
+    $count = array_fill(0, $max_val + 1,0);
+
+    /**
+     * Count all occurances of range items within the given array 
+     * Range in this instance will be from 0 - (k + 1), with k being
+     * the max value from the given array.
+     */
+    foreach($rand as $key => $val){
+
+        /**
+         * Increament value correponding to value
+         * i.e $count[2]++
+         */
+        $count[$val]++;
+
+    }
+
+    /**
+     * Store the cummulative count of each element
+     * Iterate count array and set start index position of 
+     * counted elements.
+     */
+    for($idx = 1; $idx <= $max_val; $idx++){
+
+        /**
+         * We are adding the previous index's count 
+         * to the current count. This will continue 
+         * for [k] range of elements. 
+         */
+        $count[$idx] = ($count[$idx] + $count[$idx - 1]);
+
+    }
+
+    // Find the index of each element
+    for ($idx = $array_size - 1; $idx >= 0; $idx--) {
+
+        /**
+         * So we use the value at the current index pointed to. 
+         * We then grap its cummulative value
+         * and adjust to correct position, by subtracting 1.
+         */
+        $index = ($count[$rand[$idx]] - 1); 
+
+        /**
+         * We then store the value from the original
+         * array in the adjusted postion.
+         */
+        $output[$index] = $rand[$idx];
+
+        /**
+         * We then decreament the current stored value to reflect 
+         * the moved value.
+         */
+        $count[$rand[$idx]]--;
+
+    }
+
+
+    // Copy the sorted array into the original array
+    for($idx = 0; $idx <= $array_size - 1; $idx++){
+
+        // move to correct position
+        $rand[$idx] = $output[$idx];
+
+    }
+
+    return $rand;
+
+}
+
+print_r(countingSort($randomSeq));