text: 
  #define NRANSI
  #include "nrutil.h"
  #define RX 256.0
  
  void mpmul(unsigned char w[], unsigned char u[], unsigned char v[], int n,
  	int m)
  {
  	void drealft(double data[], unsigned long n, int isign);
  	int j,mn,nn=1;
  	double cy,t,*a,*b;
  
  	mn=IMAX(m,n);
  	while (nn < mn) nn <<= 1;
  	nn <<= 1;
  	a=dvector(1,nn);
  	b=dvector(1,nn);
  	for (j=1;j<=n;j++)
  		a[j]=(double)u[j];
  	for (j=n+1;j<=nn;j++) a[j]=0.0;
  	for (j=1;j<=m;j++)
  		b[j]=(double)v[j];
  	for (j=m+1;j<=nn;j++) b[j]=0.0;
  	drealft(a,nn,1);
  	drealft(b,nn,1);
  	b[1] *= a[1];
  	b[2] *= a[2];
  	for (j=3;j<=nn;j+=2) {
  		b[j]=(t=b[j])*a[j]-b[j+1]*a[j+1];
  		b[j+1]=t*a[j+1]+b[j+1]*a[j];
  	}
  	drealft(b,nn,-1);
  	cy=0.0;
  	for (j=nn;j>=1;j--) {
  		t=b[j]/(nn>>1)+cy+0.5;
  		cy=(unsigned long) (t/RX);
  		b[j]=t-cy*RX;
  	}
  	if (cy >= RX) nrerror("cannot happen in fftmul");
  	w[1]=(unsigned char) cy;
  	for (j=2;j<=n+m;j++)
  		w[j]=(unsigned char) b[j-1];
  	free_dvector(b,1,nn);
  	free_dvector(a,1,nn);
  }
  #undef RX
  #undef NRANSI
  