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classification
Title: Type: Support negative exponents in pow() where a modulus is specified. enhancement resolved Library (Lib) Python 3.8
process
Status: Resolution: closed fixed mark.dickinson lschoe, mark.dickinson, miss-islington, pablogsal, petr.viktorin, phr, rhettinger, serhiy.storchaka, skrah, steven.daprano, tim.peters low patch

Created on 2019-02-18 20:10 by rhettinger, last changed 2022-04-11 14:59 by admin. This issue is now closed.

Pull Requests
PR 13266 merged mark.dickinson, 2019-05-12 17:57
PR 13758 merged petr.viktorin, 2019-06-02 22:39
PR 13761 merged petr.viktorin, 2019-06-03 00:00
PR 26690 merged mark.dickinson, 2021-06-12 08:22
PR 26703 merged mark.dickinson, 2021-06-13 07:35
PR 26702 merged miss-islington, 2021-06-13 07:36
Messages (24)
msg335862 - (view) Author: Raymond Hettinger (rhettinger) * Date: 2019-02-18 20:10
Having gcd() in the math module has been nice.  Here is another number theory basic that I've needed every now and then:

def multinv(modulus, value):
'''Multiplicative inverse in a given modulus

>>> multinv(191, 138)
18
>>> 18 * 138 % 191
1

>>> multinv(191, 38)
186
>>> 186 * 38 % 191
1

>>> multinv(120, 23)
47
>>> 47 * 23 % 120
1

'''
# https://en.wikipedia.org/wiki/Modular_multiplicative_inverse
# http://en.wikipedia.org/wiki/Extended_Euclidean_algorithm
x, lastx = 0, 1
a, b = modulus, value
while b:
a, q, b = b, a // b, a % b
x, lastx = lastx - q * x, x
result = (1 - lastx * modulus) // value
if result < 0:
result += modulus
assert 0 <= result < modulus and value * result % modulus == 1
return result
msg335888 - (view) Author: Tim Peters (tim.peters) * Date: 2019-02-19 05:34
- Some form of this would be most welcome!

- If it's spelled this way, put the modulus argument last?  "Everyone expects" the modulus to come last, whether in code:

x = (a+b) % m
x = a*b % m
x = pow(a, b, m)

or in math:

a^(k*(p-1)) = (a^(p-1))^k = 1^k = 1 (mod p)

- Years ago Guido signed off on spelling this

pow(value, -1, modulus)

which currently raises an exception.  Presumably

pow(value, -n, modulus)

for int n > 1 would mean the same as pow(pow(value, -1, modulus), n, modulus), if it were accepted at all.  I'd be happy to stop with -1.

- An alternative could be to supply egcd(a, b) returning (g, x, y) such that

a*x + b*y == g == gcd(a, b)

But I'm not sure anyone would use that _except_ to compute modular inverse.  So probably not.
msg335894 - (view) Author: Raymond Hettinger (rhettinger) * Date: 2019-02-19 07:23
> If it's spelled this way, put the modulus argument last?

Yes, that makes sense.

> Years ago Guido signed off on spelling this
>
>    pow(value, -1, modulus)

+1 ;-)
msg335921 - (view) Author: Mark Dickinson (mark.dickinson) * Date: 2019-02-19 10:35
+1 for the pow(value, -1, modulus) spelling. It should raise `ValueError` if `value` and `modulus` are not relatively prime.

It would feel odd to me _not_ to extend this to `pow(value, n, modulus)` for all negative `n`, again valid only only if `value` is relatively prime to `modulus`.
msg335922 - (view) Author: Mark Dickinson (mark.dickinson) * Date: 2019-02-19 10:41
Here's an example of some code in the standard library that would have benefited from the availability of `pow(x, n, m)` for arbitrary negative n: https://github.com/python/cpython/blob/e7a4bb554edb72fc6619d23241d59162d06f249a/Lib/_pydecimal.py#L957-L960

if self._exp >= 0:
exp_hash = pow(10, self._exp, _PyHASH_MODULUS)
else:
exp_hash = pow(_PyHASH_10INV, -self._exp, _PyHASH_MODULUS)

where:

_PyHASH_10INV = pow(10, _PyHASH_MODULUS - 2, _PyHASH_MODULUS)

With the proposed addition, that just becomes `pow(10, self._exp, _PyHASH_MODULUS)`, and the `_PyHASH_10INV` constant isn't needed any more.
msg335952 - (view) Author: Berry Schoenmakers (lschoe) * Date: 2019-02-19 14:39
Agreed, extending pow(value, n, modulus) to negative n would be a great addition!

To have modinv(value, modulus) next to that also makes a lot of sense to me, as this would avoid lots of confusion among users who are not so experienced with modular arithmetic. I know from working with generations of students and programmers how easy it is to make mistakes here (including lots of mistakes that I made myself;)

One would implement pow() for negative n, anyway, by first computing the modular inverse and then raising it to the power -n. So, to expose the modinv() function to the outside world won't cost much effort.

Modular powers, in particular, are often very confusing. Like for a prime modulus p, all of pow(a, -1,p), pow(a, p-2, p), pow(a, -p, p) are equal to eachother, but a common mistake is to take pow(a, p-1, p) instead. For a composite modulus things get much trickier still, as the exponent is then reduced in terms of the Euler phi function.

And, even if you are not confused by these things, it's still a bit subtle that you have to use pow(a, -1,p) instead of pow(a, p-2, p) to let the modular inverse be computed efficiently. With modinv() available separately, one would expect --and get-- an efficient implementation with minimal overhead (e.g., not implemented via a complete extended-gcd).
msg335956 - (view) Author: Mark Dickinson (mark.dickinson) * Date: 2019-02-19 15:32
> it's still a bit subtle that you have to use pow(a, -1,p) instead of pow(a, p-2, p) to let the modular inverse be computed efficiently

That's not 100% clear: the binary powering algorithm used to compute `pow(a, p-2, p)` is fairly efficient; the extended gcd algorithm used to compute the inverse may or may not end up being comparable. I certainly wouldn't be surprised to see `pow(a, p-2, p)` beat a pure Python xgcd for computing the inverse.
msg335970 - (view) Author: Berry Schoenmakers (lschoe) * Date: 2019-02-19 16:32
> ... to see `pow(a, p-2, p)` beat a pure Python xgcd for computing the inverse.

OK, I'm indeed assuming that modinv() is implemented efficiently, in CPython, like pow() is. Then, it should be considerably faster, maybe like this:

>>> timeit.timeit("gmpy2.invert(1023,p)", "import gmpy2; p=2**61-1")
0.18928535383349754
>>> timeit.timeit("gmpy2.invert(1023,p)", "import gmpy2; p=2**127-1")
0.290736872836419
>>> timeit.timeit("gmpy2.invert(1023,p)", "import gmpy2; p=2**521-1")
0.33174844290715555
>>> timeit.timeit("gmpy2.powmod(1023,p-2,p)", "import gmpy2; p=2**61-1")
0.8771009990597349
>>> timeit.timeit("gmpy2.powmod(1023,p-2,p)", "import gmpy2; p=2**127-1")
3.460449585430979
>>> timeit.timeit("gmpy2.powmod(1023,p-2,p)", "import gmpy2; p=2**521-1")
84.38728888797652
msg335976 - (view) Author: Mark Dickinson (mark.dickinson) * Date: 2019-02-19 16:49
> Then, it should be considerably faster

Why would you expect that? Both algorithms involve a number of (bigint) operations that's proportional to log(p), so it's going to be down to the constants involved and the running times of the individual operations. Is there a clear reason for your expectation that the xgcd-based algorithm should be faster?

Remember that Python has a subquadratic multiplication (via Karatsuba), but its division algorithm has quadratic running time.
msg335987 - (view) Author: Berry Schoenmakers (lschoe) * Date: 2019-02-19 17:24
> 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.
msg335994 - (view) Author: Raymond Hettinger (rhettinger) * Date: 2019-02-19 18:27
> +1 for the pow(value, -1, modulus) spelling. It should raise
> `ValueError` if `value` and `modulus` are not relatively prime.

> It would feel odd to me _not_ to extend this to
> `pow(value, n, modulus)` for all negative `n`, again
> valid only only if `value` is relatively prime to `modulus`.

I'll work up a PR using the simplest implementation.  Once that's in with tests and docs, it's fair game for someone to propose algorithmic optimizations.
msg336012 - (view) Author: Tim Peters (tim.peters) * Date: 2019-02-19 19:59
Raymond, I doubt we can do better (faster) than straightforward egcd without heroic effort anyway.  We can't even know whether a modular inverse exists without checking whether the gcd is 1, and egcd builds on what we have to do for the latter anyway.  Even if we did know in advance that a modular inverse exists, using modular exponentiation to find it requires knowing the totient of the modulus, and computing the totient is believed to be no easier than factoring.

The only "optimization" I'd be inclined to _try_ for Python's use is an extended binary gcd algorithm (which requires no bigint multiplies or divides, the latter of which is especially sluggish in Python):

https://www.ucl.ac.uk/~ucahcjm/combopt/ext_gcd_python_programs.pdf

For the rest:

- I'd also prefer than negative exponents other than -1 be supported.  It's just that -1 by itself gets 95% of the value.

- It's fine by me if `pow(a, -1, m)` is THE way to spell modular inverse.  Adding a distinct `modinv()` function too strikes me as redundnt clutter, but not damaging enough to be worth whining about.  So -0 on that.
msg336028 - (view) Author: Raymond Hettinger (rhettinger) * Date: 2019-02-20 00:02
Changing the title to reflect a focus on building-out pow() instead of a function in the math module.
msg336130 - (view) Author: Berry Schoenmakers (lschoe) * Date: 2019-02-20 17:55
In pure Python this seems to be the better option to compute inverses:

def modinv(a, m):  # assuming m > 0
b = m
s, s1 = 1, 0
while b:
a, (q, b) = b, divmod(a, b)
s, s1 = s1, s - q * s1
if a != 1:
raise ValueError('inverse does not exist')
return s if s >= 0 else s + m

Binary xgcd algorithms coded in pure Python run much slower.
msg344162 - (view) Author: Mark Dickinson (mark.dickinson) * Date: 2019-06-01 09:45
I think GH-13266 is ready to go, but I'd appreciate a second pair of eyes on it if anyone has time.
msg344260 - (view) Author: Mark Dickinson (mark.dickinson) * Date: 2019-06-02 09:24
New changeset c52996785a45d4693857ea219e040777a14584f8 by Mark Dickinson in branch 'master':
bpo-36027: Extend three-argument pow to negative second argument (GH-13266)
https://github.com/python/cpython/commit/c52996785a45d4693857ea219e040777a14584f8
msg344261 - (view) Author: Mark Dickinson (mark.dickinson) * Date: 2019-06-02 09:25
Done. Closing.
msg344271 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * Date: 2019-06-02 11:01
PR 13266 introduced a compiler warning.

Objects/longobject.c: In function ‘long_invmod’:
Objects/longobject.c:4246:25: warning: passing argument 2 of ‘long_compare’ from incompatible pointer type [-Wincompatible-pointer-types]
if (long_compare(a, _PyLong_One)) {
^~~~~~~~~~~
Objects/longobject.c:3057:1: note: expected ‘PyLongObject * {aka struct _longobject *}’ but argument is of type ‘PyObject * {aka struct _object *}’
long_compare(PyLongObject *a, PyLongObject *b)
^~~~~~~~~~~~
msg344330 - (view) Author: Petr Viktorin (petr.viktorin) * Date: 2019-06-02 22:34
I will fix the compiler warning along with another one that I just introduced.
msg344334 - (view) Author: Petr Viktorin (petr.viktorin) * Date: 2019-06-02 23:08
New changeset e584cbff1ea78e700cf9943d50467e3b58301ccc by Petr Viktorin in branch 'master':
bpo-36027 bpo-36974: Fix "incompatible pointer type" compiler warnings (GH-13758)
https://github.com/python/cpython/commit/e584cbff1ea78e700cf9943d50467e3b58301ccc
msg344345 - (view) Author: Petr Viktorin (petr.viktorin) * Date: 2019-06-03 00:28
New changeset 1e375c6269e9de4f3d05d4aa6d6d74e00f522d63 by Petr Viktorin in branch 'master':
bpo-36027: Really fix "incompatible pointer type" compiler warning (GH-13761)
https://github.com/python/cpython/commit/1e375c6269e9de4f3d05d4aa6d6d74e00f522d63
msg344454 - (view) Author: Mark Dickinson (mark.dickinson) * Date: 2019-06-03 17:27
@Petr: Thanks for the quick fix!
msg362505 - (view) Author: Mark Dickinson (mark.dickinson) * Date: 2020-02-23 12:16
msg368821 - (view) Author: paul rubin (phr) Date: 2020-05-14 10:15
https://bugs.python.org/issue457066  The old is new again ;-).
History
Date User Action Args
2021-06-13 07:36:22miss-islingtonsetnosy: + miss-islington

pull_requests: + pull_request25288
2021-06-13 07:35:45mark.dickinsonsetpull_requests: + pull_request25287
2021-06-12 08:22:41mark.dickinsonsetpull_requests: + pull_request25277
2020-05-14 10:15:56phrsetnosy: + phr
messages: + msg368821
2020-02-23 12:16:45mark.dickinsonsetmessages: + msg362505
2019-06-03 17:27:29mark.dickinsonsetmessages: + msg344454
2019-06-03 00:28:32petr.viktorinsetmessages: + msg344345
2019-06-03 00:00:33petr.viktorinsetpull_requests: + pull_request13646
2019-06-02 23:08:17petr.viktorinsetmessages: + msg344334
2019-06-02 22:39:01petr.viktorinsetpull_requests: + pull_request13639
2019-06-02 22:34:38petr.viktorinsetnosy: + petr.viktorin
messages: + msg344330
2019-06-02 11:01:47serhiy.storchakasetnosy: + serhiy.storchaka
messages: + msg344271
2019-06-02 09:25:02mark.dickinsonsetstatus: open -> closed
resolution: fixed
messages: + msg344261

stage: patch review -> resolved
2019-06-02 09:24:09mark.dickinsonsetmessages: + msg344260
2019-06-01 09:45:07mark.dickinsonsetmessages: + msg344162
2019-05-12 17:58:32mark.dickinsonsetassignee: mark.dickinson
2019-05-12 17:57:36mark.dickinsonsetkeywords: + patch
stage: patch review
pull_requests: + pull_request13174
2019-02-20 17:55:19lschoesetmessages: + msg336130
2019-02-20 00:02:14rhettingersetmessages: + msg336028
title: Consider adding modular multiplicative inverse to the math module -> Support negative exponents in pow() where a modulus is specified.