[ gcc Built-in Functions for binary ] gcc內建處理二進位函數

以下介紹的函數都是非標準的函數，他只能在GCC底下使用，不過一般的比賽環境都是支援的，所以熟記起來可以增加寫位元運算的效率

1. int __builtin_ffs (unsigned int x)
   int __builtin_ffsl (unsigned long)
   int __builtin_ffsll (unsigned long long)
   
   • 返回右起第一個1的位置
   • Returns one plus the index of the least significant 1-bit of x, or if x is zero, returns zero.
2. int __builtin_clz (unsigned int x)
   int __builtin_clzl (unsigned long)
   int __builtin_clzll (unsigned long long)
   
   • 返回左起第一個1之前0的個數
   • Returns the number of leading 0-bits in x, starting at the most significant bit position. If x is 0, the result is undefined.
    
3. int __builtin_ctz (unsigned int x)
   int __builtin_ctzl (unsigned long)
   int __builtin_ctzll (unsigned long long)
   
   • 返回右起第一個1之後的0的個數
   • Returns the number of trailing 0-bits in x, starting at the least significant bit position. If x is 0, the result is undefined.
    
4. int __builtin_popcount (unsigned int x)
   int __builtin_popcountl (unsigned long)
   int __builtin_popcountll (unsigned long long)
   
   • 返回1的個數
   • Returns the number of 1-bits in x.
    
5. int __builtin_parity (unsigned int x)
   int __builtin_parityl (unsigned long)
   int __builtin_parityll (unsigned long long)
   
   • 返回1的個數的奇偶性(1的個數 mod 2的值)
   •  Returns the parity of x, i.e. the number of 1-bits in x modulo 2. 

這種內建函數其實非常多，這邊有附上一個GCC內建函數的列表，有興趣的朋友可以參考
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

當你在GCC環境下，想直接用01寫二進位的東西，其實有簡單的方法:

• cout<<0b1010101;
• cout<<0b1010101LL;

這樣你的編譯器應該會警告你說這是GCC內建的東西(C++14以後的版本會支援它)，但是還是會正確執行，都是85，個人覺得蠻方便的

如果你是用C++11，可以用stoi,stol,stoll,stoul,stoull等函數來把二進位字串轉成int,long,long long,unsigned long,unsigned long long等，可以設定他用二進位轉換，像是這樣:

• cout<<stoi("1010101",NULL,2);
• cout<<stoll("1010101",NULL,2);

 另外有些編譯器會在<algorithm>實作std::__lg(n)這個函數，他會回傳$\floor{log_2n}$的值，可以參考這個
http://stackoverflow.com/questions/40434664/what-is-std-lg