<?

/*

Basic knowledge of how bencoding works is assumed. Details can be found
at <http://bitconjurer.org/BitTorrent/protocol.html>.


How to use these functions:

An "object" is defined to be an associative array with at least the keys
"type" and "value" present. The "type" key contains a string which is
one of "string", "integer", "list" or "dictionary". The "value" key
contains the appropriate thing, either a string, an integer, a list which
is just a flat array, or a dictionary, which is an associative array. In
the case of "list" and "dictionary", the values of the contained array
are agaib "objects".


Description of the functions:


string benc($obj);

Takes an object as argument and returns the bencoded form of it as string.
Returns the undefined/unset value on failure.

Examples:

benc(array(type => "string", value => "spam"))		returns "4:spam".
benc(array(type => "integer", value => 3))		returns "i3e".
benc(array(type => "list", value => array(
	array(type => "string", value => "spam"),
	array(type => "string", value => "eggs")
)))
						returns "l4:spam4:eggse"

benc(array(type => "dictionary", value => array(
	cow => array(type => "string", value => "moo"),
	spam => array(type => "string", value => "eggs"),
)))
					returns "d3:cow3:moo4:spam4:eggse"


object bdec($str);

Returns the object that results from bdecoding the given string. Note
that those aren't real php objects, but merely "objects" as described
above. The returned objects have two additional keys: "string" and
"strlen". They represent the bencoded form of the returned objects, as
it was given in the original bencoded string. Use this to extract
certain portions of a bencoded string without having to re-encode it
(and avoiding possible re-ordering of dictionary keys). $x["strlen"]
is always equivalent to strlen($x["string"]). The "string" attribute
of the top-level returned object will be the same as the original
bencoded string, unless there's trailing garbage at the end of the
string.

This function returns the undefined/unset value on failure.

Example:

bdec("d4:spaml11:spiced pork3:hamee")
	returns this monster:

Array
(
    [type] => dictionary
    [value] => Array
        (
            [spam] => Array
                (
                    [type] => list
                    [value] => Array
                        (
                            [0] => Array
                                (
                                    [type] => string
                                    [value] => spiced pork
                                    [strlen] => 14
                                    [string] => 11:spiced pork
                                )

                            [1] => Array
                                (
                                    [type] => string
                                    [value] => ham
                                    [strlen] => 5
                                    [string] => 3:ham
                                )

                        )

                    [strlen] => 21
                    [string] => l11:spiced pork3:hame
                )

        )

    [strlen] => 29
    [string] => d4:spaml11:spiced pork3:hamee
)


object bdec_file($filename, $maxsize);

Opens the specified file, reads its contents (up to the specified length),
and returns whatever bdec() returns for those contents. This is a simple
convenience function.

*/

function benc($obj) {
	if (!is_array($obj) || !isset($obj["type"]) || !isset($obj["value"]))
		return;
	$c = $obj["value"];
	switch ($obj["type"]) {
		case "string":
			return benc_str($c);
		case "integer":
			return benc_int($c);
		case "list":
			return benc_list($c);
		case "dictionary":
			return benc_dict($c);
		default:
			return;
	}
}

function benc_str($s) {
	return strlen($s) . ":$s";
}

function benc_int($i) {
	return "i" . $i . "e";
}

function benc_list($a) {
	$s = "l";
	foreach ($a as $e) {
		$s .= benc($e);
	}
	$s .= "e";
	return $s;
}

function benc_dict($d) {
	$s = "d";
	$keys = array_keys($d);
	sort($keys);
	foreach ($keys as $k) {
		$v = $d[$k];
		$s .= benc_str($k);
		$s .= benc($v);
	}
	$s .= "e";
	return $s;
}

function bdec_file($f, $ms) {
	$fp = fopen($f, "rb");
	if (!$fp)
		return;
	$e = fread($fp, $ms);
	fclose($fp);
	return bdec($e);
}

function bdec($s) {
	if (preg_match('/^(\d+):/', $s, $m)) {
		$l = $m[1];
		$pl = strlen($l) + 1;
		$v = substr($s, $pl, $l);
		$ss = substr($s, 0, $pl + $l);
		if (strlen($v) != $l)
			return;
		return array(type => "string", value => $v, strlen => strlen($ss), string => $ss);
	}
	if (preg_match('/^i(\d+)e/', $s, $m)) {
		$v = $m[1];
		$ss = "i" . $v . "e";
		if ($v === "-0")
			return;
		if ($v[0] == "0" && strlen($v) != 1)
			return;
		return array(type => "integer", value => $v, strlen => strlen($ss), string => $ss);
	}
	switch ($s[0]) {
		case "l":
			return bdec_list($s);
		case "d":
			return bdec_dict($s);
		default:
			return;
	}
}

function bdec_list($s) {
	if ($s[0] != "l")
		return;
	$sl = strlen($s);
	$i = 1;
	$v = array();
	$ss = "l";
	for (;;) {
		if ($i >= $sl)
			return;
		if ($s[$i] == "e")
			break;
		$ret = bdec(substr($s, $i));
		if (!isset($ret) || !is_array($ret))
			return;
		$v[] = $ret;
		$i += $ret["strlen"];
		$ss .= $ret["string"];
	}
	$ss .= "e";
	return array(type => "list", value => $v, strlen => strlen($ss), string => $ss);
}

function bdec_dict($s) {
	if ($s[0] != "d")
		return;
	$sl = strlen($s);
	$i = 1;
	$v = array();
	$ss = "d";
	for (;;) {
		if ($i >= $sl)
			return;
		if ($s[$i] == "e")
			break;
		$ret = bdec(substr($s, $i));
		if (!isset($ret) || !is_array($ret) || $ret["type"] != "string")
			return;
		$k = $ret["value"];
		$i += $ret["strlen"];
		$ss .= $ret["string"];
		if ($i >= $sl)
			return;
		$ret = bdec(substr($s, $i));
		if (!isset($ret) || !is_array($ret))
			return;
		$v[$k] = $ret;
		$i += $ret["strlen"];
		$ss .= $ret["string"];
	}
	$ss .= "e";
	return array(type => "dictionary", value => $v, strlen => strlen($ss), string => $ss);
}

?>