<?php
/**
 * Wrapper for json_encode and json_decode.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 */

/**
 * JSON formatter wrapper class
 */
class FormatJson {

	/**
	 * Skip escaping most characters above U+007F for readability and compactness.
	 * This encoding option saves 3 to 8 bytes (uncompressed) for each such character;
	 * however, it could break compatibility with systems that incorrectly handle UTF-8.
	 *
	 * @since 1.22
	 */
	const UTF8_OK = 1;

	/**
	 * Skip escaping the characters '<', '>', and '&', which have special meanings in
	 * HTML and XML.
	 *
	 * @warning Do not use this option for JSON that could end up in inline scripts.
	 * - HTML5, §4.3.1.2 Restrictions for contents of script elements
	 * - XML 1.0 (5th Ed.), §2.4 Character Data and Markup
	 *
	 * @since 1.22
	 */
	const XMLMETA_OK = 2;

	/**
	 * Skip escaping as many characters as reasonably possible.
	 *
	 * @warning When generating inline script blocks, use FormatJson::UTF8_OK instead.
	 *
	 * @since 1.22
	 */
	const ALL_OK = 3;

	/**
	 * Regex that matches whitespace inside empty arrays and objects.
	 *
	 * This doesn't affect regular strings inside the JSON because those can't
	 * have a real line break (\n) in them, at this point they are already escaped
	 * as the string "\n" which this doesn't match.
	 *
	 * @private
	 */
	const WS_CLEANUP_REGEX = '/(?<=[\[{])\n\s*+(?=[\]}])/';

	/**
	 * Characters problematic in JavaScript.
	 *
	 * @note These are listed in ECMA-262 (5.1 Ed.), §7.3 Line Terminators along with U+000A (LF)
	 *       and U+000D (CR). However, PHP already escapes LF and CR according to RFC 4627.
	 */
	private static $badChars = array(
		"\xe2\x80\xa8", // U+2028 LINE SEPARATOR
		"\xe2\x80\xa9", // U+2029 PARAGRAPH SEPARATOR
	);

	/**
	 * Escape sequences for characters listed in FormatJson::$badChars.
	 */
	private static $badCharsEscaped = array(
		'\u2028', // U+2028 LINE SEPARATOR
		'\u2029', // U+2029 PARAGRAPH SEPARATOR
	);

	/**
	 * Returns the JSON representation of a value.
	 *
	 * @note Empty arrays are encoded as numeric arrays, not as objects, so cast any associative
	 *       array that might be empty to an object before encoding it.
	 *
	 * @note In pre-1.22 versions of MediaWiki, using this function for generating inline script
	 *       blocks may result in an XSS vulnerability, and quite likely will in XML documents
	 *       (cf. FormatJson::XMLMETA_OK). Use Xml::encodeJsVar() instead in such cases.
	 *
	 * @param mixed $value The value to encode. Can be any type except a resource.
	 * @param bool $pretty If true, add non-significant whitespace to improve readability.
	 * @param int $escaping Bitfield consisting of _OK class constants
	 * @return string|bool: String if successful; false upon failure
	 */
	public static function encode( $value, $pretty = false, $escaping = 0 ) {
		if ( defined( 'JSON_UNESCAPED_UNICODE' ) ) {
			return self::encode54( $value, $pretty, $escaping );
		}
		return self::encode53( $value, $pretty, $escaping );
	}

	/**
	 * Decodes a JSON string.
	 *
	 * @param string $value The JSON string being decoded
	 * @param bool $assoc When true, returned objects will be converted into associative arrays.
	 *
	 * @return mixed: the value encoded in JSON in appropriate PHP type.
	 * `null` is returned if the JSON cannot be decoded or if the encoded data is deeper than
	 * the recursion limit.
	 */
	public static function decode( $value, $assoc = false ) {
		return json_decode( $value, $assoc );
	}

	/**
	 * JSON encoder wrapper for PHP >= 5.4, which supports useful encoding options.
	 *
	 * @param mixed $value
	 * @param bool $pretty
	 * @param int $escaping
	 * @return string|bool
	 */
	private static function encode54( $value, $pretty, $escaping ) {
		// PHP escapes '/' to prevent breaking out of inline script blocks using '</script>',
		// which is hardly useful when '<' and '>' are escaped (and inadequate), and such
		// escaping negatively impacts the human readability of URLs and similar strings.
		$options = JSON_UNESCAPED_SLASHES;
		$options |= $pretty ? JSON_PRETTY_PRINT : 0;
		$options |= ( $escaping & self::UTF8_OK ) ? JSON_UNESCAPED_UNICODE : 0;
		$options |= ( $escaping & self::XMLMETA_OK ) ? 0 : ( JSON_HEX_TAG | JSON_HEX_AMP );
		$json = json_encode( $value, $options );
		if ( $json === false ) {
			return false;
		}

		if ( $pretty ) {
			// Remove whitespace inside empty arrays/objects; different JSON encoders
			// vary on this, and we want our output to be consistent across implementations.
			$json = preg_replace( self::WS_CLEANUP_REGEX, '', $json );
		}
		if ( $escaping & self::UTF8_OK ) {
			$json = str_replace( self::$badChars, self::$badCharsEscaped, $json );
		}
		return $json;
	}

	/**
	 * JSON encoder wrapper for PHP 5.3, which lacks native support for some encoding options.
	 * Therefore, the missing options are implemented here purely in PHP code.
	 *
	 * @param mixed $value
	 * @param bool $pretty
	 * @param int $escaping
	 * @return string|bool
	 */
	private static function encode53( $value, $pretty, $escaping ) {
		$options = ( $escaping & self::XMLMETA_OK ) ? 0 : ( JSON_HEX_TAG | JSON_HEX_AMP );
		$json = json_encode( $value, $options );
		if ( $json === false ) {
			return false;
		}

		// Emulate JSON_UNESCAPED_SLASHES. Because the JSON contains no unescaped slashes
		// (only escaped slashes), a simple string replacement works fine.
		$json = str_replace( '\/', '/', $json );

		if ( $escaping & self::UTF8_OK ) {
			// JSON hex escape sequences follow the format \uDDDD, where DDDD is four hex digits
			// indicating the equivalent UTF-16 code unit's value. To most efficiently unescape
			// them, we exploit the JSON extension's built-in decoder.
			// * We escape the input a second time, so any such sequence becomes \\uDDDD.
			// * To avoid interpreting escape sequences that were in the original input,
			//   each double-escaped backslash (\\\\) is replaced with \\\u005c.
			// * We strip one of the backslashes from each of the escape sequences to unescape.
			// * Then the JSON decoder can perform the actual unescaping.
			$json = str_replace( "\\\\\\\\", "\\\\\\u005c", addcslashes( $json, '\"' ) );
			$json = json_decode( preg_replace( "/\\\\\\\\u(?!00[0-7])/", "\\\\u", "\"$json\"" ) );
			$json = str_replace( self::$badChars, self::$badCharsEscaped, $json );
		}

		if ( $pretty ) {
			return self::prettyPrint( $json );
		}
		return $json;
	}

	/**
	 * Adds non-significant whitespace to an existing JSON representation of an object.
	 * Only needed for PHP < 5.4, which lacks the JSON_PRETTY_PRINT option.
	 *
	 * @param string $json
	 * @return string
	 */
	private static function prettyPrint( $json ) {
		$buf = '';
		$indent = 0;
		$json = strtr( $json, array( '\\\\' => '\\\\', '\"' => "\x01" ) );
		for ( $i = 0, $n = strlen( $json ); $i < $n; $i += $skip ) {
			$skip = 1;
			switch ( $json[$i] ) {
				case ':':
					$buf .= ': ';
					break;
				case '[':
				case '{':
					++$indent;
					// falls through
				case ',':
					$buf .= $json[$i] . "\n" . str_repeat( '    ', $indent );
					break;
				case ']':
				case '}':
					$buf .= "\n" . str_repeat( '    ', --$indent ) . $json[$i];
					break;
				case '"':
					$skip = strcspn( $json, '"', $i + 1 ) + 2;
					$buf .= substr( $json, $i, $skip );
					break;
				default:
					$skip = strcspn( $json, ',]}"', $i + 1 ) + 1;
					$buf .= substr( $json, $i, $skip );
			}
		}
		$buf = preg_replace( self::WS_CLEANUP_REGEX, '', $buf );
		return str_replace( "\x01", '\"', $buf );
	}
}