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

Thu Dec 16 17:33:55 GMT 2021

Hi Gerd

I've followed the lists of chars at levels, some with separators, maybe
indicate totally internal nodes. The early bit must be bit flags to
describe the tree and maybe offsets into the leaves at different
levels. The repeating at the low levels seems very strange.

but wow - quite some detective work.

Ticker

On Thu, 2021-12-16 at 16:43 +0000, Gerd Petermann wrote:
> 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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