Handle positive infinity and negative infinity

[scravy]

If for whatever reason nBits is zero, the difficulty calculation (which is carried out using double types) will yield positive or negative infinity.

Infinity fails to properly serialize using univalue which will lead to broken json.

This patch will simply set it to the maximum and minimum finite values in case we have positive or negative infinity. These values are still strange enough that they allow you to diagnose the problem.

This is extracted from #577

Signed-off-by: Julian Fleischer julian@thirdhash.com

[thothd]

In which cases nBits is 0 ?

[scravy]

• When you set it to zero during development and lose a day on broken json.
• When a block was forged that way, maliciously or by accident
• When you're experimenting on Adjust difficulty #285 and you're function unluckily hits zero, try to debug it, and all you get is broken json

I would love to patch univalue to just render infinities, but it's one of these git subtree thingies.

[scravy]

Alternative: Could check for infinity before creating the univalue, same file, and then render the string "+Inf" or "-Inf" instead. The function patched here is exclusively used in this RPC call.

[scravy]

Pushed the alternative.

[scravy]

Refactored.

[thothd]

utACK 4a79f5f

[scravy]

For reviewers, in case you're not aware:

[cling]$ double x;
[cling]$ double z = x / x;
[cling]$ z == z
(bool) false

NaN is like Drölf (Drölf is 12 and 13 at the same time, and thus x == x + 1 holds true for it only).

[thothd]

utACK 4e8ae4c