一些高效代码片断

/* 来至 Quake 3 的源码 */
float CarmSqrt(float x){
        union{
                int intPart;
                float floatPart;
        } convertor;
        union{
                int intPart;
                float floatPart;
        } convertor2;
        convertor.floatPart = x;
        convertor2.floatPart = x;
        convertor.intPart = 0x1FBCF800 + (convertor.intPart >> 1);
        convertor2.intPart = 0x5f3759df - (convertor2.intPart >> 1);
        return 0.5f*(convertor.floatPart + (x * convertor2.floatPart));
}


unsigned long hash(const char *name,size_t len)
{
        unsigned long h=(unsigned long)len;
        size_t step = (len>>5)+1;
        for (size_t i=len; i>=step; i-=step)
            h = h ^ ((h<<5)+(h>>2)+(unsigned long)name[i-1]);
        return h;
}


union luai_Cast { double l_d; long l_l; };
#define lua_number2int(i,d) \
        { volatile union luai_Cast u; u.l_d = (d) + 6755399441055744.0; (i) = u.l_l; }

unsigned short alpha_blender(unsigned int x,unsigned int y,unsigned int alpha)
{
        x = (x | (x<<16)) & 0x7E0F81F;
        y = (y | (y<<16)) & 0x7E0F81F;
        unsigned int result = ((x - y) * alpha / 32 + y) & 0x7E0F81F;
        return (unsigned short)((result&0xFFFF) | (result>>16));
}


int UTF8toUTF16(int c)
{
        signed char *t=(signed char*)&c;
        int ret=*t &(0x0f | ((*t>>1) &0x1f) | ~(*t>>7));
        int i;
        assert ((*t & 0xc0) != 0x80);
        for (i=1;i<3;i++) {
                if ((t & 0xc0)!=0x80) {
                        break;
                }
                ret=ret<<6 | (t & 0x3f);
        }
        return ret;
}