Extremely long codes can cause underflow errors

[drinckes]

In a test C++ implementation, long codes were causing segmentation faults. This was due to the grid computation, where the size of the grid squares are computed in degrees latitude and degrees longitude. These start at 1/8000th of a degree, and with each step, divide by 4 (latitude) or 5 (longitude).

Fairly quickly these can get to the minimum values that can be represented as floats and cause underflow errors.

To avoid this happening, I propose to limit the length of OLC codes. This would avoid this problem while still allowing extremely precise codes. 16 digits gives roughly centimeter resolution, and with 32 digits the OLC area is only 7.86E-11 cm wide (this is in the order of a helium atom).

I don't think it matters a great deal what the limit is as long as we define one.

The changes required are:

• To modify the isValid method to reject codes with more than 32 digits or with more than 24 digits after the separator
• To limit the encode method to only return codes with a maximum of 32 digits.

[tgulacsi]

go-fuzz found similar problems, for example with codes like

+qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X88qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X88qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X88qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX297572

+qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X88qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X88qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X88qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX2975722X888X2975722X888X2975722X988X29qqX297572

[drinckes]

What I'll probably do is add a test case with a code > 32 digits that should validate to false. Then everyone gets to run their tests. :-)

[zongweil]

@drinckes is this still applicable?

[drinckes]

Probably - to languages other than C++. I think @tgulacsi is pointing out that Go is also having underflow problems without a max length.

[zongweil]

As discussed in this thread, we are going to internally limit the number of characters that we process. This will avoid underflow errors.

It will take a while to update all the libraries.

[zongweil]

I'm going to close this bug now that we have a solution going forward. #190 tracks the work of updating all the libraries.