有谁知道ACE的压缩算法？(100分)

我在用WinAce压缩工具时，发现这种工具的压缩率极高，比RAR、CAB都要高，不知有谁
知道这种压缩算法？[?][blue][/blue]

ZipTV构件包提供多种压缩算法，RAR、CAB，ace，zip等等，应有尽有！强烈推荐！

说得没错，这个软件的压缩比确实极高，而且你能发现它至少是界面部分是用Delphi编的，
算法就按小人物说的吧！

好像ziptv针对ace只有解压缩算法？
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不知道，支不支持2。0以后的呢。

ZipTV确实提供了不少的压缩解压控件,不过我想知道ACE的压缩算法原理,
不是想做一个压缩工具

你应该是学生吧，你可以到学校图书馆去查资料啊！

图书馆那么大，怎么找呀？而且里面的书太老，不一定会有呀？

gz

1. Block format
2. Block types
2.1. Archive header
2.2. File block
2.3. Recovery record

3. Archive processing
4. Building CRCs

1. Block format
The whole archive consists of blocks which vary in size.

Structure:

bytes meaning discription

2 HEAD_CRC CRC16 over block up from HEAD_TYPE
2 HEAD_SIZE size of the block from HEAD_TYPE
up to the begin
ning of the ADDSIZE block

1 HEAD_TYPE indicates type of block (see chapter 2.)
2 HEAD_FLAGS flags related to the block and its content
for all blocks these flags are valid:

bit discription

0 ADDSIZE field present
1 Block includes a comment

[4] ADDSIZE an optional field which represents the size of
an additional block without specified structure
(no HEAD_CRC, HEAD_SIZE etc.)
? OTHER FIELDS

2. Block types

2.1. Archive header
The archive header is the first block of each archive or volume.

Structure:

bytes meaning discription

2 HEAD_CRC CRC16 over block up from HEAD_TYPE
2 HEAD_SIZE size of the block from HEAD_TYPE
up to the last byte of this block

1 HEAD_TYPE archive header type is 0
2 HEAD_FLAGS contains most important information about the
archive

bit discription

0 0 (no ADDSIZE field)
1 presence of a main comment

9 SFX-archive
10 dictionary size limited to 256K
(because of a junior SFX)
11 archive consists of multiple volumes
12 main header contains AV-string
13 recovery record present
14 archive is locked
15 archive is solid

7 ACESIGN fixed string: '**ACE**' serves to find the
archive header

1 VER_EXTRACT version needed to extract archive
1 VER_CREATED version used to create the archive
1 HOST_CREATED HOST-OS for ACE used to create the archive

value host

0 MS-DOS
1 OS/2
2 Win32
3 Unix
4 MAC-OS
5 Win NT
6 Primos
7 APPLE GS
8 ATARI
9 VAX VMS
10 AMIGA
11 NEXT

1 VOLUME_NUM which volume of a multi-volume-archive is it?
4 TIME_CREATED date and time in MS-DOS format
8 RESERVED 8 bytes reserved for the future
[1] AV_SIZE size of the following AV string
[?] AV the AV string itself
[2] COMMENT_SIZE compressed size of the following comment
[?] COMMENT compressed data of comment
? RESERVED

Comments are compressed using simple LZP+huffman. Sources how to create
those compressed comments might be published sometime.

2.2. File block
Directories are stored in this type of block, too. There is no extra
block structure.

Structure:

bytes meaning discription

2 HEAD_CRC CRC16 over block up from HEAD_TYPE
2 HEAD_SIZE size of the block up from HEAD_TYPE
up to the begin
ning of the compressed data

1 HEAD_TYPE file header type is 1
2 HEAD_FLAGS
bit discription

0 1 (ADDSIZE field present)
1 presence of file comment

12 file continued from previous volume
13 file continues on the next volume
14 file encrypted with password
15 solid-flag: file compressed using data
of previous files of the archive

4 PACK_SIZE this is the ADDSIZE field;
the additional block contains compressed file data
without exception
4 ORIG_SIZE the original size of the file
4 FTIME file date and file time in MS-DOS format
4 ATTR attributes of the file
4 CRC32 checksum over the compressed file
4 TECH_INFO

bytes
1 type of compression
0 store
1 ACE_LZ77_1

1 quality of compression
0 fastest
1 fast
2 normal
3 good
4 best

2 parameter for decompression

2 RESERVED
2 FNAME_SIZE size of filename string in bytes
1 FNAME filename string (OEM)
[2] COMM_SIZE compressed size of file comment
[?] COMMENT file comment
? RESERVED
-------------------------------------------
compressed file data (size is PACK_SIZE)

2.3. Recovery record
The protection by recovery records works this way:
See the whole archive as a sequence of blocks with a defined length.
Build checksums over these blocks to determine (later) whether a block
has been damaged or not. Xor all blocks to one.
Save this xor-block and the checksums. To restore a damaged block
all undamaged blocks have to be xor'd with the xor-block.

Structure:

2 HEAD_CRC CRC16 over block up from HEAD_TYPE
2 HEAD_SIZE size of the block up from HEAD_TYPE

1 HEAD_TYPE header type of recovery records is 2
2 HEAD_FLAGS
bit discription

0 1 (ADDSIZE field present)
4 REC_BLK_SIZE ADDSIZE field;
size of recovery data
7 ACESIGN string: '**ACE**';
allows search for this block with destroyed
archive structure
4 REL_STRT relative start (to this block) of the data this
block is mode of
4 NUM_BLKS number of blocks the data is splitten in
4 CL_SIZE size of these blocks
2 REC_CRC CRC16 over recovery data
------------------------
recovery data:

2 1st CRC16 CRC over the first block of the archive
2 2nd CRC16 CRC over the second block of the archive
. .
. .
2 NUM_BLKSth CRC16 CRC over the last block (up to the recovery
record) of the archive
CL_SIZE XOR-DATA contains the xor'd data of all the blocks
the archive has been splitten in for this
process

3. Archive processing
For processing an archive you first need to get the start offset of it
in a certain file. The way todo
so is:

1.) search for the acesign ('**ACE**')
2.) read a block from (acesign_position-7)
(because the acesign is at offset 7 in the archive header)
3.) build the checksum of this block and check it against HEAD_CRC;
if theydo
not match go to step 1.)

After you got the start you can read one block after another like this:
1.) read HEAD_CRC and HEAD_SIZE
2.) read HEAD_SIZE bytes;
build the CRC of them and check it against HEAD_CRC;
the archive is broken if the CRCsdo
not match
3.) interpret the contents of the block
4.) skip ADDSIZE bytes if ADDSIZE is present
do
so until the EOF of the archive.

See next chapter how to build CRCs.

4. Building CRCs

This chapter contains sources how to build a "CRC32". The full 32 bits
are needed at the file checksums only. In the headers there are only
the lower 16 bits of a CRC32 value saved.

To initialize "getcrc" just call "make_crc_table". To get a CRC of a
block inititalize your CRC variable with CRC_MASK. then
use getcrc to
update this variable.

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