Fast Iterative GCD - algorithm

Question

I have GCD(n, i) where i=1 is increasing in loop by 1 up to n. Is there any algorithm which calculate all GCD's faster than naive increasing and compute GCD using Euclidean algorithm?
PS I've noticed if n is prime I can assume that number from 1 to n-1 would give 1, because prime number would be co-prime to them. Any ideas for other numbers than prime?

Answer 1

C++ implementation, works in O(n * log log n) (assuming size of integers are O(1)):
#include <cstdio>
#include <cstring>
using namespace std;
void find_gcd(int n, int *gcd) {
// divisor[x] - any prime divisor of x
// or 0 if x == 1 or x is prime
int *divisor = new int[n + 1];
memset(divisor, 0, (n + 1) * sizeof(int));
// This is almost copypaste of sieve of Eratosthenes, but instead of
// just marking number as 'non-prime' we remeber its divisor.
// O(n * log log n)
for (int x = 2; x * x <= n; ++x) {
if (divisor[x] == 0) {
for (int y = x * x; y <= n; y += x) {
divisor[y] = x;
}
}
}
for (int x = 1; x <= n; ++x) {
if (n % x == 0) gcd[x] = x;
else if (divisor[x] == 0) gcd[x] = 1; // x is prime, and does not divide n (previous line)
else {
int a = x / divisor[x], p = divisor[x]; // x == a * p
// gcd(a * p, n) = gcd(a, n) * gcd(p, n / gcd(a, n))
// gcd(p, n / gcd(a, n)) == 1 or p
gcd[x] = gcd[a];
if ((n / gcd[a]) % p == 0) gcd[x] *= p;
}
}
}
int main() {
int n;
scanf("%d", &n);
int *gcd = new int[n + 1];
find_gcd(n, gcd);
for (int x = 1; x <= n; ++x) {
printf("%d:\t%d\n", x, gcd[x]);
}
return 0;
}

Answer 2

SUMMARY
The possible answers for the gcd consist of the factors of n.
You can compute these efficiently as follows.
ALGORITHM
First factorise n into a product of prime factors, i.e. n=p1^n1*p2^n2*..*pk^nk.
Then you can loop over all factors of n and for each factor of n set the contents of the GCD array at that position to the factor.
If you make sure that the factors are done in a sensible order (e.g. sorted) you should find that the array entries that are written multiple times will end up being written with the highest value (which will be the gcd).
CODE
Here is some Python code to do this for the number 1400=2^3*5^2*7:
prime_factors=[2,5,7]
prime_counts=[3,2,1]
N=1
for prime,count in zip(prime_factors,prime_counts):
N *= prime**count
GCD = [0]*(N+1)
GCD[0] = N
def go(i,n):
"""Try all counts for prime[i]"""
if i==len(prime_factors):
for x in xrange(n,N+1,n):
GCD[x]=n
return
n2=n
for c in xrange(prime_counts[i]+1):
go(i+1,n2)
n2*=prime_factors[i]
go(0,1)
print N,GCD

Answer 3

Binary GCD algorithm:
https://en.wikipedia.org/wiki/Binary_GCD_algorithm
is faster than Euclidean algorithm:
https://en.wikipedia.org/wiki/Euclidean_algorithm
I implemented "gcd()" in C for type "__uint128_t" (with gcc on Intel i7 Ubuntu), based on iterative Rust version:
https://en.wikipedia.org/wiki/Binary_GCD_algorithm#Iterative_version_in_Rust
Determining number of trailing 0s was done efficiently with "__builtin_ctzll()". I did benchmark 1 million loops of two biggest 128bit Fibonacci numbers (they result in maximal number of iterations) against gmplib "mpz_gcd()" and saw 10% slowdown. Utilizing the fact that u/v values only decrease, I switched to 64bit special case "_gcd()" when "<=UINT64_max" and now see speedup of 1.31 over gmplib, for details see:
https://www.raspberrypi.org/forums/viewtopic.php?f=33&t=311893&p=1873552#p1873552
inline int ctz(__uint128_t u)
{
unsigned long long h = u;
return (h!=0) ? __builtin_ctzll( h )
: 64 + __builtin_ctzll( u>>64 );
}
unsigned long long _gcd(unsigned long long u, unsigned long long v)
{
for(;;) {
if (u > v) { unsigned long long a=u; u=v; v=a; }
v -= u;
if (v == 0) return u;
v >>= __builtin_ctzll(v);
}
}
__uint128_t gcd(__uint128_t u, __uint128_t v)
{
if (u == 0) { return v; }
else if (v == 0) { return u; }
int i = ctz(u); u >>= i;
int j = ctz(v); v >>= j;
int k = (i < j) ? i : j;
for(;;) {
if (u > v) { __uint128_t a=u; u=v; v=a; }
if (v <= UINT64_MAX) return _gcd(u, v) << k;
v -= u;
if (v == 0) return u << k;
v >>= ctz(v);
}
}