php 二分查找

假设数组已经排好顺序

* binarysearch.php

<?php
function binarysearch(array $a, $item, callable $c = null) {
    $low = 0;
    $high = count($a) - 1;

    if (null == $c) {
        $c = function($a, $b) {
            return $a - $b;
        };
    }
    
    while ($low <= $high) {
        $mid = floor( ($high-$low)/2  + $low );
        if ($c($a[$mid], $item) < 0) {
            $low = $mid + 1;
        } else if ($c($a[$mid], $item) > 0) {
            $high = $mid - 1;
        } else {
            return $mid;
        }
    }
    return -1;
}

test:

* index.php

<?php
include './binarysearch.php';

$a = ['b', 'e', 'h', 'i', 'n', 'p', 'r', 's', 'y'];

$c = function($a, $b) {
    return strcmp($a, $b);
};

echo binarysearch($a, 'b', $c).PHP_EOL;
echo binarysearch($a, 'p', $c).PHP_EOL;
echo binarysearch($a, 'y', $c).PHP_EOL;

0

5

8