<?php
/**
 * Random_* Compatibility Library 
 * for using the new PHP 7 random_* API in PHP 5 projects
 * 
 * The MIT License (MIT)
 * 
 * Copyright (c) 2015 Paragon Initiative Enterprises
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

if (!defined('RANDOM_COMPAT_READ_BUFFER')) {
    define('RANDOM_COMPAT_READ_BUFFER', 8);
}

/**
 * Unless open_basedir is enabled, use /dev/urandom for
 * random numbers in accordance with best practices
 * 
 * Why we use /dev/urandom and not /dev/random
 * @ref http://sockpuppet.org/blog/2014/02/25/safely-generate-random-numbers
 * 
 * @param int $bytes
 * 
 * @throws Exception
 * 
 * @return string
 */
function random_bytes($bytes)
{
    static $fp = null;
    /**
     * This block should only be run once
     */
    if (empty($fp)) {
        /**
         * We use /dev/urandom if it is a char device.
         * We never fall back to /dev/random
         */
        $fp = fopen('/dev/urandom', 'rb');
        if (!empty($fp)) {
            $st = fstat($fp);
            if (($st['mode'] & 0170000) !== 020000) {
                fclose($fp);
                $fp = false;
            }
        }

        if (!empty($fp)) {
            /**
             * stream_set_read_buffer() does not exist in HHVM
             * 
             * If we don't set the stream's read buffer to 0, PHP will
             * internally buffer 8192 bytes, which can waste entropy
             * 
             * stream_set_read_buffer returns 0 on success
             */
            if (function_exists('stream_set_read_buffer')) {
                stream_set_read_buffer($fp, RANDOM_COMPAT_READ_BUFFER);
            }
            if (function_exists('stream_set_chunk_size')) {
                stream_set_chunk_size($fp, RANDOM_COMPAT_READ_BUFFER);
            }
        }
    }

    try {
        $bytes = RandomCompat_intval($bytes);
    } catch (TypeError $ex) {
        throw new TypeError(
            'random_bytes(): $bytes must be an integer'
        );
    }

    if ($bytes < 1) {
        throw new Error(
            'Length must be greater than 0'
        );
    }

    /**
     * This if() block only runs if we managed to open a file handle
     * 
     * It does not belong in an else {} block, because the above 
     * if (empty($fp)) line is logic that should only be run once per
     * page load.
     */
    if (!empty($fp)) {
        $remaining = $bytes;
        $buf = '';

        /**
         * We use fread() in a loop to protect against partial reads
         */
        do {
            $read = fread($fp, $remaining); 
            if ($read === false) {
                /**
                 * We cannot safely read from the file. Exit the
                 * do-while loop and trigger the exception condition
                 */
                $buf = false;
                break;
            }
            /**
             * Decrease the number of bytes returned from remaining
             */
            $remaining -= RandomCompat_strlen($read);
            $buf .= $read;
        } while ($remaining > 0);
        
        /**
         * Is our result valid?
         */
        if ($buf !== false) {
            if (RandomCompat_strlen($buf) === $bytes) {
                /**
                 * Return our random entropy buffer here:
                 */
                return $buf;
            }
        }
    }

    /**
     * If we reach here, PHP has failed us.
     */
    throw new Exception(
        'Error reading from source device'
    );
}
