# 2019q1W7上:方鈺學
contributed <`JulianATA`>

>有時間會回頭補詳細解釋！
>[name=Julian Fang]
> 好啊，我等你
> [name=jserv]

## 測驗 `1` :
### `find_next_bit()` :

static inline size_t find_next_bit(const unsigned long *bitmap,
                                   size_t bits,
                                   size_t start)
{
    unsigned long t;
    size_t l = BITS_TO_LONGS(bits);
    size_t first_long = start / BITS_PER_LONG;
    size_t long_lower = start - (start % BITS_PER_LONG);

    if (start >= bits)
        return bits;

    t = bitmap[first_long] & BITMAP_FIRST_WORD_MASK(start);
    for (size_t i = first_long + 1; !t && i < l; i++) {
       long_lower += BITS_PER_LONG;
       t = bitmap[i];
       t ^= 0UL;
    }

    if (!t)
        return bits;

    size_t pos = long_lower + bitops_ffs(t) - 1;
    if(pos >= bits)
        return bits;
    return pos;
}

## 測驗 `2` :
### `main` :

int main(void) {
    // sequential check
    for (int i = 1; i < MAX_TEST_BITS; i++) {
        bitmap_fill(test, i);
        assert(find_next_bit(test, i, 0) == 0);

        bitmap_zero(test, i);
        assert(find_next_bit(test, i, 0) == i);
    }
    // random check
    srand(time(NULL));
    int _t;
    for (int i = 1; i < 1000; i++){
        _t = rand()%MAX_TEST_BITS;
        bitmap_fill(test,_t);
        assert(find_next_bit(test, _t, 0) == 0);

        bitmap_zero(test, _t);
        assert(find_next_bit(test, _t, 0) == _t);
    }
    return 0;
}

## 測驗 `3` :
### `bitsops_ffs()` :

static inline size_t bitops_ffs(unsigned long x) {
    return log2(x & -x) +1;
}

## 測驗 `4` :
### `bitsops_ffz()` :

#include <math.h>
#include <stdio.h>


/**
 * bitops_ffz() - find (least significant) first zero bit plus one
 * @x: unsigned long to check
 *
 * Return: plus-one index of first zero bit; zero when x is ULONG_MAX
 */
static inline size_t bitops_ffz(unsigned long x) {
    return log((-x & (x+1)))+1;
}

int main()
{
    unsigned long n = 31;
    printf("%zu", bitops_ffz(n));
    return 0;
}

:::warning
為何需要用到 `log` 呢？
:::

###### tags: `Cprogramming` `LinuxKernel`