Much faster implementation of Kaprekar's function

kaprekar.c

/*
 *  Attempt to find fixed points of a particularly interesting function.
 *
 *  Kaprekar's function sorts the digits of n ascending and descending, then
 *  returns the descending minus the ascending. The fixed points of this
 *  function, as well as the attractor classes are very interesting.
 */

#include<stdio.h>
#include<limits.h>

void kaprekar(unsigned int, unsigned int *);
void reverse(unsigned int, unsigned int *);
void sort_digits(unsigned int, unsigned int *);

int main(int argc, char *argv[]) {
    for (unsigned int i = 0; i <= UINT_MAX/10; i++) {
        printf("%d", i);
        unsigned int k;
        kaprekar(i, &k);
        if (i == k)
            printf("\n"); // Newline to leave fixedpoint in console output
        else
            printf("\x1B[G"); // Return to beginning of line to overwrite
    }
    return 0;
}

void kaprekar(unsigned int x, unsigned int *result) {
    unsigned int ascending, descending;
    sort_digits(x, &descending);
    reverse(descending, &ascending);
    *result = descending-ascending;
}

void reverse(unsigned int x, unsigned int *result) {
    *result = 0;
    while (x > 0) {
        *result = *result*10 + x%10;
        x /= 10;
    }
}

void sort_digits(unsigned int x, unsigned int *result) {
    unsigned int mask[10] = {1, 1, 1, 1, 1, 1, 1, 1, 1, 1};
    *result = 0;
    while (x > 0) {
        unsigned int d = x%10;
        unsigned int rem = *result % mask[d];
        *result = (*result - rem) * 10 + d*mask[d] + rem;

        x /= 10; // Pop digit
        for (int i = d+1; i < 10; i++) { // increment masks after inserted digit
            mask[i] *= 10;
        }
    }
}