' --------------------------------------------------------------------------------------------------------------------
'	DESCRIPTION: 	State-Space implementation of HP Filter.
'
'	DATE: 				 03 / 03 / 2017
'
'	AUTHOR:			Graeme Walsh
' --------------------------------------------------------------------------------------------------------------------

' ********************************************************************************************************************
' Pre-liminary set-up
' ********************************************************************************************************************
  
  wfcreate(wf=example) Q 1980Q1 2017Q4

' ********************************************************************************************************************
' Create Simulated Data
' ********************************************************************************************************************

  rndseed 123456

  series e1 = nrnd
  series Y = 0

  model AR2
  AR2.append Y = 3.20 + 0.22 * Y(-1) + 0.15 * Y(-2) + e1
  AR2.solveopt(s=d,d=d)
  AR2.solve

  series Y = Y_0

' *******************************************************************************************************************
' EViews Built-in HP Filter function
' *******************************************************************************************************************

  hpf(lambda=1600,power=2) Y HPT @ HPC

' ********************************************************************************************************************
' State-space model
' ********************************************************************************************************************

' HP filter smoothing parameter
  scalar lambda = 1600
  
' Create the sspace object
  sspace hp_mod  

' Measurement equation
  hp_mod.append @signal y = 1 * tau + 0 * beta +  eps 
  hp_mod.append @ename eps 
  hp_mod.append @evar var(eps) = 1

' State equation (tau)
  hp_mod.append @state tau = 1 * tau(-1) + 1 * beta(-1) + eta_tau 
  hp_mod.append @ename eta_tau 
  hp_mod.append @evar var(eta_tau) = 0  

' State equation (beta)
  hp_mod.append @state beta = 0 * tau(-1) + 1 * beta(-1) + eta_beta 
  hp_mod.append @ename eta_beta 
  hp_mod.append @evar var(eta_beta) = 1 / lambda
    
' Estimate model by maximum likelihood
  hp_mod.ml

' Create state estimates
  hp_mod.makestates(t=smooth) *f1
  hp_mod.makestates(t=pred) *f2
  hp_mod.makestates(t=filt) *f3

' Create gap estimates
  series CYCLE1 = Y-TAUF1
  series CYCLE2 = Y-TAUF2
  series CYCLE3 = Y-TAUF3
 
' ********************************************************************************************************************
' Compare EViews built-in with State-Space Model
' ********************************************************************************************************************

  show HPT TAUF1		' Trend comparison
  show HPC CYCLE1	' Cycle comparison
  
' -------------------------------------------------------------------------------------------------------------------
'	END OF PROGRAM
' -------------------------------------------------------------------------------------------------------------------