2的最小整数对数

short log2US16(unsigned short a)
{
    short zero = 0;

    if (a == 0) return 0;

    if (!(0xFF00 & (a << zeros))) zero += 8;
    if (!(0xF000 & (a << zeros))) zero += 4;
    if (!(0xC000 & (a << zeros))) zero += 2;
    if (!(0x8000 & (a << zeros))) zero += 1;

    return (15 - zero); 
}