[C] Move a range of bits to a new range in a number

Description:

此function可以將某數某範圍的bits移到另一個範圍

void move_bits(int *a, int l, int new_l, int num){
    if(num == 0)
        return;
    //create get_mask
    int get_mask = 0;
    int count = num;
    //create get mask
    while(count){
        get_mask = (get_mask << 1) + 1;
        count--;
    }
    //cover get_mask on a and move to target range and become set_mask
    get_mask = get_mask << l;
    int set_mask = get_mask & (*a);
    if(l > new_l)
        set_mask = set_mask >> l - new_l;
    else
        set_mask = set_mask << new_l - l;
    //create erase mask
    int erase_mask = 0;
    count = num;
    while(count){
        erase_mask = (erase_mask << 1) + 1;
        count--;
    }
    erase_mask = ~(erase_mask << new_l);
    //cover erase mask to a to erase 1234 bits to 0
    *a = erase_mask & (*a);
    //cover set_mask to a
    *a = set_mask | (*a);
}

若是要移到另一數的某範圍

void move_bits(int *a, int *b, int l, int new_l, int num){
    if(num == 0)
        return;
    //create get_mask
    int get_mask = 0;
    int count = num;
    //create get mask
    while(count){
        get_mask = (get_mask << 1) + 1;
        count--;
    }
    //cover get_mask on a and move to target range
    get_mask = get_mask << l;
    int set_mask = get_mask & (*a);
    if(l > new_l)
        set_mask = set_mask >> l - new_l;
    else
        set_mask = set_mask << new_l - l;
    //create erase mask
    int erase_mask = 0;
    count = num;
    while(count){
        erase_mask = (erase_mask << 1) + 1;
        count--;
    }
    erase_mask = ~(erase_mask << new_l);
    //cover erase mask to a to erase 1234 bits to 0
    *b = erase_mask & (*b);
    printf("%x %x\n", erase_mask, set_mask);
    //cover set_mask to a
    *b = set_mask | (*b);
}

Explanation:

先用get_mask取出舊範圍的bits，再將他移到新的範圍變成set_mask，接著用erase_mask將新範圍的bits清除成0，最後再用set_mask將bits複製過去到新的範圍。