From f889760f6163fdda460d0bc18ae804f08f967fcb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20Villemot?= <sebastien@dynare.org>
Date: Tue, 29 Oct 2013 18:46:54 +0800
Subject: [PATCH] Reactivate and fix the test for the balanced growth path.

If the homogeneized equation evaluates to zero, then we skip the
test (otherwise the 2nd derivative of the log is infinite, and the test fails
while it should not necessarily).

Closes #506
---
 preprocessor/DynamicModel.cc | 39 +++++++++++++++++++++---------------
 preprocessor/ModFile.cc      |  4 ++--
 2 files changed, 25 insertions(+), 18 deletions(-)

diff --git a/preprocessor/DynamicModel.cc b/preprocessor/DynamicModel.cc
index d19727fa4..2fd7f54c4 100644
--- a/preprocessor/DynamicModel.cc
+++ b/preprocessor/DynamicModel.cc
@@ -3757,22 +3757,28 @@ DynamicModel::testTrendDerivativesEqualToZero(const eval_context_t &eval_context
         || symbol_table.getType(it->first.first) == eLogTrend)
       for (int eq = 0; eq < (int) equations.size(); eq++)
         {
-          expr_t testeq = AddLog(AddMinus(equations[eq]->get_arg1(), // F: a = b -> ln(a - b)
-                                          equations[eq]->get_arg2()));
-          testeq = testeq->getDerivative(it->second); // d F / d Trend
-          for (deriv_id_table_t::const_iterator endogit = deriv_id_table.begin();
-               endogit != deriv_id_table.end(); endogit++)
-            if (symbol_table.getType(endogit->first.first) == eEndogenous)
-              {
-                double nearZero = testeq->getDerivative(endogit->second)->eval(eval_context); // eval d F / d Trend d Endog
-                if (fabs(nearZero) > ZERO_BAND)
+          expr_t homogeneq = AddMinus(equations[eq]->get_arg1(),
+                                      equations[eq]->get_arg2())
+
+          // Do not run the test if the term inside the log is zero
+          if (fabs(homogeneq->eval(eval_context)) > ZERO_BAND)
+            {
+              expr_t testeq = AddLog(homogeneq); // F = log(lhs-rhs)
+              testeq = testeq->getDerivative(it->second); // d F / d Trend
+              for (deriv_id_table_t::const_iterator endogit = deriv_id_table.begin();
+                   endogit != deriv_id_table.end(); endogit++)
+                if (symbol_table.getType(endogit->first.first) == eEndogenous)
                   {
-                    cerr << "ERROR: trends not compatible with balanced growth path; the second-order cross partial of equation " << eq + 1 << " w.r.t. trend variable "
-                         << symbol_table.getName(it->first.first) << " and endogenous variable "
-                         << symbol_table.getName(endogit->first.first) << " is not null. " << endl;
-                    exit(EXIT_FAILURE);
+                    double nearZero = testeq->getDerivative(endogit->second)->eval(eval_context); // eval d F / d Trend d Endog
+                    if (fabs(nearZero) > ZERO_BAND)
+                      {
+                        cerr << "ERROR: trends not compatible with balanced growth path; the second-order cross partial of equation " << eq + 1 << " w.r.t. trend variable "
+                             << symbol_table.getName(it->first.first) << " and endogenous variable "
+                             << symbol_table.getName(endogit->first.first) << " is not null. " << endl;
+                        exit(EXIT_FAILURE);
+                      }
                   }
-              }
+            }
         }
 }
 
@@ -4134,8 +4140,9 @@ DynamicModel::transformPredeterminedVariables()
 void
 DynamicModel::detrendEquations()
 {
-  for (nonstationary_symbols_map_t::const_iterator it = nonstationary_symbols_map.begin();
-       it != nonstationary_symbols_map.end(); it++)
+  // We go backwards in the list of trend_vars, to deal correctly with I(2) processes
+  for (nonstationary_symbols_map_t::const_reverse_iterator it = nonstationary_symbols_map.rbegin();
+       it != nonstationary_symbols_map.rend(); ++it)
     for (int i = 0; i < (int) equations.size(); i++)
       {
         BinaryOpNode *substeq = dynamic_cast<BinaryOpNode *>(equations[i]->detrend(it->first, it->second.first, it->second.second));
diff --git a/preprocessor/ModFile.cc b/preprocessor/ModFile.cc
index 61b551d48..631dc4fa4 100644
--- a/preprocessor/ModFile.cc
+++ b/preprocessor/ModFile.cc
@@ -416,8 +416,8 @@ ModFile::computingPass(bool no_tmp_terms)
   // Mod file may have no equation (for example in a standalone BVAR estimation)
   if (dynamic_model.equation_number() > 0)
     {
-      /*if (nonstationary_variables)
-        trend_dynamic_model.runTrendTest(global_eval_context);*/
+      if (nonstationary_variables)
+        trend_dynamic_model.runTrendTest(global_eval_context);
 
       // Compute static model and its derivatives
       dynamic_model.toStatic(static_model);