<?php

namespace Mdanter\Ecc\Util;

class BinaryString
{
    /**
     * Multi-byte-safe string length calculation
     *
     * @param string $str
     * @return int
     */
    public static function length($str)
    {
        // Premature optimization: cache the function_exists() result
        static $exists = null;
        if ($exists === null) {
            $exists = function_exists('mb_strlen');
        }

        // If it exists, we need to make sure we're using 8bit mode
        if ($exists) {
            return mb_strlen($str, '8bit');
        }
        return strlen($str);
    }

    /**
     * Multi-byte-safe substring calculation
     *
     * @param string $str
     * @param int $start
     * @param int $length (optional)
     * @return string
     */
    public static function substring($str, $start = 0, $length = null)
    {
        // Premature optimization: cache the function_exists() result
        static $exists = null;
        if ($exists === null) {
            $exists = function_exists('mb_substr');
        }

        // If it exists, we need to make sure we're using 8bit mode
        if ($exists) {
            return mb_substr($str, $start, $length, '8bit');
        } elseif ($length !== null) {
            return substr($str, $start, $length);
        }
        return substr($str, $start);
    }
    
    /**
     * Equivalent to hash_equals() in PHP 5.6 
     * 
     * @param string $knownString
     * @param string $userString
     * 
     * @return bool
     */
    public static function constantTimeCompare($knownString, $userString)
    {
        if (PHP_VERSION_ID >= 50600) {
            return hash_equals($knownString, $userString);
        }
        
        // We have to roll our own
        $kLen = self::length($knownString);
        $uLen = self::length($userString);
        if ($kLen !== $uLen) {
            return false;
        }
        $result = 0;
        for ($i = 0; $i < $kLen; $i++) {
            $result |= (ord($knownString[$i]) ^ ord($userString[$i]));
        }
        // They are only identical strings if $result is exactly 0...
        return 0 === $result;
    }
}