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+/* $Id: CoinPresolveFixed.hpp 1510 2011-12-08 23:56:01Z lou $ */
+// Copyright (C) 2002, International Business Machines
+// Corporation and others. All Rights Reserved.
+// This code is licensed under the terms of the Eclipse Public License (EPL).
+
+#ifndef CoinPresolveFixed_H
+#define CoinPresolveFixed_H
+#define FIXED_VARIABLE 1
+
+/*! \class remove_fixed_action
+ \brief Excise fixed variables from the model.
+
+ Implements the action of virtually removing one or more fixed variables
+ x_j from the model by substituting the value sol_j in each constraint.
+ Specifically, for each constraint i where a_ij != 0, rlo_i and rup_i
+ are adjusted by -a_ij*sol_j and a_ij is set to 0.
+
+ There is an implicit assumption that the variable already has the correct
+ value. If this isn't true, corrections to row activity may be incorrect.
+ If you want to guard against this possibility, consider make_fixed_action.
+
+ Actual removal of the empty column from the matrix is handled by
+ drop_empty_cols_action. Correction of the objective function is done there.
+*/
+class remove_fixed_action : public CoinPresolveAction {
+ public:
+ /*! \brief Structure to hold information necessary to reintroduce a
+ column into the problem representation.
+ */
+ struct action {
+ int col; ///< column index of variable
+ int start; ///< start of coefficients in #colels_ and #colrows_
+ double sol; ///< value of variable
+ };
+ /// Array of row indices for coefficients of excised columns
+ int *colrows_;
+ /// Array of coefficients of excised columns
+ double *colels_;
+ /// Number of entries in #actions_
+ int nactions_;
+ /// Vector specifying variable(s) affected by this object
+ action *actions_;
+
+ private:
+ /*! \brief Constructor */
+ remove_fixed_action(int nactions,
+ action *actions,
+ double * colels,
+ int * colrows,
+ const CoinPresolveAction *next);
+
+ public:
+ /// Returns string "remove_fixed_action".
+ const char *name() const;
+
+ /*! \brief Excise the specified columns.
+
+ Remove the specified columns (\p nfcols, \p fcols) from the problem
+ representation (\p prob), leaving the appropriate postsolve object
+ linked as the head of the list of postsolve objects (currently headed
+ by \p next).
+ */
+ static const remove_fixed_action *presolve(CoinPresolveMatrix *prob,
+ int *fcols,
+ int nfcols,
+ const CoinPresolveAction *next);
+
+ void postsolve(CoinPostsolveMatrix *prob) const;
+
+ /// Destructor
+ virtual ~remove_fixed_action();
+};
+
+
+/*! \relates remove_fixed_action
+ \brief Scan the problem for fixed columns and remove them.
+
+ A front end to collect a list of columns with equal bounds and hand them to
+ remove_fixed_action::presolve() for processing.
+*/
+
+const CoinPresolveAction *remove_fixed(CoinPresolveMatrix *prob,
+ const CoinPresolveAction *next);
+
+
+/*! \class make_fixed_action
+ \brief Fix a variable at a specified bound.
+
+ Implements the action of fixing a variable by forcing both bounds to the same
+ value and forcing the value of the variable to match.
+
+ If the bounds are already equal, and the value of the variable is already
+ correct, consider remove_fixed_action.
+*/
+class make_fixed_action : public CoinPresolveAction {
+
+ /// Structure to preserve the bound overwritten when fixing a variable
+ struct action {
+ double bound; ///< Value of bound overwritten to fix variable.
+ int col ; ///< column index of variable
+ };
+
+ /// Number of preserved bounds
+ int nactions_;
+ /// Vector of preserved bounds, one for each variable fixed in this object
+ const action *actions_;
+
+ /*! \brief True to fix at lower bound, false to fix at upper bound.
+
+ Note that this applies to all variables fixed in this object.
+ */
+ const bool fix_to_lower_;
+
+ /*! \brief The postsolve object with the information required to repopulate
+ the fixed columns.
+ */
+ const remove_fixed_action *faction_;
+
+ /*! \brief Constructor */
+ make_fixed_action(int nactions, const action *actions, bool fix_to_lower,
+ const remove_fixed_action *faction,
+ const CoinPresolveAction *next)
+ : CoinPresolveAction(next),
+ nactions_(nactions), actions_(actions),
+ fix_to_lower_(fix_to_lower),
+ faction_(faction)
+ {}
+
+ public:
+ /// Returns string "make_fixed_action".
+ const char *name() const;
+
+ /*! \brief Perform actions to fix variables and return postsolve object
+
+ For each specified variable (\p nfcols, \p fcols), fix the variable to
+ the specified bound (\p fix_to_lower) by setting the variable's bounds
+ to be equal in \p prob. Create a postsolve object, link it at the head of
+ the list of postsolve objects (\p next), and return the object.
+ */
+ static const CoinPresolveAction *presolve(CoinPresolveMatrix *prob,
+ int *fcols,
+ int nfcols,
+ bool fix_to_lower,
+ const CoinPresolveAction *next);
+
+ /*! \brief Postsolve (unfix variables)
+
+ Back out the variables fixed by the presolve side of this object.
+ */
+ void postsolve(CoinPostsolveMatrix *prob) const;
+
+ /// Destructor
+ virtual ~make_fixed_action() {
+ deleteAction(actions_,action*);
+ delete faction_;
+ }
+};
+
+/*! \relates make_fixed_action
+ \brief Scan variables and fix any with equal bounds
+
+ A front end to collect a list of columns with equal bounds and hand them to
+ make_fixed_action::presolve() for processing.
+*/
+
+const CoinPresolveAction *make_fixed(CoinPresolveMatrix *prob,
+ const CoinPresolveAction *next) ;
+
+/*! \brief Transfer costs from singleton variables
+ \relates make_fixed_action
+
+ Transfers costs from singleton variables in equalities onto the other
+ variables. Will also transfer costs from one integer variable to other
+ integer variables with zero cost if there's a net gain in integer variables
+ with non-zero cost.
+
+ The relation to make_fixed_action is tenuous, but this transform should be
+ attempted before the initial round of variable fixing.
+*/
+void transferCosts(CoinPresolveMatrix * prob);
+#endif