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 #include <stdio.h>
 #include <stdint.h>
 #include <stdlib.h>
 
 /* http://marc-b-reynolds.github.io/math/2017/09/18/ModInverse.html */
 static uint32_t modinv(uint32_t a)
 {
 	uint32_t x;
 	x = a;		//  3 bits
 	x *= 2 - a*x;	//  6
 	x *= 2 - a*x;	// 12
 	x *= 2 - a*x;	// 24
 	x *= 2 - a*x;	// 48 / retaining bottom 32
 	return x;
 }
 
 /* http://number-none.com/blow/blog/programming/2016/07/08/fabian-on-lcg-fast-forward.html */
 uint32_t skip(uint32_t x, uint32_t a, uint32_t c, unsigned n)
 {
 	while (n) {
     		if (n & 1)
         		x = a * x + c;
 		c *= a + 1;
 		a *= a;
 		n >>= 1;
 	}
 	return x;
 }
 
 int main(int argc, char **argv)
 {
 	srandom(time(NULL));
 	unsigned i, s = 1, a = random() | 1UL, c =  random() | 1UL;
 
 	/* move forward one step at a time */
 	for (i = 0; i < 100; i++)
 		s = s * a + c;
 	printf("%08x\n", s);
 	/* skip forwaward */
 	printf("%08x\n", skip(1, a, c, 100));
 	/* you could loop through the full LCG cycle */
 	printf("%08x\n", skip(s, a, c, -100));
 	/* or inverse of LCG is still LCG */
 	a = modinv(a);
 	c = -c * a;
 	/* skip backward */
 	printf("%08x\n", skip(s, a, c, 100));
 	/* move backward one step at a time */
 	for (i = 0; i < 100; i++)
 		s = s * a + c;
 	printf("%08x\n", s);
 }