[mkgmap-dev] Please try and report results of Huffman compression

Sat Jan 15 15:59:24 GMT 2022

Hi all,

during the last weeks Ticker and I decoded Garmin's String compression of the global index for the PC (the *_mdr.img) .
In the mdr2 branch I've coded support for that in mkgmap and I think its ready for a first beta test.

It works with codepages 1250 ..1258 or 65001. 
I've no idea how the 2-byte pages work, but Garmin seems to support that, too. Maybe I'll work on this later.

The branch version produces around 200 lines of diagnostic output to show what happens when compression is enabled.
This will be changed to debug level later.

The encoding is enabled by default, special option --x-no-mdr15-compression can be used to turn it off.

Note that the size of the *_mdr.img file will be smaller (which is esp. intersting for those who get close to the 2G limit)
Note also that a zipped *_mdr.img can and will be larger since zip cannot remove as much redundancy. 
Note also that this branch includes the changes from the faster-mp branch and introduces a few changes 
which are relavant for --lower-case maps.

How to test?
Install the map in MapSource and check if you see any unexpected characters in the result lists or maybe even crashes while searching.
If so, please try the same input files with the --x-no-mdr15-compression to verify if this is caused by compression or maybe other changes.

Please don't yet publish the maps, but let me know if you see anything unexpected. Some changes reg. lower-case in this branch are still experimental.

The binary is here:
https://www.mkgmap.org.uk/download/mkgmap-mdr2-r4865.zip

If you see errors which are caused by compression, please attach the diagnostic output.

@Ticker: Please review the source.

Gerd