Convert a string to number and back to string?

Question

I would like to know how I can convert a short ASCII string to a number (int, float, or numeric string). I saw a couple of posts here mentioned perfect hashes which seems like it might be what I need. However, I'm not quite understanding the math for this.

How could you convert an ASCII string into a sequence of numbers and then back to a string?

As a side note, breaking a string down into it's ASCII character numbers is easy enough.

foreach(str_split($string) as $char) $number .= ord($char);

Update

After more reading I came up with this. However, I'm wondering if there are anyways to shorten the number sequence so it's not quite as long.

class intnum
{
    public static $charset = array(
        32 => ' ', 33 => '!', 34 => '"', 35 => '#', 36 => '$',
        37 => '%', 38 => '&', 39 => "'", 40 => '(', 41 => ')',
        42 => '*', 43 => '+', 44 => ',', 45 => '-', 46 => '.',
        47 => '/', 48 => '0', 49 => '1', 50 => '2', 51 => '3',
        52 => '4', 53 => '5', 54 => '6', 55 => '7', 56 => '8',
        57 => '9', 58 => ':', 59 => ';', 60 => '<', 61 => '=',
        62 => '>', 63 => '?', 64 => '@', 65 => 'A', 66 => 'B',
        67 => 'C', 68 => 'D', 69 => 'E', 70 => 'F', 71 => 'G',
        72 => 'H', 73 => 'I', 74 => 'J', 75 => 'K', 76 => 'L',
        77 => 'M', 78 => 'N', 79 => 'O', 80 => 'P', 81 => 'Q',
        82 => 'R', 83 => 'S', 84 => 'T', 85 => 'U', 86 => 'V',
        87 => 'W', 88 => 'X', 89 => 'Y', 90 => 'Z', 91 => '[',
        92 => '\\', 93 => ']', 94 => '^', 95 => '_', 96 => '`',
        97 => 'a', 98 => 'b', 99 => 'c', 100 => 'd', 101 => 'e',
        102 => 'f', 103 => 'g', 104 => 'h', 105 => 'i', 106 => 'j',
        107 => 'k', 108 => 'l', 109 => 'm', 110 => 'n', 111 => 'o',
        112 => 'p', 113 => 'q', 114 => 'r', 115 => 's', 116 => 't',
        117 => 'u', 118 => 'v', 119 => 'w', 120 => 'x', 121 => 'y',
        122 => 'z', 123 => '{', 124 => '|', 125 => '}'
    );

    public static function fromNumber($number)
    {
        $string = '';
        while($number)
        {
            $value = substr($number, 0, 2);
            $number = substr($number, 2);

            if($value < 32)
            {
                $value .= substr($number, 0, 1);
                $number = substr($number, 1);
            }

            $string .= self::$charset[ (int) $value];
        }
        return $string;
    }

    public static function fromString($string)
    {
        $number = '';
        foreach(str_split($string) as $char) $number .= ord($char);
        return $number;
    }
}

$string = 'this is my test string to convert';

$number = intnum::fromString($string);
$string = intnum::fromNumber($number);

Curie

Why not use the code you just posted? – Yam

@Brad, how do I get the string back? – Curie

Answer 1

A string-to-number encoder as one-liner (PHP 5.3 style):

$numbers = implode(array_map(function ($n) { return sprintf('%03d', $n); },
                          unpack('C*', $str)));

It simply converts every byte into its decimal number equivalent, zero-padding it to a fixed length of 3 digits so it can be unambiguously converted back.

The decoder back to a string:

$str = implode(array_map('chr', str_split($numbers, 3)));

Example text:

Wörks wíth all ストリングズ
087195182114107115032119195173116104032097108108032227130185227131136227131170227131179227130176227130186

Lucre

hmmm, doesn't seem to work with the unicode characters you provided. – Curie

Works fine, as long as you make sure you interpret the result in the same encoding as it was input. – Lucre

See codepad.org/wXA9ViFu (written in PHP 5.2 style, since Codepad doesn't run 5.3 yet). – Lucre

Works in PHP 8 ? – Houser

Answer 2

You can't just ORD chars into a string of numbers and expect it to come back because some chars may be on 2 characters and others 3.

For example:

Kang-HO will give you: 10797106103457279

Now how do you know it's not: 10-79-71-0-61-0-34-57-27-9?

You need to either pad all your numbers in 3 number codes and thus get: 107097106103045072079 and then break it apart in blocks of 3 numbers and then ASC them back...

Retrorocket

@PhilLello, do you see any way to alter my code above using hex? – Curie

Answer 3

Well, if you want to convert your string into a sequence of integers you must use always a fixed block of numbers. In this case 3 since ASCII uses a 8 bit words, therefore, the maximun possible integer is 2^8-1 = 255.

You should fill the unsed space with 0:

function zero_fill($num){
    if($num <= 9) $num = "00".$num;
    elseif($num <= 99) $num = "0".$num;
    return $num;
}

You can use the function you have created in conjuction with this one, and to recover the string, take block of 3 integers and convert them back to its correspondant ASCII character.

foreach(str_split($numberSeq, 3) as $asciiIntValue) $stringBack .= chr($asciiIntValue);

Scarfskin

"11bit words"? 2^11-1 = 127? I think you derailed a little there. – Lucre

Was about to write it too, 2^11 = 2048... you meant 2^7 but then again, ASCII has 255 characters buddy so it's 2^8... – Retrorocket