#include <vector>
#include <map>
#include <cstdio>
#include <string>

#include <sys/mman.h>

class JIT
{
    typedef unsigned char byte;
    std::vector<byte> code;
    
    std::map<std::string, double> vars;
    
private:
    void Emit(byte a) { code.push_back(a); }
    void Emit(byte a,byte b) { Emit(a);Emit(b); }
    void EmitRef(double& r)
    {
        unsigned x = (unsigned)&r;
        Emit(x); Emit(x>>8); Emit(x>>16); Emit(x>>24);
    }
public:
    void DefineVar(const std::string& name, double value)
    {
        vars[name] = value;
    }
    
    // Compilation
    void Load(const std::string& name)
    {
        Emit(0xDD,0x05); EmitRef(vars[name]);
    }
    void Plus()
    {
        Emit(0xDE,0xC1);
    }
    void Mul()
    {
        Emit(0xDE,0xC9);
    }
    void Minus()
    {
        Emit(0xDE,0xE9);
    }
    double Exec()
    {
        double result;
        Emit(0xDD,0x1D); EmitRef(result); //fstp
        Emit(0xC3); //ret
        
        unsigned len = code.size();
        void* ptr = (void*)&code[0];
        
        mprotect(ptr,len,PROT_EXEC);
        ((void(*)(void))ptr) ();
        mprotect(ptr,len,PROT_READ|PROT_WRITE);
        
        return result;
    }
};

int main(void)
{
    JIT jit;
    
    jit.DefineVar("x", 1.0);
    jit.DefineVar("y", 2.0);
    jit.DefineVar("z", 300.0);
    
    // Testing expression: (x+y)*z - x
    
    jit.Load("x");
    jit.Load("y");
    jit.Plus();
    jit.Load("z");
    jit.Mul();
    jit.Load("x");
    jit.Minus();
    double value = jit.Exec();
    
    std::printf("(x+y)*z-x = %g\n", value);
}