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

Mon Dec 20 08:49:24 GMT 2021

Hi Gerd

The tree is arranged so that letters at any level always have the
highest values, ie come in from the right, and there are no gaps so no
need for bit-flags. All it needs is the count of letters at each level
to determine the tree.

For levels 7 to 18 this is consistent and the count can be determined
from the first byte in the "struct for {level}".

Levels 19 and 20 are slightly out, but I think you found some
strangeness here, including something indicate using Mdr3x data.

Level 6 count might be mixed into the following

I haven't been able to work out levels 1-5 yet, but somewhere would
hope to find counts 0,0,1,7,9 maybe cumulative, maybe backwards, maybe
without the first 2 values, maybe starting from the cumulative level 6
value.

Ticker

On Sun, 2021-12-19 at 21:12 +0000, Gerd Petermann wrote:
> Hi Ticker,
> 
> my thought for the struct bytes was that the 1-bits might represent
> the position of leafs, but
> the bit counts don't match.
> 
> Gerd
> 
> ________________________________________
> Von: mkgmap-dev <mkgmap-dev-bounces at lists.mkgmap.org.uk> im Auftrag
> von Ticker Berkin <rwb-mkgmap at jagit.co.uk>
> Gesendet: Sonntag, 19. Dezember 2021 10:12
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev]       [mkgmap-svn]    Commit  r572:  
> MDR16   is      some    kind    of      codebook.
> 
> Hi Gerd
> 
> It looks like the order of the letter patterns approaches canonical
> Huffman (but not quite as far as I can see). With this, only the
> number
> of codes of each length is required to form the tree.
> 
> The "struct for {level}" looks like 2 numbers. Both increasing as
> levels go from 20 to 6. The first, a single byte from #00 to #3d -
> this
> could be give the accumulated lengths at each level (but I didn't get
> it to match). The second, variable length, from #000006 to #180c00
> and
> I've no idea what it could mean.
> 
> Levels 1 - 5 seem to be handled differently. with the repeat * 4,
> repeat * 2, no repeat and the padding.
> 
> Ticker
> 
> 
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