Compute modular inverse

Given two positive numbers $$\x\$$ and $$\n\$$ with $$\x<2^n\$$, write the shortest possible function to compute $$\x^{-1} \mod 2^n\$$. In other words, find $$\y\$$ such that $$\xy=1 \mod 2^n\$$.

Your function must complete in a reasonable time for at least $$\n=64\$$, so exhaustive search will not work.

If the inverse does not exist, you must indicate that to the caller somehow (throw an exception, return a sentinel value, etc).

If you're wondering where to start, try the Extended Euclidean Algorithm.

• this is going to be a single statement in some math softwares Jan 29 '11 at 23:45
• @st0le: Right, and you wouldn't be allowed to use such a function in such systems. :-D Jan 30 '11 at 0:20
• Exponentiation has higher precedence than modulo, correct? Mar 4 '20 at 22:08

Python, 29 bytes

lambda x,n:pow(x,2**n-1,2**n)

This returns 0 for even x. It uses Euler’s theorem, with the observation that 2^n − 1 is divisible by 2^(n − 1) − 1, via Python’s builtin fast modular exponentiation. This is plenty fast enough for n up to 7000 or so, where it starts taking more than about a second.

Python 3.8 (pre-release), 25 bytes

lambda a,b:pow(a,-1,2**b)

Try it online!

Mathematica - 22

f=PowerMod[#,-1,2^#2]&

f[x,n] returns y with x*y=1 mod 2^n, otherwise x is not invertible modulo 2^n

GolfScript (23 chars)

{:^((1${\.**2^?%}+*}:f; The sentinel result for a non-existent inverse is 0. This is a simple application of Euler's theorem. $x^{\varphi(2^n)} \equiv 1 \pmod {2^n}$, so $x^{-1} \equiv x^{2^{n-1}-1} \pmod {2^n}$ Unfortunately that's rather too big an exponential to compute directly, so we have to use a loop and do modular reduction inside the loop. The iterative step is $x^{2^k-1} = \left(x^{2^{k-1}-1}\right)^2 \times x$ and we have a choice of base case: either k=1 with {1\:^(@{\.**2^?%}+*}:f; or k=2 with {:^((1${\.**2^?%}+*}:f;

I'm working on another approach, but the sentinel is more difficult.

The key observation is that we can build the inverse up bit by bit: if $xy \equiv 1 \pmod{2^{k-1}}$ then $xy \in \{ 1, 1 + 2^{k-1} \} \pmod{2^k}$, and if $x$ is odd we have $x(y + xy-1) \equiv 1 \pmod{2^k}$. (If you're not convinced, check the two cases separately). So we can start at any suitable base case and apply the transformation $y' = (x+1)y - 1$ a suitable number of times.

Since $0x \equiv 1 \pmod {2^0}$ we get, by induction

$x\left(\frac{1 - (x+1)^n}{x}\right) \equiv 1 \pmod {2^n}$

where the inverse is the sum of a geometric sequence. I've shown the derivation to avoid the rabbit-out-of-a-hat effect: given this expression, it's easy to see that (given that the bracketed value is an integer, which follows from its derivation as a sum of an integer sequence) the product on the left must be in the right equivalence class if $x+1$ is even.

That gives the 19-char function

{1$)1$?@/~)2@?%}:f;

which gives correct answers for inputs which have an inverse. However, it's not so simple when $x$ is even. One potentially interesting option I've found is to add x&1 rather than 1.

{1$.1&+1$?@/~)2@?%}:f;

This seems to give sentinel values of either $0$ or $2^{n-1}$, but I haven't yet proved that.

Taking that one step further, we can ensure a sentinel of $0$ for even numbers by changing the expression $1 - (x+1)^n$ into $1 - 1^n$:

{1$.1&*)1$?@/~)2@?%}:f;

That ties with the direct application of Euler's theorem for code length, but is going to have worse performance for large $n$. If we take the arguments the other way round, as n x f, we can save one character and get to 22 chars:

{..1&*)2\$?\/~)2@?%}:f;

Python 95 89

c is your function. Returns 0 if there is no inverse (i.e. when x is even).

p=lambda x,y,m:y and p(x,y/2,m)**2*x**(y&1)%m or 1
c=lambda x,n:[0,p(x,2**n-1,2**n)][x%2]

Ruby - 88 characters

Use the function f.

def e a,b;a%b==0?[0,1]:(x,y=e(b,a%b);[y,x-(y*(a/b))])end
def f x,n;e(x,2**n)*(x%2)end

Simply the recursive function from the linked wiki page, returns 0 on error.

• You can save some characters by inlining e: (e=->a,b{...})[x,2**n]. Can also save a character by testing a%b<1 instead of a%b==0. Jun 12 '14 at 14:19

_!1=1
x!n|r<-x!div(n+1)2=(2-r*x)*rmod2^n

Using an algorithm based on Hensel’s lemma that doubles the number of digits in every iteration, this runs in under a second for n up to about 30 million!

Pyth, 9 bytes

.^Et^2Q^2

Try it here!

Takes input in reverse order. Or, 9 bytes too: .^EtK^2QK.

Explanation

.^Et^2Q^2  - Full program.

.^         - Pow function. The same in Python (pow).
E        - The second input.
^2Q    - And 2 ^ first input.
t       - Decremented.
^2  - And 2 ^ first input again.

Jelly, 5 bytes

2*æi@

Try it online!

Takes input with $$\n\$$ first and $$\x\$$ second, returns $$\0\$$ if no such inverse exists. The Footer in the TIO link reverses the input for you and formats the test cases.

How it works

2*æi@ - Main link. Takes n on the left and x on the right
2*    - Yield 2*n
æi@ - Modular inverse of x, modulo 2*n, or if none exists, 0.

GAP, 39 bytes

f:=function(x,n)return 1/x mod 2^n;end;

f(x,n) returns the inverse of x modulo 2^n and gives an error message

Error, ModRat: for <r>/<s> mod <n>, <s>/gcd(<r>,<s>) and <n> must be coprime

if no inverse exists.

GolfScript, 10 9 bytes

2\?:q(?q%

Try it online!

A port of Mr. XCoder's solution, to GS. Stack manipulation is a little messy, could probably be done with a little tomfoolery. Takes in input as X N.

V 1.1 : Improved stack manip, it sucks a lot less now.

2\?:q(?q% #Multiplicative Inverse
2\        #Move "2" to the second pos on the stack. X 2 N
?       #Exponentiate. X 2^N
:q     #Remember, q is 2^N.
(    #Decrement. X 2^N-1
?   #Exponentiate. X^(2^N-1)
q? #Mod by 2^N.

# X^([2^N] -1) % 2^N is the multiplicative inverse, since
# X^([2^N] -1)*X = X^(2^N)
# which is 1 mod 2^N if X and 2^N are coprime.

If there is no M.I., it will return 0.

Python 3, 77 57 bytes

f=lambda x,n,b=1,i=1:n and f(x,n-1,b-(b&i)*~-x,i+i)or b%i

Outgolfed all you snek users (except for the pow(x, -1, 2**n) cheapshots) with nothing but pure algorithm.

Try it online! (Only uses Python 3.8 for comparison against pow(a,-1,2**n))

Ungolfed version:

def func(value, shift, acc = 1, mask = 1):
if value != 0:
return func(value, shift - 1, acc - (acc & mask) * (value - 1), mask << 1)
else:
return acc & (mask - 1)

Or, as a loop with size hacks removed:

def func(value, exponent):
value -= 1
acc = 1
for i in range(exponent):
if acc & (1 << i):
acc -= value << i
return acc & ((1 << i) - 1)

Explanation

The function is a lambda called f. It takes two positive integers, and returns either the multiplicative modular inverse or zero.

You may be wondering what the heck is going on. This uses a different, faster, and smaller algorithm for modular inverse for powers of 2 instead of the Euclidian approach.

I'm not going to go too far into the details of why it works, as it is really complex and explained better by others. Translation: even I don't fully understand it.

This set of algorithms is explained here on algassert with a related algorithm explained here on Crypto SE with some interesting papers linked.

This algorithm actually uses no true multiplication (just shifts), the multiplication is just a shortcut for size.

This naturally returns 0 for even numbers, as the odd subtraction (we start with x - 1) causes a chain reaction of trailing with 0b0, which, when masked off, turns the result to zero. No need for a manual sentinel.

0000 0001 - 0000 1101 -> 1111 0010
1111 0010 - 0001 1010 -> 1110 0000