Prevent overflow

[minad]

disabled the overestimate test for the INT_MAX-1 number for now since this number is not allowed anymore.

[minad]

@czurnieden could you do me a favor and recompute the biggest number table for the overestimate test which is disabled now?

[czurnieden]

could you do me a favor and recompute the biggest number table for the overestimate test which is disabled now?

If you want to make the maximum size dependent on MP_DIGIT_BIT we would need several tables, one for each size/MP_xBIT and two for MP_16BIT because sizeof(int) can be 2.

That may take a day or two ;-)

[minad]

Uuh could we do it somehow differently?

[minad]

@czurnieden I added a randomized test for now.

[minad]

@sjaeckel Right, I added a workaround using the tommath_c89.h header.

[minad]

@sjaeckel Good to merge?

[minad]

@sjaeckel Ping. Can we merge this?

[czurnieden]

What is your use case, may I ask?

A new and improved mp_log_n.
And also extended to full bigint if my calculations are not too much off.

It nagged me from the start on, that it has a lot of full bigint operations with up to twice the number of digits of the input, the trial exponentiations can overflow MP_MAX_DIGIT_COUNT * MP_DIGIT_BIT (the squarings too, but you could put a lid on them), and that the base is, although not theoretically but in praxi restricted to a small native integer instead of the actual upper limit (MP_MAX_DIGIT_COUNT * MP_DIGIT_BIT) - 1 .

You can get that down to one trial exponentiation that does not overflow and up to two small trials. One of the trials is by multiplying with the base which can overflow but that overflow is usable by the trial, that's why I wanted the distinct error. The other trial is a division by the base. It is exact, but as long as we don't have "exact division" also slower than the multiplication for bigint bases.

That together with a rather large "hidden constant" might make it significantly slower than the old method for small input but I have no benchmarks at this stage.

So don't hold your breath ;-)