[mkgmap-dev] What's the maximum size of global index MDR size?

Mon Dec 13 12:33:21 GMT 2021

Hi Ticker,

I am pretty sure that the original data doesn't contain lower case characters.
The characters B and b in name "Baca Pri Podbrdu" both change when the last byte in MDR16 (0x42)
is changed.

I've looked at several suggestions how the tree could be stored, nothing seems to match the pattern
in MDR 16 so far. Of course we cannot even be sure that huffman encoding is used.

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, 13. Dezember 2021 13:14
An: Development list for mkgmap
Betreff: Re: [mkgmap-dev] What's the maximum size of global index MDR size?

Hi Gerd

I've build and examined this. I assume that the table came from a map
that used most of the available characters, so not seeing these
precludes the theory of it being flattened shape/contents lists in any
simple representation. Can you tell if the original map was mixed case?
If it wasn't then it can't be a simple normalized frequency count as
there would be an area of zeros for the lower case letters.

Ticker

On Mon, 2021-12-13 at 10:25 +0000, Gerd Petermann wrote:
> Hi Ticker,
>
> attached is my experimental code patch for mkgmap.
> It changes mkgmap to write the MDR 16 section ad MDR15 section with
> compression set to 1 in the MDR header.
> The Mdr15 content are the first 32 bytes of the original table.
>
> I use it with the attached OSM file and options --index --gmapi --
> code-page=1252
>
> Hope it helps. Maybe you can find the download link to the adria map
> somewhere
> in the archive, my downloaded file is called "AdriaTOPO 2.40 HR.exe"
> Unfortunately my links to Nabble no longer work.
>
> 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, 13. Dezember 2021 11:08
> An: Development list for mkgmap
> Betreff: Re: [mkgmap-dev] What's the maximum size of global index MDR
> size?
>
> Hi Gerd
>
> Does Mdr16 look like some form of a flattened Huffman tree? A common
> representation is a stream of bits describing the shape and a stream
> of
> chars giving the contents.
>
> If you send me an example + the little bit at the start of Mdr15
> you've
> been using I'll have a look.
>
> Ticker
>
> On Sun, 2021-12-12 at 16:20 +0000, Gerd Petermann wrote:
> > Hi all,
> >
> > yes, Mdr16 obviously contains some kind of codebook that is used to
> > decode the strings in Mdr15.
> >
> > I've modified the code to write a copy of the MDR 16 section that I
> > found in the Adria Topo map
> > and changed Mdr15 code so that it always returns 1 as string offset
> > and copies the first 24 bytes of the Mdr15 section. The original
> > index contains offsets 1,11,16,20,24,27,... into Mdr15
> > so the first string seems quite long compared to the next ones.
> > If created a map that contains a single city POI with the name abc
> > and MapSource displays Abc as expected (did not use --lower-case)
> > Next I start to search for cities.
> > No matter what character I type in the city name field the string
> > that is displayed is
> > "Baca Pri Podbrdu" (not sure about Upper/Lowercase yet)
> > When I change the code to return offset 11 instead of 1 the string
> > "Bavsica" is displayed instead of "Baca Pri Podbrdu"
> >
> > These strings can be found in the Adria Topo map.
> >
> > When I modify the content of the MDR 16 table, esp. the last byte,
> > I
> > see different strings displayed.
> > E.g. when I change the last byte 0x42 ('B') to 0x58 ('X') the
> > string
> > "Baca Pri Podbrdu" changes to "Xaca Pri Podxrdu"
> > So, I am now trying to understand the meaning of that table. It is
> > different in the three maps, all use
> > codepage 1252 and the MDR 16 sizes are 165, 177 and 212. It's very
> > likely that the content
> > depends on the frequency of the characters in the uncompressed MDR
> > 15
> > file.
> >
> > Gerd
>
>
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