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

Thu Dec 23 19:53:27 GMT 2021

Hi Ticker,

reg. "stat": my rule is a bit different but the result is the same:
for all odd values:
depth = usedBits = stat >> 1;

val is the 2nd byte.
for even stat values and rows where stat == val << 1 the decoder
has to read  exactly 1 more bit to decide which character was encoded.
I've not yet understood how the stat or the val field can be used to find the correct entry
in the byte array with the characters.

So, maybe the stat value should be shifted to the right to make some sense.

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, 23. Dezember 2021 15:56
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev]       [mkgmap-svn]    Commit  r572:   MDR16   is      some    kind    of      codebook.

Hi Gerd

Maybe the "is always 8" is the character width.

I think I understand parts of it.

Read 5 bits and look up, giving 2 bytes.
if first 3/5/6/9/b the final char in the second byte and return (b-
first)/2 to the bit stream. Otherwise the combination somehow indicate
a minimum number of more bits to read and where to start searching.
maybe the 'struct for level' 1 or 2 bytes is a bit flag of which levels
to look in. I notice (in topo_fr) it is normally a single or adjacent
bits except for 0xa, but there are no chars at level 19

prefix 0 doesn't quite follow the rules

Ticker

On Thu, 2021-12-23 at 09:40 +0000, Gerd Petermann wrote:
> Hi Ticker,
>
> ouch, didn't even read that one :(  Would have saved me a lot
> of time as my first idea was a "jump table".
> Attached is the new patch and the corrected mdr16 outputs produced
> with this patch.
>
> So, besides the fields with ??? the only open question is the meaning
> of the struct bytes in the first table.
> I guess it will become clear when I try to use the lookup table in
> the decoder.
>
> 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, 23. Dezember 2021 09:31
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev]       [mkgmap-svn]    Commit  r572:
> MDR16   is      some    kind    of      codebook.
>
> Hi Gerd
>
> I guessed that it was the \0 that cut the file off.
>
> You mean something like answer 5 here:
>
> https://stackoverflow.com/questions/759707/efficient-way-of-storing-huffman-tree
>
>
> I looked at this earlier trying to work out if it was relevant but
> didn't make it fit - I should have tried harder.
>
> I still haven't spotted anything to determine the length of the 2 /3
> byte in before and in the struct for {length}
>
> Ticker
>
> On Thu, 2021-12-23 at 01:17 +0000, Gerd Petermann wrote:
> > Hi Ticker,
> >
> > sorry, just noticed that something went wrong with copy&paste
> > because
> > of the \0 character.
> > Anyway, I think I understand the meaning of the part with the
> > prefixes.
> > I assume that Garmin reads the first 5 bits and uses this value as
> > an
> > index into a table
> > which directly follows the first array. This 2nd table is a 32x2
> > lookup
> > table, where the 2nd byte gives the value
> > and the first byte some kind of status info which is used to
> > position
> > the bit reader.
> > This would explain the repeating characters. Something like this:
> >
> > --------- MDR 16 (decompression codebook Huffman tree) ------------
> > ----
> > ---------
> > 000002b6 | 000000 | 4a                      | ???
>
>
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