/src/com/adobe/serialization/json/JSONTokenizer.as

/*

  Copyright (c) 2008, Adobe Systems Incorporated

  All rights reserved.



  Redistribution and use in source and binary forms, with or without 

  modification, are permitted provided that the following conditions are

  met:



  * Redistributions of source code must retain the above copyright notice, 

    this list of conditions and the following disclaimer.

  

  * Redistributions in binary form must reproduce the above copyright

    notice, this list of conditions and the following disclaimer in the 

    documentation and/or other materials provided with the distribution.

  

  * Neither the name of Adobe Systems Incorporated nor the names of its 

    contributors may be used to endorse or promote products derived from 

    this software without specific prior written permission.



  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS

  IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,

  THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR

  PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR 

  CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,

  EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,

  PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR

  PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF

  LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING

  NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS

  SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

*/



package com.adobe.serialization.json {



	public class JSONTokenizer {

	

		/** The object that will get parsed from the JSON string */

		private var obj:Object;

		

		/** The JSON string to be parsed */

		private var jsonString:String;

		

		/** The current parsing location in the JSON string */

		private var loc:int;

		

		/** The current character in the JSON string during parsing */

		private var ch:String;

		

		/**

		 * Constructs a new JSONDecoder to parse a JSON string 

		 * into a native object.

		 *

		 * @param s The JSON string to be converted

		 *		into a native object

		 */

		public function JSONTokenizer( s:String ) {

			jsonString = s;

			loc = 0;

			

			// prime the pump by getting the first character

			nextChar();

		}

		

		/**

		 * Gets the next token in the input sting and advances

		* the character to the next character after the token

		 */

		public function getNextToken():JSONToken {

			var token:JSONToken = new JSONToken();

			

			// skip any whitespace / comments since the last 

			// token was read

			skipIgnored();

						

			// examine the new character and see what we have...

			switch ( ch ) {

				

				case '{':

					token.type = JSONTokenType.LEFT_BRACE;

					token.value = '{';

					nextChar();

					break

					

				case '}':

					token.type = JSONTokenType.RIGHT_BRACE;

					token.value = '}';

					nextChar();

					break

					

				case '[':

					token.type = JSONTokenType.LEFT_BRACKET;

					token.value = '[';

					nextChar();

					break

					

				case ']':

					token.type = JSONTokenType.RIGHT_BRACKET;

					token.value = ']';

					nextChar();

					break

				

				case ',':

					token.type = JSONTokenType.COMMA;

					token.value = ',';

					nextChar();

					break

					

				case ':':

					token.type = JSONTokenType.COLON;

					token.value = ':';

					nextChar();

					break;

					

				case 't': // attempt to read true

					var possibleTrue:String = "t" + nextChar() + nextChar() + nextChar();

					

					if ( possibleTrue == "true" ) {

						token.type = JSONTokenType.TRUE;

						token.value = true;

						nextChar();

					} else {

						parseError( "Expecting 'true' but found " + possibleTrue );

					}

					

					break;

					

				case 'f': // attempt to read false

					var possibleFalse:String = "f" + nextChar() + nextChar() + nextChar() + nextChar();

					

					if ( possibleFalse == "false" ) {

						token.type = JSONTokenType.FALSE;

						token.value = false;

						nextChar();

					} else {

						parseError( "Expecting 'false' but found " + possibleFalse );

					}

					

					break;

					

				case 'n': // attempt to read null

				

					var possibleNull:String = "n" + nextChar() + nextChar() + nextChar();

					

					if ( possibleNull == "null" ) {

						token.type = JSONTokenType.NULL;

						token.value = null;

						nextChar();

					} else {

						parseError( "Expecting 'null' but found " + possibleNull );

					}

					

					break;

					

				case '"': // the start of a string

					token = readString();

					break;

					

				default: 

					// see if we can read a number

					if ( isDigit( ch ) || ch == '-' ) {

						token = readNumber();

					} else if ( ch == '' ) {

						// check for reading past the end of the string

						return null;

					} else {						

						// not sure what was in the input string - it's not

						// anything we expected

						parseError( "Unexpected " + ch + " encountered" );

					}

			}

			

			return token;

		}

		

		/**

		 * Attempts to read a string from the input string.  Places

		 * the character location at the first character after the

		 * string.  It is assumed that ch is " before this method is called.

		 *

		 * @return the JSONToken with the string value if a string could

		 *		be read.  Throws an error otherwise.

		 */

		private function readString():JSONToken {

			// the token for the string we'll try to read

			var token:JSONToken = new JSONToken();

			token.type = JSONTokenType.STRING;

			

			// the string to store the string we'll try to read

			var string:String = "";

			

			// advance past the first "

			nextChar();

			

			while ( ch != '"' && ch != '' ) {

								

				// unescape the escape sequences in the string

				if ( ch == '\\' ) {

					

					// get the next character so we know what

					// to unescape

					nextChar();

					

					switch ( ch ) {

						

						case '"': // quotation mark

							string += '"';

							break;

						

						case '/':	// solidus

							string += "/";

							break;

							

						case '\\':	// reverse solidus

							string += '\\';

							break;

							

						case 'b':	// bell

							string += '\b';

							break;

							

						case 'f':	// form feed

							string += '\f';

							break;

							

						case 'n':	// newline

							string += '\n';

							break;

							

						case 'r':	// carriage return

							string += '\r';

							break;

							

						case 't':	// horizontal tab

							string += '\t'

							break;

						

						case 'u':

							// convert a unicode escape sequence

							// to it's character value - expecting

							// 4 hex digits

							

							// save the characters as a string we'll convert to an int

							var hexValue:String = "";

							

							// try to find 4 hex characters

							for ( var i:int = 0; i < 4; i++ ) {

								// get the next character and determine

								// if it's a valid hex digit or not

								if ( !isHexDigit( nextChar() ) ) {

									parseError( " Excepted a hex digit, but found: " + ch );

								}

								// valid, add it to the value

								hexValue += ch;

							}

							

							// convert hexValue to an integer, and use that

							// integrer value to create a character to add

							// to our string.

							string += String.fromCharCode( parseInt( hexValue, 16 ) );

							

							break;

					

						default:

							// couldn't unescape the sequence, so just

							// pass it through

							string += '\\' + ch;

						

					}

					

				} else {

					// didn't have to unescape, so add the character to the string

					string += ch;

					

				}

				

				// move to the next character

				nextChar();

				

			}

			

			// we read past the end of the string without closing it, which

			// is a parse error

			if ( ch == '' ) {

				parseError( "Unterminated string literal" );

			}

			

			// move past the closing " in the input string

			nextChar();

			

			// attach to the string to the token so we can return it

			token.value = string;

			

			return token;

		}

		

		/**

		 * Attempts to read a number from the input string.  Places

		 * the character location at the first character after the

		 * number.

		 * 

		 * @return The JSONToken with the number value if a number could

		 * 		be read.  Throws an error otherwise.

		 */

		private function readNumber():JSONToken {

			// the token for the number we'll try to read

			var token:JSONToken = new JSONToken();

			token.type = JSONTokenType.NUMBER;

			

			// the string to accumulate the number characters

			// into that we'll convert to a number at the end

			var input:String = "";

			

			// check for a negative number

			if ( ch == '-' ) {

				input += '-';

				nextChar();

			}

			

			// the number must start with a digit

			if ( !isDigit( ch ) )

			{

				parseError( "Expecting a digit" );

			}

			

			// 0 can only be the first digit if it

			// is followed by a decimal point

			if ( ch == '0' )

			{

				input += ch;

				nextChar();

				

				// make sure no other digits come after 0

				if ( isDigit( ch ) )

				{

					parseError( "A digit cannot immediately follow 0" );

				}

			}

			else

			{

				// read numbers while we can

				while ( isDigit( ch ) ) {

					input += ch;

					nextChar();

				}

			}

			

			// check for a decimal value

			if ( ch == '.' ) {

				input += '.';

				nextChar();

				

				// after the decimal there has to be a digit

				if ( !isDigit( ch ) )

				{

					parseError( "Expecting a digit" );

				}

				

				// read more numbers to get the decimal value

				while ( isDigit( ch ) ) {

					input += ch;

					nextChar();

				}

			}

			

			// check for scientific notation

			if ( ch == 'e' || ch == 'E' )

			{

				input += "e"

				nextChar();

				// check for sign

				if ( ch == '+' || ch == '-' )

				{

					input += ch;

					nextChar();

				}

				

				// require at least one number for the exponent

				// in this case

				if ( !isDigit( ch ) )

				{

					parseError( "Scientific notation number needs exponent value" );

				}

							

				// read in the exponent

				while ( isDigit( ch ) )

				{

					input += ch;

					nextChar();

				}

			}

			

			// convert the string to a number value

			var num:Number = Number( input );

			

			if ( isFinite( num ) && !isNaN( num ) ) {

				token.value = num;

				return token;

			} else {

				parseError( "Number " + num + " is not valid!" );

			}

            return null;

		}



		/**

		 * Reads the next character in the input

		 * string and advances the character location.

		 *

		 * @return The next character in the input string, or

		 *		null if we've read past the end.

		 */

		private function nextChar():String {

			return ch = jsonString.charAt( loc++ );

		}

		

		/**

		 * Advances the character location past any

		 * sort of white space and comments

		 */

		private function skipIgnored():void {

			skipWhite();

			skipComments();

			skipWhite();

		}

		

		/**

		 * Skips comments in the input string, either

		 * single-line or multi-line.  Advances the character

		 * to the first position after the end of the comment.

		 */

		private function skipComments():void {

			if ( ch == '/' ) {

				// Advance past the first / to find out what type of comment

				nextChar();

				switch ( ch ) {

					case '/': // single-line comment, read through end of line

						

						// Loop over the characters until we find

						// a newline or until there's no more characters left

						do {

							nextChar();

						} while ( ch != '\n' && ch != '' )

						

						// move past the \n

						nextChar();

						

						break;

					

					case '*': // multi-line comment, read until closing */



						// move past the opening *

						nextChar();

						

						// try to find a trailing */

						while ( true ) {

							if ( ch == '*' ) {

								// check to see if we have a closing /

								nextChar();

								if ( ch == '/') {

									// move past the end of the closing */

									nextChar();

									break;

								}

							} else {

								// move along, looking if the next character is a *

								nextChar();

							}

							

							// when we're here we've read past the end of 

							// the string without finding a closing */, so error

							if ( ch == '' ) {

								parseError( "Multi-line comment not closed" );

							}

						}



						break;

					

					// Can't match a comment after a /, so it's a parsing error

					default:

						parseError( "Unexpected " + ch + " encountered (expecting '/' or '*' )" );

				}

			}

			

		}

		

		

		/**

		 * Skip any whitespace in the input string and advances

		 * the character to the first character after any possible

		 * whitespace.

		 */

		private function skipWhite():void {

			

			// As long as there are spaces in the input 

			// stream, advance the current location pointer

			// past them

			while ( isWhiteSpace( ch ) ) {

				nextChar();

			}

			

		}

		

		/**

		 * Determines if a character is whitespace or not.

		 *

		 * @return True if the character passed in is a whitespace

		 *	character

		 */

		private function isWhiteSpace( ch:String ):Boolean {

			return ( ch == ' ' || ch == '\t' || ch == '\n' || ch == '\r' );

		}

		

		/**

		 * Determines if a character is a digit [0-9].

		 *

		 * @return True if the character passed in is a digit

		 */

		private function isDigit( ch:String ):Boolean {

			return ( ch >= '0' && ch <= '9' );

		}

		

		/**

		 * Determines if a character is a digit [0-9].

		 *

		 * @return True if the character passed in is a digit

		 */

		private function isHexDigit( ch:String ):Boolean {

			// get the uppercase value of ch so we only have

			// to compare the value between 'A' and 'F'

			var uc:String = ch.toUpperCase();

			

			// a hex digit is a digit of A-F, inclusive ( using

			// our uppercase constraint )

			return ( isDigit( ch ) || ( uc >= 'A' && uc <= 'F' ) );

		}

	

		/**

		 * Raises a parsing error with a specified message, tacking

		 * on the error location and the original string.

		 *

		 * @param message The message indicating why the error occurred

		 */

		public function parseError( message:String ):void {

			throw new JSONParseError( message, loc, jsonString );

		}

	}

	

}