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  1. (I already asked this on SuperUser but didn't get any reply so far, so I have little hope of getting some useful insight over here, but let's try anyway...)

    Let's say there is a ZIP or RAR archive on a file sharing network, an old archive which has been out there for a long time, containing hundreds of small files (JPG pictures typically), and some parts are missing, like 20MB out of 500MB, there is no longer a single complete source and it's unlikely there will ever be, so anyone attempting to download it will get stuck with a large unusable file (well, the complete files inside can still be extracted, but most users either wait for the file to complete or delete it altogether after a while).

    But I may have all the individual files contained in those missing parts, found in other similar archives, or acquired from another source. The goal would be to sort of “revive” such a broken archive, in a case like this where only a small part is missing, so that it can be shared again.

    If an archive is stored without compression, such a process is tedious enough (I've done this a few times recently, painstakingly copying each file with a hexadecimal editor and reconstructing each individual file's header, then verifying that the hash code matched that of the original archive), but it gets really tricky if compression is involved, as it is not possible to simply copy and paste the contents of the missing files, they have to first be compressed with the exact same parameters as the incomplete archive, so that the actual binary content can match.

    For instance I have an incomplete ZIP file with a size of 372MB, missing 18MB. I identified a picture set contained within the missing part in another, larger archive: fortunately the timestamps seem to be exactly the same, but unfortunately the compression parameters aren't the same, the compressed sizes are different and the binary contents won't match. So I uncompressed that set, and attempted to re-compress it as ZIP using WinRAR 5.40, testing with all the available parameters, and checked if the output matched (each file should have the exact same compressed size and the same binary content when examined with the hex editor), but I couldn't get that result. So the incomplete archive was probably created with a different software and/or version, using a different compression algorithm.

    Now, is it possible, by examining the file's header, to know exactly what specific application was used to create it, and with which exact parameters ? Do the compression algorithms get updated with each new version of a particular program, or only with some major updates ? Are the ZIP algorithms in WinRAR different from those in WinZIP, or 7Zip, or other implementations ? Does the hardware have any bearing on the outcome of ZIP / RAR compression — for instance if using a mono-core or multi-core CPU, or a CPU featuring or not featuring a specific set of instructions, or the amount or RAM — or even the operating system environment ? (In which case it would be a nigh impossible task.)

    The header of the ZIP file mentioned above is as follows :
    50 4B 03 04 14 00 02 00 08 00 B2 7A B3 2C 4C
    5D 98 15 F1 4F 01 00 65 50 01 00 1F 00 00 00
    I tried to search information about the ZIP format header structure, but so far came up with nothing conclusive with regards to what I'm looking for.

    The problem seems even more complicated with RAR files (I also have a few with “holes”), as they don't seem to have a complete index of their contents in the header or footer (like ZIP archives have), if I'm not mistaken, each file is referenced only by its own header, and without the complete list of missing files it's almost certainly a fool's errand. But I managed to find several versions of the rar.exe CLI compressor, with which I could quickly run tests in the hope of finding the right one, whereas WinZIP (which, I would guess, is most likely the software used to create a ZIP archive around 2003, based on the files' timestamps) apparently only works within the GUI, and installing a bunch of older versions just to run such tests would be totally unpractical and unreasonable for what is already a quite foolish endeavour in the first place !

    Last edited by abolibibelot; 14th Nov 2019 at 14:55.
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  2. Member DB83's Avatar
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    Maybe this reply over-simplifies the 'solution'

    Have you attempted a 'repair' of the archive ? >> Run WinRAR >> Highlight file >> Choose Repair
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  3. Well, of course that won't work, since the goal is to reconstruct the original archive file, with the same hash code, so that people having had that file in their queue for months or years can finally complete it, and share it in turn (if it so happens that I do have all the missing files from the archive, then I don't really need to complete it for myself). Repairing is useful for extracting all files that are complete from an archive which is incomplete, for instance if file number 150 out of 500 is corrupted because of a large "hole" of missing data, usually the extraction with WinRAR stops right away, and hundreds of files aren't recovered, whereas the repair process creates a new archive with a modified structure which makes it possible to extract all valid files beyond the "hole", but it still won't make it possible to recover the files which have even a few bytes located in that "hole" (if the archive had recovery record data it's theoretically possible to actually repair mild corruption, but with several megabytes missing it's useless). What I'm looking for here is way more advanced, and perhaps even impossible... As I wrote, I managed to do it for archives created in "store only" mode, i.e. with no compression ; the problem with actually compressed archives is that the exact outcome of file compression varies a lot, apparently, and I'm trying to find out how to re-generate the exact same compression pattern as in those archives created 15+ years ago, without knowing how they were created in the first place.
    Last edited by abolibibelot; 14th Nov 2019 at 18:19.
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  4. not possible, just compress the missing files in a new archiveand move on..

    compression dictionaries differ acc to total files input.. compressor version etc..
    so you will never reconstruct the original with your current setup
    Last edited by teodz1984; 14th Nov 2019 at 18:38.
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  5. compression dictionaries differ acc to total files input.. compressor version etc..
    so you will never reconstruct the original with your current setup
    Could you elaborate a bit ? You mean that, for instance, even with the same compressor version on the same setup, with a per-file compression scheme (as opposed to the so-called "solid" mode), compressing 100 files and then compressing 99 out of those 100 will produce a different result ? I did a quick test : compressed 8 small files in ZIP "good" (with WinRAR 5.40 again), then compressed 7 out of those 8, the order in which the files are positioned within each archive is different, but the binary contents for each file present in both archives are exactly identical, so it would seem like each file is processed independently in a reproducible way regardless of the total number of files.
    Is there a place where I could find a thorough list of the factors which affect the outcome of file compression ? Do you know if hardware is such a factor ?

    and move on..
    Yeah, that's somethin' I have trouble with generally speakin'... é_č
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    compare same files compressed with different versions of winzip, winrar etc..
    each produces a different result that is not binary .
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  7. as you can see in the documentation, algorithms have have changed over the years, so only way to recreate is have thw orinal software do the recompression
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  8. Dinosaur Supervisor KarMa's Avatar
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    7zip should allow you to extract incomplete archives, obviously it will just stop once the missing block is reached.
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