#include <string.h>

void matrixmult(float *mat, float *vec) {
  float tmp[4];
  memcpy(tmp, vec, sizeof(tmp));

  for (int i = 0; i < 4; i++) {
    double sum = 0;
    for (int j = 0, jj=0; j < 4; j++, jj+=4) {
      sum += mat[i+jj] * tmp[j];
    }
    vec[i] = sum;
  }
}