[issue36027] Consider adding modular multiplicative inverse to the math module

[Berry Schoenmakers]

Berry Schoenmakers <l.a.m.schoenmakers at tue.nl> added the comment:

> Is there a clear reason for your expectation that the xgcd-based algorithm should be faster?

Yeah, good question. Maybe I'm assuming too much, like assuming that it should be faster;) It may depend a lot on the constants indeed, but ultimately the xgcd style should prevail.

So the pow-based algorithm needs to do log(p) full-size muls, plus log(p) modular reductions. Karatsuba helps a bit to speed up the muls, but as far as I know it only kicks in for quite sizeable inputs. I forgot how Python is dealing with the modular reductions, but presumably that's done without divisions.

The xgcd-based algorithm needs to do a division per iteration, but the numbers are getting smaller over the course of the algorithm. And, the worst-case behavior occurs for things involving Fibonacci numbers only. In any case, the overall bit complexity is quadratic, even if division is quadratic. There may be a few expensive divisions along the way, but these also reduce the numbers a lot in size, which leads to good amortized complexity for each iteration.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36027>
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