<?php

/**
 * A class to encode & decode bencoded data.
 *
 */
class bencoder
{
	/**
	 * Pass mixed PHP data and expect a bencoded string to be returned.
	 *
	 * @param mixed $data
	 * @return string
	 */
	public static function encode($data)
	{
		if (is_array($data))
		{
			ksort($data);
			
			if (is_string(key($data)))
			{
				return self::encode_dictionary($data);
			}
			else
			{
				return self::encode_list($data);
			}
		}
		else if (is_numeric($data))
		{
			return self::encode_int($data);
		}
		else if (is_string($data))
		{
			return self::encode_string($data);
		}
		else
		{
			throw new Exception('invalid data');
		}
	}
	
	private static function encode_string($data)
	{
		return strlen($data) . ":" . $data;
	}
	
	private static function encode_int($data)
	{
		return 'i' . $data . 'e';
	}
	
	private static function encode_list(array $data)
	{
		$return = 'l';
		
		foreach ($data as $datum)
		{
			$return .= self::encode($datum);
		}
		
		$return .= 'e';
		
		return $return;
	}
	
	private static function encode_dictionary(array $data)
	{
		$return = 'd';

		foreach ($data as $key => $value)
		{
			$return .= self::encode_string($key);
			$return .= self::encode($value);	
		}
		
		$return .= 'e';
		
		return $return;
	}
	
	/**
	 * Pass in a bencoded string and get back an int, string, or array
	 * depending on the bencoded data.
	 *
	 * @param $data string
	 * @return mixed
	 */
	public static function decode($data)
	{
		// every time something external calls decode we will reset the pointer
		// it will be left alone for internal recursive calls
		return self::_decode($data, TRUE);
	}
	
	/**
	 * Does the actual decoding. This is private so that the caller cannot
	 * request the pointer to be reset.
	 *
	 * @param $data string
	 * @param $reset_pointer boolean
	 * @return mixed
	 */
	private static function _decode($data, $reset_pointer = FALSE)
	{
		static $pointer = 0;
		
		if ($reset_pointer) $pointer = 0;

		// decoding a string
		if (is_numeric($data[$pointer]))
		{
			$strlength = '';

			while (is_numeric($data[$pointer]))
			{
				$strlength .= $data[$pointer];
				$pointer++;
			}

			$pointer++;

			$strlength = (int)$strlength;

			$decoded = substr($data, $pointer, $strlength);
			$pointer += $strlength;
		}
		else
		{
			switch ($data[$pointer])
			{
				// decoding an integer
				case 'i':

					$number = '';

					$pointer++; // advance past the 'i'

					while (is_numeric($data[$pointer]))
					{
						$number .= $data[$pointer];	
						$pointer++;
					}

					$pointer++; // advance past the ending 'e'

					$decoded = (int)$number;

					break;

				// decoding a list
				case 'l':

					$decoded = array();
					
					$pointer++; // advance past the 'l'

					while (isset($data[$pointer]) && $data[$pointer] != 'e')
					{
						$decoded[] = self::_decode($data);
					}
					
					$pointer++; // advance past the ending 'e'

					break;

				// decoding a dictionary
				case 'd':

					$decoded = array();
					
					$pointer++; // advance past the 'd'
					
					while (isset($data[$pointer]) && $data[$pointer] != 'e')
					{
						$decoded[self::_decode($data)] = self::_decode($data);
					}
					
					$pointer++; // advance past ending 'e'

					break;

				// encountered invalid bencoding
				default:

					throw new Exception('invalid data: [' . $pointer . '] = ' . $data[$pointer]);
			}
		}
		
		return $decoded;
	}
}

//
// TESTS:
//

// ENCODING

// int
$data = 3;
var_dump(bencoder::encode($data));

// list
$data = array(45,754,5434);
var_dump(bencoder::encode($data));

// dictionary
$data = array
	(
		'cow' => 'moo',
		'spam' => 'eggs'
	);	
var_dump(bencoder::encode($data));

// list nested in dictionary
$data = array
	(
		'spam' => array ('a','b')
	);
var_dump(bencoder::encode($data));

// nested lists
$data = array
	(
		array(1,2,3),
		array(3,2,1)
	);
var_dump(bencoder::encode($data));


// DECODING:

// string
$data = '4:spam';
var_dump(bencoder::decode($data));

// int
$data = 'i5435e';
var_dump(bencoder::decode($data));

// list
$data = 'li45ei754ei5434ee';
var_dump(bencoder::decode($data));

// nested lists
$data = 'lli1ei2ei3eeli3ei2ei1eee';
var_dump(bencoder::decode($data));

// list nested in dictionary
$data = 'd4:spaml1:a1:bee';
var_dump(bencoder::decode($data));