<?php
# tinyurl_util.php - tinyurl helper functions.
# Version 0.1  Tue, 04 Jul 2006 20:53:53 -0600
#
# Copyright (C) 2006  Dwayne C. Litzenberger <dlitz@dlitz.net>
# 
# 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.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

# These functions help you implement a tinyurl service (for example, http://tinyurl.com/)

# tinyurl_next_id - Given a tinyurl identifier, return the next identifer in the sequence.
function tinyurl_next_id($id)
{
    # Canonicalize the id
    $id = tinyurl_canonical_id($id);
    
    # Set the list of base characters.
    # NB: must be regexp-safe, url-safe, and filename-safe
    $basechars = 'acefjkmnpqrsuxy01234689';
    
    # Convert the id into an array of numbers
    $idlen = strlen($id);
    $ida = array();
    for ($i = 0; $i < $idlen; $i++) {
        $ida[] = strpos($basechars, substr($id, $i, 1));
    }

    while (true) {
        # Increment the array of numbers
        $p = count($ida)-1;
        $num_basechars = strlen($basechars);
        while (true) {
            $ida[$p]++;
            if ($ida[$p] < $num_basechars) {
                break;
            }
            $ida[$p] = 0;
            $p--;
            if ($p < 0) {
                array_unshift($ida, 0);
                $p++;
            }
        }
        
        # Convert the array of numbers back into an id
        $idlen = count($ida);
        $retval = array();
        for ($i = 0; $i < $idlen; $i++) {
            $retval[] = $basechars[$ida[$i]];
        }
        $retval = implode('', $retval);
 
        # Political correctness: Never return certain blacklisted ids.
        # A lot of ids that would be blacklisted here aren't because they're
        # impossible to create (canonically) given the base characters.
        $tryagain = false;
        foreach (array('ass', 'arse', 'cock', 'fuck', 'pussy') as $npcword) {
            if (strpos($retval, $npcword) !== false) {
                $tryagain = true;
                break;
            }
        }
        if ($tryagain) {
            continue;
        }

        break;
    }
    
    return $retval;
}

# tinyurl_canonical_id - Given a tinyurl identifier, return the canonical
# tinyurl identifier.  This is needed since, for example, the identifiers "a1"
# and "Al" are considered equivalent, in order to make the tinyurls easier to
# read if handwritten, for example.
function tinyurl_canonical_id($id)
{
    # Lowercase
    $id = strtolower($id);

    # Remove invalid chars
    $id = preg_replace('/[^0-9a-z]/s', '', $id);

    # Replace character-equivalents
    $id = strtr($id, 'odli7tbgh5wvz', '0a111f69nsuu2');

    return $id;
}