Task: Prime decomposition
C: #include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>

typedef uint32_t pint;
typedef uint64_t xint;
typedef unsigned int uint;
#define PRIuPINT PRIu32     
#define PRIuXINT PRIu64     
#define MAX_FACTORS 63      

uint8_t *pbits;

#define MAX_PRIME (~(pint)0)
#define MAX_PRIME_SQ 65535U
#define PBITS (MAX_PRIME / 30 + 1)

pint next_prime(pint);
int is_prime(xint);
void sieve(pint);

uint8_t bit_pos[30] = {
    0, 1<<0, 0, 0, 0,    0,
    0, 1<<1, 0, 0, 0, 1<<2,
    0, 1<<3, 0, 0, 0, 1<<4,
    0, 1<<5, 0, 0, 0, 1<<6,
    0,    0, 0, 0, 0, 1<<7,
};

uint8_t rem_num[] = { 1, 7, 11, 13, 17, 19, 23, 29 };

void init_primes()
{
    FILE *fp;
    pint s, tgt = 4;

    if (!(pbits = malloc(PBITS))) {
        perror("malloc");
        exit(1);
    }

    if ((fp = fopen("primebits", "r"))) {
        fread(pbits, 1, PBITS, fp);
        fclose(fp);
        return;
    }

    memset(pbits, 255, PBITS);
    for (s = 7; s <= MAX_PRIME_SQ; s = next_prime(s)) {
        if (s > tgt) {
            tgt *= 2;
            fprintf(stderr, "sieve %"PRIuPINT"\n", s);
        }
        sieve(s);
    }
    fp = fopen("primebits", "w");
    fwrite(pbits, 1, PBITS, fp);
    fclose(fp);
}

int is_prime(xint x)
{
    pint p;
    if (x > 5) {
        if (x < MAX_PRIME)
            return pbits[x/30] & bit_pos[x % 30];

        for (p = 2; p && (xint)p * p <= x; p = next_prime(p))
            if (x % p == 0) return 0;

        return 1;
    }
    return x == 2 || x == 3 || x == 5;
}

void sieve(pint p)
{
    unsigned char b[8];
    off_t ofs[8];
    int i, q;

    for (i = 0; i < 8; i++) {
        q = rem_num[i] * p;
        b[i] = ~bit_pos[q % 30];
        ofs[i] = q / 30;
    }

    for (q = ofs[1], i = 7; i; i--)
        ofs[i] -= ofs[i-1];

    for (ofs[0] = p, i = 1; i < 8; i++)
        ofs[0] -= ofs[i];

    for (i = 1; q < PBITS; q += ofs[i = (i + 1) & 7])
        pbits[q] &= b[i];
}

pint next_prime(pint p)
{
    off_t addr;
    uint8_t bits, rem;

    if (p > 5) {
        addr = p / 30;
        bits = bit_pos[ p % 30 ] << 1;
        for (rem = 0; (1 << rem) < bits; rem++);
        while (pbits[addr] < bits || !bits) {
            if (++addr >= PBITS) return 0;
            bits = 1;
            rem = 0;
        }
        if (addr >= PBITS) return 0;
        while (!(pbits[addr] & bits)) {
            rem++;
            bits <<= 1;
        }
        return p = addr * 30 + rem_num[rem];
    }

    switch(p) {
        case 2: return 3;
        case 3: return 5;
        case 5: return 7;
    }
    return 2;
}

int decompose(xint n, xint *f)
{
    pint p = 0;
    int i = 0;

    
    if (n <= MAX_PRIME && is_prime(n)) {
        f[0] = n;
        return 1;
    }

    while (n >= (xint)p * p) {
        if (!(p = next_prime(p))) break;
        while (n % p == 0) {
            n /= p;
            f[i++] = p;
        }
    }
    if (n > 1) f[i++] = n;
    return i;
}

int main()
{
    int i, len;
    pint p = 0;
    xint f[MAX_FACTORS], po;

    init_primes();

    for (p = 1; p < 64; p++) {
        po = (1LLU << p) - 1;
        printf("2^%"PRIuPINT" - 1 = %"PRIuXINT, p, po);
        fflush(stdout);
        if ((len = decompose(po, f)) > 1)
            for (i = 0; i < len; i++)
                printf(" %c %"PRIuXINT, i?'x':'=', f[i]);
        putchar('\n');
    }

    return 0;
}

Java: public boolean prime(BigInteger i);