From 5496c87aa289adaf18148dc435706dc6e233d7a1 Mon Sep 17 00:00:00 2001
From: Laurent Perron <lperron@google.com>
Date: Thu, 12 Aug 2021 15:23:02 +0200
Subject: [PATCH] minor improvement in glop presolve

---
 ortools/glop/BUILD           |  1 +
 ortools/glop/lp_solver.cc    |  2 +-
 ortools/glop/preprocessor.cc |  8 ++++++++
 ortools/glop/preprocessor.h  | 10 +++++-----
 4 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/ortools/glop/BUILD b/ortools/glop/BUILD
index 8c404ac8b0..998246752c 100644
--- a/ortools/glop/BUILD
+++ b/ortools/glop/BUILD
@@ -326,6 +326,7 @@ cc_library(
         ":revised_simplex",
         ":status",
         "//ortools/base",
+        "//ortools/base:iterator_adaptors",
         "//ortools/lp_data",
         "//ortools/lp_data:base",
         "//ortools/lp_data:lp_data_utils",
diff --git a/ortools/glop/lp_solver.cc b/ortools/glop/lp_solver.cc
index 2837a1ecc5..04406d28e0 100644
--- a/ortools/glop/lp_solver.cc
+++ b/ortools/glop/lp_solver.cc
@@ -217,7 +217,7 @@ ProblemStatus LPSolver::SolveWithTimeLimit(const LinearProgram& lp,
     RunRevisedSimplexIfNeeded(&solution, time_limit);
   }
 
-  if (postsolve_is_needed) preprocessor.RecoverSolution(&solution);
+  if (postsolve_is_needed) preprocessor.DestructiveRecoverSolution(&solution);
   const ProblemStatus status = LoadAndVerifySolution(lp, solution);
 
   // LOG some statistics that can be parsed by our benchmark script.
diff --git a/ortools/glop/preprocessor.cc b/ortools/glop/preprocessor.cc
index 9b7a979d1f..189b10f3c0 100644
--- a/ortools/glop/preprocessor.cc
+++ b/ortools/glop/preprocessor.cc
@@ -17,6 +17,7 @@
 #include <limits>
 
 #include "absl/strings/str_format.h"
+#include "ortools/base/iterator_adaptors.h"
 #include "ortools/base/strong_vector.h"
 #include "ortools/glop/revised_simplex.h"
 #include "ortools/glop/status.h"
@@ -177,6 +178,13 @@ void MainLpPreprocessor::RunAndPushIfRelevant(
 }
 
 void MainLpPreprocessor::RecoverSolution(ProblemSolution* solution) const {
+  SCOPED_INSTRUCTION_COUNT(time_limit_);
+  for (const auto& p : gtl::reversed_view(preprocessors_)) {
+    p->RecoverSolution(solution);
+  }
+}
+
+void MainLpPreprocessor::DestructiveRecoverSolution(ProblemSolution* solution) {
   SCOPED_INSTRUCTION_COUNT(time_limit_);
   while (!preprocessors_.empty()) {
     preprocessors_.back()->RecoverSolution(solution);
diff --git a/ortools/glop/preprocessor.h b/ortools/glop/preprocessor.h
index 385d6ebc34..6889cbeaa1 100644
--- a/ortools/glop/preprocessor.h
+++ b/ortools/glop/preprocessor.h
@@ -110,6 +110,10 @@ class MainLpPreprocessor : public Preprocessor {
 
   bool Run(LinearProgram* lp) final;
   void RecoverSolution(ProblemSolution* solution) const override;
+  // Like RecoverSolution but destroys data structures as it goes to reduce peak
+  // RAM use. After calling this the MainLpPreprocessor object may no longer be
+  // used.
+  void DestructiveRecoverSolution(ProblemSolution* solution);
 
  private:
   // Runs the given preprocessor and push it on preprocessors_ for the postsolve
@@ -119,11 +123,7 @@ class MainLpPreprocessor : public Preprocessor {
                             LinearProgram* lp);
 
   // Stack of preprocessors currently applied to the lp that needs postsolve.
-  //
-  // TODO(user): This is mutable so that the preprocessor can be freed as soon
-  // as their RecoverSolution() is called. Make RecoverSolution() non-const or
-  // remove this optimization?
-  mutable std::vector<std::unique_ptr<Preprocessor>> preprocessors_;
+  std::vector<std::unique_ptr<Preprocessor>> preprocessors_;
 
   // Initial dimension of the lp given to Run(), for displaying purpose.
   EntryIndex initial_num_entries_;