<?php

/**
 * Binary search algorithm iterative approach
 *
 * Be careful collection must be ascending sorted, otherwise result will be unpredictable
 *
 * Examples:
 * binarySearchIterative([0, 5, 7, 10, 15], 0);
 * 0
 * binarySearchIterative([0, 5, 7, 10, 15], 15);
 * 4
 * binarySearchIterative([0, 5, 7, 10, 15], 5);
 * 1
 * binarySearchIterative([0, 5, 7, 10, 15], 6);
 *
 * @param array $list
 * @param int $target
 * @return int
 */
function binarySearchIterative($list, $target)
{
    $first = 0;
    $last = count($list)-1;


    while ($first <= $last) {
        $mid = ($first + $last) >> 1;


        if ($list[$mid] == $target) {
            return $mid;
        } elseif ($list[$mid] > $target) {
            $last = $mid - 1;
        } elseif ($list[$mid] < $target) {
            $first = $mid + 1;
        }
    }

    return null;
}

/**
 * Binary search algorithm in PHP by recursion
 *
 * Be careful collection must be ascending sorted, otherwise result will be unpredictable
 *
 * Examples:
 * binarySearchByRecursion([0, 5, 7, 10, 15], 0, 0, 4);
 * 0
 * binarySearchByRecursion([0, 5, 7, 10, 15], 15, 0, 4);
 * 4
 * binarySearchByRecursion([0, 5, 7, 10, 15], 5, 0, 4);
 * 1
 * binarySearchByRecursion([0, 5, 7, 10, 15], 6, 0, 4);
 *
 * @param Array $list a sorted array list of integers to search
 * @param integer $target an integer number to search for in the list
 * @param integer $start an integer number where to start searching in the list
 * @param integer $end an integer number where to end searching in the list
 * @return integer the index where the target is found (or null if not found)
 */
function binarySearchByRecursion($list, $target, $start, $end)
{
    if (count($list) == 0) {
        return null;
    }

    if (count($list) < 2) {
        return $list[0] == $target ? 0 : null;
    }

    if ($start > $end)
        return null;


    $mid = ($start + $end) >> 1;


    if ($list[$mid] == $target) {
        return $mid;
    } elseif ($list[$mid] > $target) {
        return binarySearchByRecursion($list, $target, $start, $mid-1);
    } elseif ($list[$mid] < $target) {
        return binarySearchByRecursion($list, $target, $mid+1, $end);
    }

    return null;
}