[mkgmap-dev] [PATCH] Bug in label encoding

Mon Feb 8 20:52:55 GMT 2010

Am 08.02.2010 10:46, schrieb Steve Ratcliffe:
> On 07/02/10 23:47, Ronny Klier wrote:
>> I think there is a bug in label encoding in Format6Encoder. For some
>> string length the last encoded byte is not stored.
>>
>> E.g. having a string "10007" the encoded byte buffer looks like this
>>
>> [0] [0x86]
>> [1] [0x8]
>> [2] [0x20]
>> [3] [0x9f]
>> [4] [0xf0]
>>
>> The number of stored bytes is 4. So the 0xf0 will not show up in the
>> final image file.
>
> I believe the code is correct and the 0xf0 is not required and omitted
> on purpose.
>
> A 5 byte string where each character is encoded as 6 bits requires 30
> bits of storage which is 3.75 bytes.  There is then a end-of-string
> marker which is written as the 6 bit value 0x3f.  However, the end of
> string marker is actually variable length and as long as the first two
> bits are written you can drop the rest.  So the whole thing fits into
> 4 bytes which is what is written.
>
> The code is this way because originally I did not realise that the
> string terminator was effectively a two bit quantity ie. if the first
> two bits are both 1 then the strings ends and you stop reading and
> throw away any remaining part of the byte.  You could probably write 2
> bits and then round the byte count up instead of writing 6 and
> rounding down.
>
> ..Steve

OK, I got this wrong. I thought the label section could be continously 
read. Every label ending with 0x3f and next label starting at next byte 
boundary.