Webcentre Ltd LHA FAQ

TCompLHA Frequently Asked Questions

Last updated: September 2015

Contents:

**** Purchasing and Licensing:
* Do I need to pay to use TCompLHA?

**** Performance and Capabilities
* With what archiving programs is TCompLHA compatible?
* Does TCompLHA require any external DLLs to operate?
* Can I create a self-extracting EXE with TCompLHA?
* Is TCompLHA "industrial strength", or does it have hidden size
limitations or problems?
* Can I create archives which span across several disks?
* How good is the data compression I get with TCompLHA?
* I have some files which don't get any smaller when compressed
using TCompLHA. Why not?
* Why don't "protected" Paradox tables compress well with TCompLHA?
* Is TCompLHA thread-safe?
* Is TCompLHA Year-2000 compliant?

**** How To:
* Can I seek into the middle of a compressed file and decompress
just part of it?
* Does TCompLHA handle updates and deletions from an archive
nicely, or does it leave "holes"?
* How can I speed up access to files in an archive?
* Can TCompLHA do an "update only newer files" operation by checking dates?

**** Problems (also see the Troubleshooting section of CompLHA.hlp):
* I compressed a file using TCompLHA, but I can't seem to decompress it.
Why not?
* I compressed something into a stream, but I can't seem to expand it.
Why not?
* How do I eliminate the registration popup message? Is any other
functionality affected in an unregistered copy of TCompLHA?
* Do you have any demo programs written specifically for C++ Builder, or
are they all Pascal?
* My application's memory use seems to increase every time I decompress
files. Why?
* What should I do if I see an "EUnableToCompress" exception?

============ And here are the answers: =================

**** Purchasing and Licensing:

* Do I need to pay to use TCompLHA?
No. You may freely use the source code in your projects and freely distribute finished executable
programs which contain its object code. However, you may NOT redistribute or resell the
source units themselves without express prior permission from the copyright holder.

**** Performance and Capabilities

* With what archiving programs is TCompLHA compatible?
TCompLHA has been thoroughly tested with LHArc 1.13 and LHA 2.13 for DOS and
the popular WinZip shell utility. In summary:
1. TCompLHA can COMPRESS files into archives which are compatible with
any of the above programs
2. TCompLHA can EXPAND files which were compressed with LHArc or with LHA
and with WinZip writing to "LZH" files
3. TCompLHA can EXPAND files compressed with any version of our
TCompress
product, provided they don't use custom compression formats or archive key
protection.

* Does TCompLHA require any external DLLs to operate?
None whatsoever. TCompLHA is written in 100% native Delphi.

* Can I create a self-extracting EXE with TCompLHA?
Yes. See the SFX.DPR project (read the comments at the top) and MAKEEXE.DPR.

* Is TCompLHA "industrial strength", or does it have hidden size limitations
or problems?
Hundreds of developers are using our compression products for an extremely
wide range of applications, with file sizes ranging from kilobytes to tens of gigabytes. They, and we, believe them to be a trouble-free solution for the
range of compression problems for which they were designed. Limitations tend to
be wholly a factor of the deployment platform (e.g. system resources, free
disk space etc). Version 8.0 supports files and archives with an overall size
bigger than the 2GB "OS-determined" limit which applied to earlier versions
(there is a new upper limit, of course, but that is 8 exabytes = 2^63 bytes).

* Can I create archives which span across several disks?
Yes -- use the TSegLHA component. It's also great for simple backup/restore
applications -- you don't need to worry if the archive size will increase
beyond one floppy disk.

* How good is the data compression I get with TCompLHA?
All data compression is dependent on the method used and the nature of the
input data. Because it is compatible with LHArc and LHA, you should get
compression results which are identical to those programs and similar to
those achieved by other compression utilities.

* I have some files which don't get any smaller when compressed using
TCompLHA. Why not?
See the question above. Some files have data which is already so complex that
they won't compress significantly, or at all. If you have colh5 set as your
CompressionMethod, you should get roughly the same level of compression as
common compression utilities will give you.

* Why don't "protected" Paradox tables compress well with TCompLHA?
When you protect a Paradox table, the entropy or "randomness" of its data
increases significantly -- so much so that all compression algorithms become
much less efficient. You'll encounter this whether you use TCompLHA or any
other compression utility.

* Is TCompLHA thread-safe?
Yes, the code can be used in multi-threaded applications.
To use it effectively, you need to create an instance of the component
within each new thread -- as per the example code in the LHADemo
sample application.

* Is TCompLHA Year-2000 compliant
Yes. No date-based calculations are performed,
and all date storage formats used are compliant
with Windows definitions, which support four year digits.

**** How To:

* Can I seek into the middle of a single compressed file in the archive,
and decompress just part of it?
Generally speaking, no compression method will permit this. However, it can
be "faked" in either of the following ways:
1. Decompress the entire file into a temporary TMemorystream using
the ExpandStreamFromArchive method, then use the TStream seek and read methods
to access any part of it. This is not useful if the file is huge of course, but
decompressing to a temporary TFilestream instead might be viable for some
situations.
2. Instead of compressing it as a single file, use CompressStreamToArchive to
compress it as a series of "chunks" or file segments (each chunk can be given
a name in the archive which is its number, e.g. "1", "2" etc). With this
approach you can randomly select any chunk and decompress it individually. For
performance-critical applications, also see the question below on speeding up
access to files in archives.

* Does TCompLHA handle updates and deletions from an archive nicely, or does
it leave "holes"?
Unless the AllowDuplicates property is True, archives are rewritten to
eliminate holes and remove old files. AllowDuplicates will cause new additions
to be directly appended to an archive without checking, which could lead to
duplicate files in the archive, but not "holes".

* How can I speed up access to files in an archive?
For small archives, this is hardly necessary --- finding and decompressing
any file is generally very fast. But if your archive gets very large, the
sequential search needed to find a given file to decompress it can be a time-
waster. A solution to this is to use the Scan method once, when you
first access your archive (as the LHAdemo example application does). This
process also creates a TFileInformation list with information about each file,
including its position in the archive. With that information, you can open the
archive as a TFileStream, seek directly to the appropriate place and use the
ExpandFilesFromStream method to expand just that file. Time savings on large
archives by using this method can be very significant indeed.

* Can TCompLHA do an "update only newer files" operation by checking dates?
Yes. To do this you would start with a Scan of the archive to create a list of the existing archive files and their dates, then Compress the files you want to add with the following logic in the OnCheckFile event:

if mode=cmLHACompress then
with CompLHA do
begin
count := FileList.indexof(newfilepath); { note: may need to search case INsensitive }
if (count >=0) and
(datetime <= TCompLHAFileInfo(FileInformation[count]).DateTime) then
newFilePath := CompLHASkipFlag;
end;

**** Problems (also see the Troubleshooting section of CompLHA.hlp):

* I compressed a file using TCompLHA, but I can't seem to decompress it. Why
not?
The most likely reason is that you are not specifying the correct filename (as
stored in the archive). This can be double-checked by using the LHADemo program
to look at the files in your archive. The filename you specify should match the
filepath which is stored in the archive. Review the CompLHA.hlp help section on
"Filename Handling in Compressed Archives" to see some examples of how to
control what filenames and paths are stored in the archive.

* I compressed something into a stream, but I can't seem to expand it.
Why not?
Don't forget to either close and reopen the stream or (more commonly)
seek back to the beginning of the stream before trying to expand from it!

* How do I eliminate the registration popup message? Is any other
functionality affected in an unregistered copy of TCompLHA?
When you register TCompLHA, you will be given values to set into the RegName
and RegNumber properties of your TCompLHA components. If you set these
exactly as provided to you, they will eliminate the popup. Apart from the
popup message, unregistered copies of TCompLHA behave exactly the same as
registered ones.

* Do you have any demo programs written specifically for C++ Builder, or are
they all Delphi?
At this point, all the demonstrations are in Delphi (with a special variant of one for Kylix).

* My application's memory use seems to increase every time I decompress
files. Why?
If you are directly using the ScanCompressedFile or ScanCompressedStream
methods, see the notes in their section of CompLHA.hlp about the
TCompLHAFileInfo information and the FreeFileList method, which will allow
you to eliminate memory loss.

* What should I do if I see an "EUnableToCompress" exception?
Nothing, unless you are directly calling the DoCompress method (see the help). All other methods handle this exception internally, meaning that the only time you'll see this exception is if you have "Break On Exception" turned on in the IDE.

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