Fibonacci numbers with GMP lib

EDIT: solved. ans var needed to be set to 0 in every iteration.That was a mistake I overlooked.A mpz_set_ui(ans,0); at the beginning of each loop solves the problem.Thanks to Oli Charlesworth for his effort!

I'm coding a fibonacci function in C using the GMP library,I use the matrix multiplication algorithm,but for some unexpected reason,I get wrong results(I strongly believe that开发者_Python百科 the algorithm is right).It must have something to do with Gmp structs and inits.Any ideas?

/*********************************************************
Find n th Fibonacci number.Matrix algorithm.
+--          --+     +--     --+
|f(n+1) f(n)   |     | 1     1 |  ^n
|              |  == |         |
|f(n)   f(n-1) |     | 1     0 |
+--          --+     +--     --+
*********************************************************/

int
fastfib(mpz_t result,int n)
{


    int cons[2][2]={{1,1},{1,0}};      //"constant" matrix
    mpz_t mat[2][2];                   //actual matrix holding fibonacci numbers
    mpz_t ans;                         //holds f(n+1)   
    mpz_t return_val;                  //used for calculations
    mpz_t temp;                        //used for calculations as well

    //initialize (automatically set to zero)
    mpz_init(ans);
    mpz_init(return_val);
    mpz_init(temp);
    mpz_init(mat[0][0]);
    mpz_init(mat[0][1]);
    mpz_init(mat[1][0]);
    mpz_init(mat[1][1]);

    //start with n=1
    mpz_set_ui(mat[0][0],1);
    mpz_set_ui(mat[1][0],1);
    mpz_set_ui(mat[0][1],1);
    mpz_set_ui(mat[1][1],0);

    //some trivial cases
    if(n==0){
        mpz_set_ui(result,0);
        return 0;
    }
    if(n==1){
        mpz_set_ui(result,1);
        return 0;
    }
    if(n==2){
        mpz_set_ui(result,1);
        return 0;
    }

    n--;


    while(n>1){

        //fib[n+1]
        //ans=mat[0][0]*cons[0][0]+mat[0][1]*cons[1][0];

        mpz_set_ui(ans,0);

        mpz_mul_ui(temp,mat[0][0],cons[0][0]);
        mpz_add(ans,ans,temp);
        mpz_mul_ui(temp,mat[0][1],cons[1][0]);
        mpz_add(ans,ans,temp);

        //update matrix


        mpz_set(mat[1][1],mat[1][0]);   //mat[1][1]=mat[1][0];
        mpz_set(mat[1][0],mat[0][0]);   //mat[0][1]=mat[1][0]=mat[0][0];
        mpz_set(mat[0][1],mat[0][0]);
        mpz_set(mat[0][0],ans);         //mat[0][0]=ans;

        n--;
    }


    //clear vars
    mpz_clear(ans);
    mpz_clear(return_val);
    mpz_clear(temp);

    mpz_set(result,mat[0][0]);
    return 0;

}

Some debug info

results for n=2       // expected
2 1                   //  2 1
1 1                   //  1 1

results for n=3
5 2                   //  3 2
2 1                   //  2 1

results for n=4
12 5                  //  5 3
5 2                   //  3 2

results for n=5
29 12                 //  8 5
12 5                  //  5 3

results for n=6
70 29                 //  13 8
29 12                 //  8  5

70

solved. ans var needed to be set to 0 in every iteration.That was a mistake I overlooked.A mpz_set_ui(ans,0); at the beginning of each loop solves the problem.Thanks to Oli Charlesworth for his effort!