# clz contributed by <`Sisker1111`> ###### tags: `進階電腦系統理論與實作` ## CLZ實作: ```cpp= int clz(uint32_t x) { int n = 32, c = 16; do { uint32_t y = x >> c; if (y) { n -= c; x = y; } c >>= 1; } while (c); return (n - x); } ``` ## CTZ實作: CTZ 可以通過把 x 的每個 Bits 反轉，在實施 CLZ. ```cpp= /// reverse uint32_t reverse(uint32_t x){ uint32_t n = x; n = ((n & 0xffff0000) >> 16) | ((n & 0x0000ffff) << 16); n = ((n & 0xff00ff00) >> 8) | ((n & 0x00ff00ff) << 8); n = ((n & 0xf0f0f0f0) >> 4) | ((n & 0x0f0f0f0f) << 4); n = ((n & 0xcccccccc) >> 2) | ((n & 0x33333333) << 2); n = ((n & 0xaaaaaaaa) >> 1) | ((n & 0x55555555) << 1); return n; } /// CTZ int ctz(uint32_t x) { x = reverse(x); int n = 32, c = 16; do { uint32_t y = x >> c; if (y) { n -= c; x = y; } c >>= 1; } while (c); return (n - x); } ``` 或者使用以下方式: ```cpp= uint32_t ctz(uint32_t x) { if(x==0) return 0; int n=0; if(!(x&0x0000FFFF)) { n+=16; x>>=16;} if(!(x&0x000000FF)) { n+=8; x>>=8;} if(!(x&0x0000000F)) { n+=4; x>>=4;} if(!(x&0x00000003)) { n+=2; x>>=2;} if(!(x&0x00000001)) { n+=1; x>>=1;} return n; } ```