#include<iostream>

#include<fstream>

using namespace std;

unsigned char exp[];         //exp[i] = 3^i

unsigned char log[];         //log[i] = log3(i)

unsigned char GFmul(unsigned char a, unsigned char b){

    //GF(2^8) 乘法

    unsigned char result = ;

    if((b&) == )result = a;

    b >>= ;

    for(int i = ; i < ; i ++){

        if(a > ){

            a = (a << ) ^ 0x1b;

        }

        else{

            a <<= ;

        }

        if((b&) == ){

            result ^= a;

        }

        b >>= ;

    }

    return result;

}

void generateMulTab(){

    //选择生成元3作为构造乘法表的基础

    const int N = ;

    unsigned char tmp = ;

    for(int i = ; i < ; i ++){

        tmp = GFmul(tmp, N);

        exp[i] = tmp;

        log[tmp] = i;

    }

}

unsigned char GFfastMul(unsigned char x, unsigned char y){

    //利用exp和log来查表实现乘法

    if(x ==  || y == )return ;

    //x = 3^m, y = 3 ^ n;   x * y = 3^m * 3^n = 3^(m+n)

    int m = log[x], n = log[y];

    return exp[(m+n)>?(m+n-):(m+n)];

}

int main(){

    //单元测试，乘法打表

    generateMulTab();

    int count[];

    for(int i = ; i < ; i ++)count[i] = ;

    unsigned char x, y;

    x = ;

    do{

        y = ;

        do{

            count[GFfastMul(x, y)] ++;

            y ++;

        }while(y != );

        x ++;

    }while(x != );

    ofstream write("Test.txt");

    for(int i = ; i < ; i ++)write<<i<<"\t"<<count[i]<<endl;

    write.close();

    return ;

}