// To compute the determinant of a k by k matrix where A[i, j]
// is located at b + i*s + j.
// k > 0
// the matrix is corrupted.
typedef double R;
R det(R * b, int k, int s){
  if(k==1) return *b;
  for(int j=0; j<k; ++j) if (b[s*j] != 0.0) {
     if(j) {R * q=b+j*s; for(int n=0; n<k; ++n) {
        R t = q[n]; q[n] = b[n]; b[n] = -t;}}
    {R t = 1./ *b;
  for(int i=1; i<k; ++i) {R * q = b+i*s; R a = t * *q;
    for(int r=1; r<k; ++r) q[r] -= a * b[r];}
  return *b * det(b+s+1, k-1, s);}}
  return 0;}

#if 1
#include <math.h>
#include <stdio.h>
int c = 0;
void tst(int n, R m[n][n], R a) {
  R b = det(&(m[0][0]), n, n);
  if(fabs(a-b)> 1e-12) printf("fail %d %e\n", c, b);
  ++c;}
int main(){
{R a[][2] = {{0, 0}, {0, 0}}; tst(2, a, 0);}
{R a[][2] = {{0, 0}, {1, 0}}; tst(2, a, 0);}
{R a[][2] = {{0, 1}, {1, 0}}; tst(2, a, -1);}
{R a[][2] = {{0, 1}, {0, 2}}; tst(2, a, 0);}
{R a[][3] = {{1, 3, -2}, {3, -4, 2}, {2, 3, -4}}; tst(3, a, 24);}}
#endif