Preliminary mp_int -> double conversion

mp_get_double.c

#include "tommath.h"
#include <stdlib.h>
#include <math.h>

double mp_get_double(mp_int *a) {
    double d = 0.0;
    if (USED(a) == 0)
        return d;
    if (USED(a) == 1)
        return SIGN(a) == MP_NEG ? (double) -mp_get_int(a) : (double) mp_get_int(a);

    int i;
    for (i = USED(a) - 1; DIGIT(a, i) == 0 && i > 0; i--) {
        /* do nothing */
    }
    d = (double) DIGIT(a, i);
    if (SIGN(a) == MP_NEG)
        d *= -1;
    i--;
    if (i == -1) {
        return d;
    }
    d *= pow(2.0, DIGIT_BIT);
    d += (double) DIGIT(a, i);

    d *= pow(2.0, DIGIT_BIT * i);
    return d;
}

int main(void) {
    mp_int *b;
    mp_init(b);
    mp_set_int(b, 12);
    mp_mul_2d(b, 130, b);

    double d = mp_get_double(b);
    printf("%.10e\n", d);
}