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

Mon Dec 20 09:38:52 GMT 2021

Hi Gerd

Are you OK with the idea of the letters at any level being to the
right. Then the letters at this level take decreasing values. When
exhausted, the next lower slot will be the start of the next level, so
subtract 1 to find it and append '1' to the bit-string to get the
starting position for the next level.

in the test map there is, say:

000002cf | 000019 | 11                      | struct level: 17
000002d0 | 00001a | 15 50 00 00             | struct for 17
000002d4 | 00001e | 10                      | struct level: 16
000002d5 | 00001f | 19 80 00 00             | struct for 16
000002d9 | 000023 | 0f                      | struct level: 15
000002da | 000024 | 1c c0 00 00             | struct for 15

The differences between the first byte after the level indicator ie 15,
19 & 1c) give 4 and 3, which are the number of letters at level 17 and
level 16

Ticker

On Mon, 2021-12-20 at 09:18 +0000, Gerd Petermann wrote:
> Hi Ticker,
> please explain. What counters do you see? What do they 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: Montag, 20. Dezember 2021 09:49
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev]       [mkgmap-svn]    Commit  r572:   MDR16  
> is      some    kind    of      codebook.
> 
> 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
> > 
> > 
> > _______________________________________________
> > mkgmap-dev mailing list
> > mkgmap-dev at lists.mkgmap.org.uk
> > https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
> > _______________________________________________
> > mkgmap-dev mailing list
> > mkgmap-dev at lists.mkgmap.org.uk
> > https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
> 
> 
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