BitsReader: Read bits from given ByteArray

July 15th, 2008 Ntt.cc

I am working on SWF file decompiling according to the swf_file_format_spec_v9.pdf provided by adobe these days.  After I decompiled the first 8 bytes data of a sample SWF file. I found that the FrameSize is defined as a RECT structure below:

1. RECT
2. Field Type Comment
3. Nbits UB[5] Bits in each rect value field
4. Xmin SB[Nbits] x minimum position for rect
5. Xmax SB[Nbits] x maximum position for rect
6. Ymin SB[Nbits] y minimum position for rect
7. Ymax SB[Nbits] y maximum position for rect

I have to read the first 5 bits from a ByteArray contains 9 bytes as the Nbits, then read 15 bits each time to get the positions for the rect.

That’s really a hard work for me to get bits from the binary data, I have never do such thing before.

I wrote a lot of codes, and tried many ways to get what I want . But I found all of them are not “beautiful” enough. At last, I got this simple class to catch what I want.

This class is used to read any amount,less than 53, bits from a given ByteArray.

Demo | Download Full Project

Download: BitsReader.as

1. package com.edison.utils
2. {
3.     import flash.utils.ByteArray;
4.    
5.     public class BitsReader
6.     {   
7.         private var _source:ByteArray;
8.        
9.         private var maxBitsLength:uint;
10.        
11.         private const BYTE_LEGNTH:uint = 8;
12.        
13.         private const NUMBER_LENGTH:uint = 53;
14.        
15.         private const INT_LENGTH:uint = 32;
16.        
17.         public function get source():ByteArray
18.         {
19.             return _source;
20.         }    
21.        
22.         private var _position:uint = 0;
23.        
24.         public function get position():uint       
25.         {
26.             return _position;
27.         }
28.        
29.         public function set position(value:uint):void
30.         {
31.             if(value > maxBitsLength)
32.             {
33.                 _position = maxBitsLength;
34.             }
35.             else
36.             {
37.                 _position = value;
38.             }
39.         }
40.        
41.         public function BitsReader(source:ByteArray)
42.         {
43.             maxBitsLength = source.length*BYTE_LEGNTH;
44.             _source = source;
45.         }
46.        
47.         public function read(length:uint):Number
48.         {
49.             //make sure the length of the return value is less or equal than the length of Number;
50.             if(length> NUMBER_LENGTH)
51.             {
52.                 length = NUMBER_LENGTH;
53.             }
54.            
55.             //make sure the length of the return value if less or equal the amount of all the bits.
56.             if(length>maxBitsLength)
57.             {
58.                 length = maxBitsLength;
59.             }
60.            
61.             //if there is not enought bits to return, return all the left.
62.             if(length>maxBitsLength - position)
63.             {
64.                 length = maxBitsLength - position;
65.             }
66.                    
67.             var result:Number = 0;
68.            
69.             var byteLength:uint = (position+length)/BYTE_LEGNTH;
70.            
71.             var leftBitsLength:uint = (position+length)%BYTE_LEGNTH;           
72.            
73.             var startByteIndex:uint = 0;
74.             var extractFromFirstByte:Boolean = false;
75.                        
76.             if(position!=0)
77.             {
78.                 startByteIndex = Math.ceil(position/BYTE_LEGNTH)-1;
79.                 extractFromFirstByte = true;
80.             }
81.             source.position = startByteIndex;
82.            
83.             var byte:uint = 0;
84.             for(var i:uint = startByteIndex ;i<byteLength;i++)
85.             {
86.                 result <<= BYTE_LEGNTH;
87.                 byte = source.readUnsignedByte();
88.                
89.                 if(extractFromFirstByte)
90.                 {                   
91.                     result ^= (byte<< (position%BYTE_LEGNTH+INT_LENGTH-BYTE_LEGNTH)) >>> (position%BYTE_LEGNTH+INT_LENGTH-BYTE_LEGNTH) ;
92.                     position += (BYTE_LEGNTH - position%BYTE_LEGNTH);
93.                     extractFromFirstByte = false;
94.                 }
95.                 else
96.                 {                   
97.                     result ^= byte;
98.                     position += BYTE_LEGNTH;
99.                 }               
100.             }           
101.             result <<= leftBitsLength;
102.             byte = source.readUnsignedByte();
103.            
104.             result ^= byte>>(BYTE_LEGNTH-leftBitsLength);
105.            
106.             position += leftBitsLength;
107.                        
108.             return result;
109.         }
110.        
111.     }
112. }

This is a sample of BitsReader, quite easy.

1. var rect:ByteArray = new ByteArray();
2. uncompressedData.readBytes(rect,0,9);
3.  
4. var bits:BitsReader = new BitsReader(rect);
5. bits.read(5);
6. bits.read(15);
7. trace(”file width: “,bits.read(15)/20,”pixel”);
8. bits.read(15);
9. trace(”file height: “,bits.read(15)/20,”pixel”);

Now, you can have a try with this class .

Share and Enjoy:

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Posted in Flash, Flex Tags: ActionScript, Add new tag, bits, BitsReader, ByteArray, RECT 3,567 views

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7 Responses to “BitsReader: Read bits from given ByteArray”

1. 远古的迷茫…… says:

   July 16, 2008 at 9:10 am

   BitsReader:从ByteArry中读取一定个数的二进制位…

    
   I am working on SWF file decompiling according to the swf_file_format_spec_v9.pdf provided by adobe these days.  After I decompiled the first 8 bytes data of a sample SWF file. I found that the FrameSize is defined as a RECT structure bel…
2. hasselquist.net » Blog Archive » How to Highlight the Text in Flex(with Full Source Code) says:

   July 27, 2008 at 1:56 am

   [...] BitsReader: Read bits from given ByteArray (1) [...]
3. Yogesh Puri says:

   August 6, 2008 at 4:23 pm

   Hi

   Thanks for sharing this class as i was in really need of the same. I am writing a SWF reader in AIR and tried to use this class. However i am facing two errors on following lines while compiling

   LINE 91:::
   # result ^= (byte> (position%BYTE_LEGNTH+INT_LENGTH-BYTE_LEGNTH) ;

   LINE 101:::
   # result (BYTE_LEGNTH-leftBitsLength);

   Can you please suggest what are the issues in these lines or what is the correct code
4. Ntt.cc says:

   August 6, 2008 at 4:44 pm

   Sorry , there is something wrong with the codes in this article.

   LINE 91:::
   # result ^= (byte> (position%BYTE_LEGNTH+INT_LENGTH-BYTE_LEGNTH) ;

   it should be :
   result ^= (byte<>> (position%BYTE_LEGNTH+INT_LENGTH-BYTE_LEGNTH) ;

   LINE 101:::
   # result (BYTE_LEGNTH-leftBitsLength);

   it should be :
   result ^= byte>>(BYTE_LEGNTH-leftBitsLength);
5. Yogesh Puri says:

   August 6, 2008 at 8:12 pm

   Hi

   Thanks for the fix and project download.
6. Flex/AIR中如何以Byte为单位读取BMP格式图片的代码 - 中文Flex例子 says:

   August 12, 2008 at 6:34 pm

   [...] 在BitsReader: Read bits from given ByteArray中专门介绍过针对SWF的Byte读去操作，Flex/Air本身其实支持png, gif, jpg,swf 格式的载入，不过却不支持BMP。这个其实是相当奇怪的，不过既然这样设计，那也没办法。只能自己写代码来解决BMP载入的问题了。好在Flex cookbook上已经提供了现成的代码并且有相关的说明，有需要的直接去取来用吧。下面是地址： [...]
7. hasselquist.net » Blog Archive » A trick of ActionScript 3: Using break “labelname” to break from outer loops says:

   August 21, 2008 at 10:09 am

   [...] BitsReader: Read bits from given ByteArray (6) [...]