[mkgmap-dev] [mkgmap-svn] Commit r572: MDR16 is some kind of codebook.

Thu Dec 16 16:43:38 GMT 2021

Hi Ticker,

I looked at a few more old maps (donations from a mkgmap user)
with MDR16. They all seem to match the
pattern with the prefixes 0x0b, 0x09, and 0x07 and the repeating of information.

Maybe you get an idea what the rest means.

Gerd

________________________________________
Von: Gerd Petermann <gpetermann_muenchen at hotmail.com>
Gesendet: Donnerstag, 16. Dezember 2021 15:42
An: Development list for mkgmap
Betreff: AW: [mkgmap-dev]       [mkgmap-svn]    Commit  r572:   MDR16   is      some    kind    of      codebook.

Hi Ticker,

the order of the chars seems to depend on the depth in the tree at which they appear.
Something like this happens beginning at offset 5c in MDR 16:
write 0x0b
write 1st value at depth 5
write 0x0b
write 2nd value at depth 5
write 0x0b
write 3rd value at depth 5
...
write 0x0b
write last value at depth 5

repeat 2 times:
write 0x09
write 1st value at depth 4
repeat 2 times:
write 0x09
write 2nd value at depth 4
...
repeat 2 times:
write 0x09
write last value at depth 4

repeat 4 times:
write 0x07
write 1st value at depth 3 (there is only one in this tree)

No prefix, no repeats for the rest:
write values at level depth 20
write values at level depth 19
...
write values at level depth 6

This decribes all bytes in MDR 16 from offset 5c to the end.

No idea yet what the prefixes mean.
Gerd

________________________________________
Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag von Ticker Berkin <rwb-mkgmap at jagit.co.uk>
Gesendet: Donnerstag, 16. Dezember 2021 10:04
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev]       [mkgmap-svn]    Commit  r572:   MDR16   is      some    kind    of      codebook.

Hi Gerd

I found similar/same algos. I'm trying to thing of other ways that
might come up with what we see. I'll continue research.

Ticker

On Wed, 2021-12-15 at 20:18 +0000, Gerd Petermann wrote:
> Hi Ticker,
>
> do you have a link for me? None of the methods to store the tree that
> I found would produce such a data structure.
> This is what I found so far:
> https://web.stanford.edu/class/archive/cs/cs106b/cs106b.1208/assignments/assign6/warmup
> https://stackoverflow.com/questions/759707/efficient-way-of-storing-huffman-tree
>
> Gerd

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