逆向算法

Question

我使用此算法從此處將MongoID壓縮為20個字符： 將PHP字符串從24個字符壓縮（縮短）為20個字符 。 但是，我在扭轉它並找回我的24個字符方面遇到困難。

有人可以給我一些指導。

我使用的代碼：

$padded = str_repeat('0', 20 - strlen($purchase_id)) . $purchase_id;
$leastSignificant = base_convert(substr($padded, 14, 10), 32, 16); // will be 8 chars most

$middleSignificant = base_convert(substr($padded, 4, 10), 32, 16); // will be 8 chars most

$highSignificant = base_convert(substr($padded, 0, 4), 32, 16); // will be 4 chars most

// Concatenate, and make sure everything is correctly padded
$result = str_repeat('0', 4 - strlen($highSignificant)) . $highSignificant .
          str_repeat('0', 8 - strlen($middleSignificant )) . $middleSignificant .
          str_repeat('0', 8 - strlen($leastSignificant )) . $leastSignificant;

我收到錯誤消息： Warning: str_repeat(): Second argument has to be greater than or equal to 0

我得到了與原始MongoID正確的MinimumSignificant和highSignificant值。 但不是中間重要的。 我得到25個字符，而不是24個字符。

Answer 1

得到它了！

// Pad with 0's to make sure you have 20 chars
$padded = str_repeat('0', 20 - strlen($purchase_id)) . $purchase_id;

$leastSignificant = base_convert(substr($padded, 12, 8), 32, 16); // will be 10 chars most
    var_dumped($leastSignificant,false);

$middleSignificant = base_convert(substr($padded, 4, 8), 32, 16); // will be 10 chars most
    var_dumped($middleSignificant,false);

$highSignificant = base_convert(substr($padded, 0, 4), 32, 16); // will be 4 chars most
    var_dumped($highSignificant,false);

// Concatenate, and make sure everything is correctly padded
$result = str_repeat('0', 4 - strlen($highSignificant)) . $highSignificant .
          str_repeat('0', 10 - strlen($middleSignificant )) . $middleSignificant .
          str_repeat('0', 10 - strlen($leastSignificant )) . $leastSignificant;