function [d,t]=hpfast(y,s)
d=[];t=[];
[nargout,nargin] = argn(0)
// Copyright (C) 2001 Michel Juillard
// 
// Fast hpfiltering:
// 
// [c,t]=hpfast(y,lambda)
// 
// Inputs:
// 
// y:    vector (nx1) containing the series you wish to filter
// lambda: smoothing parameter (default: quarterly 1600)
// 
// Outputs:
// 
// c:   Cyclical component (deviations from trend)
// t:   Trend component
// 
if nargin<2 then
  s = 1600;
end
iopt = 0;
y = y(:);
 
//! unknown arg type, using mtlb_length 
n = mtlb_length(y);
d1 = zeros(n,1);
t = zeros(n,1);
 
v = zeros(3*n,6);
if iopt~=1 then
  ss = s;
  nn = n;
  v11 = 1;
  v22 = 1;
  v12 = 0;
  for i = 3:n
    x = v11;
    z = v12;
    v11 = 1/s+4*(x-z)+v22;
    v12 = 2*x-z;
    v22 = x;
    det1 = v11*v22-v12*v12;
    v(i,1) = v22/det1;
    v(i,3) = v11/det1;
    v(i,2) = -v12/det1;
    x = v11+1;
    z = v11;
    v11 = v11-v11*v11/x;
    v22 = v22-v12*v12/x;
    v12 = v12-z*v12/x;
  end
end
// 
//  Forward pass 
// 
m1 = y(2);
m2 = y(1);
for i = 3:n
  x = m1;
  m1 = 2*m1-m2;
  m2 = x;
  t(i-1,1) = v(i,1)*m1+v(i,2)*m2;
  d1(i-1,1) = v(i,2)*m1+v(i,3)*m2;
  det1 = v(i,1)*v(i,3)-v(i,2)*v(i,2);
  v11 = v(i,3)/det1;
  v12 = -v(i,2)/det1;
  z = (y(i)-m1)/(v11+1);
  m1 = m1+v11*z;
  m2 = m2+v12*z;
end
t(n,1) = m1;
t(n-1,1) = m2;
// 
// Backward pass
// 
m1 = y(n-1);
m2 = y(n);
for i = n-2:-1:1
  i1 = i+1;
  ib = n-i+1;
  x = m1;
  m1 = 2*m1-m2;
  m2 = x;
  // 
  // Combine info for y(.lt.i) with info for y(.ge.i)
  // 
  if i>2 then
    e1 = v(ib,3)*m2+v(ib,2)*m1+t(i,1);
    e2 = v(ib,2)*m2+v(ib,1)*m1+d1(i,1);
    b11 = v(ib,3)+v(i1,1);
    b12 = v(ib,2)+v(i1,2);
    b22 = v(ib,1)+v(i1,3);
    det1 = b11*b22-b12*b12;
    t(i,1) = (-b12*e1+b11*e2)/det1;
  end
  // 
  // End of combining
  // 
  det1 = v(ib,1)*v(ib,3)-v(ib,2)*v(ib,2);
  v11 = v(ib,3)/det1;
  v12 = -v(ib,2)/det1;
  z = (y(i)-m1)/(v11+1);
  m1 = m1+v11*z;
  m2 = m2+v12*z;
end
t(1,1) = m1;
t(2,1) = m2;
d = y-t;