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Heatshrink compressed drives? - Tamp ISO compression test (Announce)

posted by mceric, Germany, 30.01.2025, 02:21

> > If you ever feel bored, maybe you could consider building a heatshrinked
> > version of SHSUCDHD?
>
> If you've got the RAM I think you'd be better off just using gzip and
> SHSUCDRD. But anyway, here's a
> demonstration
> compressor (Windows binary), using Tamp...

Interestingly short :-)

So your image file format consists of a 4 byte header and an array of 32-bit compressed sector offsets? I fail to see where the total number of sectors in the image is stored, though?

If you already have test-compressed a few CD images you had around, could you share only the resulting compressed-image headers WITHOUT the compressed sector contents themselves, as a small file on your page? I am curious about the distribution of compressed sector sizes for typical ISOs :-)

Of course it would be possible to "compress" the table of file offsets of compressed sectors, but that would complicate access later and it is not necessary for the evaluation of compressors like TAMP.

A simple scheme would be to only store the absolute offset of every 16th compressed sector, followed by only the 16 lower bits for the next 15 offsets. Saves roughly half of the header size. To "decompress" the offset of sector x, one has to read the full offset F of sector (x and not 15), then, if (x and 15) is non-zero, read the low bits L for sector (x and 15) and compute F + (L if L was read) + (65536 if L was read and L < low bits of F). This should work for sector sizes of up to 4096 bytes, such as ISO sectors of 2048 bytes.

But I am getting distracted :-)

I am quite curious what the compressed sector size distributions are in your experience, also in relation to overall compression factors for the images as a whole.

Thank you!

---
FreeDOS / DOSEMU2 / ...

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