solvers/ipopt/asc_ipopt.c

/*  ASCEND modelling environment
    Copyright (C) 2007 Carnegie Mellon University

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2, or (at your option)
    any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place - Suite 330,
    Boston, MA 02111-1307, USA.
*//**
    @file
    Connection of the IPOPT optimisation solver into ASCEND.

    THIS IS STILL VERY MUCH UNDER DEVELOPMENT AND INCOMPLETE. I'VE ACTUALLY
    ONLY JUST STARTED WRITING IT by starting with asc_tron.c and modifying.

    The IPOPT solver is documented at http://projects.coin-or.org/Ipopt/
*//*
    ASCEND wrapper for IPOPT originally by John Pye, Jun 2007.
*/

#include <utilities/config.h>

#ifndef ASC_WITH_IPOPT
# error "ASC_WITH_IPOPT must be defined in order to build this." 
#endif

#include <solver/solver.h>

#include <system/calc.h>
#include <system/relman.h>
#include <system/slv_stdcalls.h>
#include <system/block.h>

#include <utilities/ascConfig.h>
#include <utilities/ascPanic.h>
#include <utilities/ascMalloc.h>
#include <utilities/ascDynaLoad.h>
#include <utilities/mem.h>
#include <utilities/ascEnvVar.h>
#include <general/tm_time.h>
#include <general/env.h>

#include <ipopt/IpStdCInterface.h>

ASC_DLLSPEC SolverRegisterFn ipopt_register;

/*------------------------------------------------------------------------------
  DATA STRUCTURES AND FORWARD DEFS
*/

/**
    Documentation of solver options for IPOPT is at
    http://www.coin-or.org/Ipopt/documentation/node1.html
*/
enum{
    IPOPT_PARAM_TOL
    ,IPOPT_PARAM_MAX_ITER
    ,IPOPT_PARAM_SAFEEVAL
    ,IPOPT_PARAM_MU_STRATEGY
    ,IPOPT_PARAMS
};

#define SYS(s) ((IpoptSystem *)(s))

struct IpoptSystemStruct{

    /*
        Problem definition
    */
    slv_system_t                slv;     /* slv_system_t back-link */
    struct rel_relation         *obj;    /* Objective function: NULL = none */
    struct rel_relation         *old_obj;/* Objective function: NULL = none */
    struct var_variable         **vlist; /* Variable list (NULL terminated) */
    struct rel_relation         **rlist; /* Relation list (NULL terminated) */

    var_filter_t vfilt;
    rel_filter_t rfilt;

    /*
        Solver information
    */
    int32                  presolved;    /* ? Has the system been presolved */
    int32                  resolve;      /* ? Has the system been resolved */
    slv_parameters_t       p;            /* Parameters */
    slv_status_t           s;            /* Status (as of iteration end) */

    int32                  cap;          /* Order of matrix/vectors */
    int32                  rank;         /* Symbolic rank of problem */
    int32                  vused;        /* Free and incident variables */
    int32                  vtot;         /* length of varlist */
    int32                  rused;        /* Included relations */
    int32                  rtot;         /* length of rellist */
    double                 clock;        /* CPU time */

    int32 calc_ok;
    double obj_val;

    void *parm_array[IPOPT_PARAMS];
    struct slv_parameter pa[IPOPT_PARAMS];

    /*
        IPOPT DATA
    */
    Index n;                          /* number of variables */
    Index m;                          /* number of constraints */

    int nnzJ; /* number of non zeros in the jacobian of the constraints */
    int nnzH; /* number of non-zeros in the hessian of the objective */

    Number* x_L;                  /* lower bounds on x */
    Number* x_U;                  /* upper bounds on x */
    Number* g_L;                  /* lower bounds on g */
    Number* g_U;                  /* upper bounds on g */

    IpoptProblem nlp;             /* IpoptProblem */

    enum ApplicationReturnStatus status; /* Solve return code */
    Number* x;                    /* starting point and solution vector */
    Number* mult_x_L;             /* lower bound multipliers at the solution */
    Number* mult_x_U;             /* upper bound multipliers at the solution */
    Index i;                      /* generic counter */
};

typedef struct IpoptSystemStruct IpoptSystem;

static int ipopt_get_default_parameters(slv_system_t server, SlvClientToken asys, slv_parameters_t *parameters);

static void ipopt_iteration_begins(IpoptSystem *sys);
static void ipopt_iteration_ends(IpoptSystem *sys);

/*------------------------------------------------------------------------------
  SYSTEM SETUP/DESTROY, STATUS AND SOLVER ELIGIBILITY
*/

static SlvClientToken ipopt_create(slv_system_t server, int32*statusindex){
    IpoptSystem *sys;

    sys = ASC_NEW_CLEAR(IpoptSystem);
    if(sys==NULL){
        *statusindex = 1;
        return sys;
    }

    sys->p.parms = sys->pa;
    sys->p.dynamic_parms = 0;
    ipopt_get_default_parameters(server,(SlvClientToken)sys,&(sys->p));
    sys->p.whose = (*statusindex);

    sys->presolved = 0;
    sys->resolve = 0;

    sys->n = -1;
    sys->m = -1;

    sys->s.ok = TRUE;
    sys->s.calc_ok = TRUE;
    sys->s.costsize = 0;
    sys->s.cost = NULL; /*redundant, but sanity preserving */
    sys->s.block.number_of = 1;
    sys->s.block.current_block = 0;
    sys->s.block.current_reordered_block = 0;
    sys->s.block.current_size = 0;
    sys->s.block.previous_total_size = 0;
    sys->s.block.iteration = 0;
    sys->s.block.funcs = 0;
    sys->s.block.jacs = 0;
    sys->s.block.cpu_elapsed = 0;
    sys->s.block.functime = 0;
    sys->s.block.jactime = 0;
    sys->s.block.residual = 0;

    sys->rfilt.matchbits = (REL_INCLUDED | REL_EQUALITY | REL_ACTIVE);
    sys->rfilt.matchvalue = (REL_INCLUDED | REL_EQUALITY | REL_ACTIVE);
    sys->vfilt.matchbits =  (VAR_ACTIVE | VAR_INCIDENT | VAR_SVAR | VAR_FIXED);
    sys->vfilt.matchvalue = (VAR_ACTIVE | VAR_INCIDENT | VAR_SVAR);

    sys->vlist = slv_get_solvers_var_list(server);
    sys->rlist = slv_get_solvers_rel_list(server);

    sys->rtot = slv_get_num_solvers_rels(server);
    sys->vtot = slv_get_num_solvers_vars(server);

    sys->obj = slv_get_obj_relation(server);
    if(sys->vlist == NULL) {
        ASC_FREE(sys);
        ERROR_REPORTER_HERE(ASC_PROG_ERR,"IPOPT called with no variables.");
        *statusindex = -2;
        return NULL;
    }
    if(sys->rlist == NULL && sys->obj == NULL) {
        ASC_FREE(sys);
        ERROR_REPORTER_HERE(ASC_PROG_ERR,"IPOPT called with no relations or objective.");
        *statusindex = -1;
        return NULL;
    }
    
    /* do nothing with the objective list, pars, bounds, extrels, etc etc */

    slv_check_var_initialization(server);
    *statusindex = 0;
    return((SlvClientToken)sys);
}

static int32 ipopt_destroy(slv_system_t server, SlvClientToken asys){
    UNUSED_PARAMETER(server);
    ERROR_REPORTER_HERE(ASC_PROG_ERR,"ipopt_destroy not implemented");
    return 1;
}   


static int ipopt_get_status(slv_system_t server, SlvClientToken asys
        ,slv_status_t *status
){
    IpoptSystem *sys;
    (void)server;  /* stop gcc whine about unused parameter */

    sys = SYS(asys);
    //if (check_system(sys)) return 1;
    mem_copy_cast(&(sys->s),status,sizeof(slv_status_t));
    return 0;
}

static int32 ipopt_eligible_solver(slv_system_t server){
    UNUSED_PARAMETER(server);
    
    /// TODO check that there is a MAXIMIZE or MINIMIZE statement
    /// TODO check that there are no discrete-valued variables
    /// TODO check that there are no WHENs or CONDITIONALs
    /// TODO check anything else?

    ERROR_REPORTER_HERE(ASC_PROG_WARNING,"ipopt_eligible_solver not implemented");
    return 1;
}

/*------------------------------------------------------------------------------
  SOLVER PARAMETERS
*/

static
int32 ipopt_get_default_parameters(slv_system_t server, SlvClientToken asys
        ,slv_parameters_t *parameters
){
    IpoptSystem *sys = NULL;
    struct slv_parameter *new_parms = NULL;
    int32 make_macros = 0;

    if(server != NULL && asys != NULL) {
        sys = SYS(asys);
        make_macros = 1;
    }

    if(parameters->parms == NULL) {
        new_parms = ASC_NEW_ARRAY_OR_NULL(struct slv_parameter,IPOPT_PARAMS);
        if(new_parms == NULL) {
            return -1;
        }
        parameters->parms = new_parms;
        parameters->dynamic_parms = 1;
    }

    parameters->num_parms = 0;

    slv_param_int(parameters,IPOPT_PARAM_MAX_ITER
        ,(SlvParameterInitInt){{"max_iter"
            ,"Maximum number of iterations",2
            ,"The algorithm terminates with an error message if the number of iterations exceeded this number."
        }, 3000, 0, 100000000}
    );

    slv_param_real(parameters,IPOPT_PARAM_TOL
        ,(SlvParameterInitReal){{"tol"
            ,"Desired convergence tolerance (relative)",2
            ,"Determines the convergence tolerance for the algorithm. The algorithm"
            " terminates successfully, if the (scaled) NLP error becomes smaller"
            " than this value, and if the (absolute) criteria according to "
            " 'dual_inf_tol', 'primal_inf_tol', and 'cmpl_inf_tol' are met. (This"
            " is epsilon_tol in Eqn. (6) in implementation paper). See also "
            " 'acceptable_tol' as a second termination criterion. Note, some other"
            " algorithmic features also use this quantity to determine thresholds"
            " etc."
        }, 1e-8, 0, 1e20}
    );

    slv_param_char(parameters,IPOPT_PARAM_MU_STRATEGY
        ,(SlvParameterInitChar){{"mu_strategy"
            ,"Update strategy for barrier parameter",6
            ,"Determines which barrier parameter update strategy is to be used."
            " 'monotone' is the monotone (Fiacco-McCormick) strategy;"
            " 'adaptive' is the adaptive update strategy."
        }, "monotone"}, (char *[]){
            "monotone","adaptive",NULL
        }
    );

    slv_param_bool(parameters,IPOPT_PARAM_SAFEEVAL
        ,(SlvParameterInitBool){{"safeeval"
            ,"Use safe evaluation?",1
            ,"Use 'safe' function evaluation routines (TRUE) or allow ASCEND to "
            "throw SIGFPE errors which will then halt integration (FALSE)."
        }, FALSE}
    );

    asc_assert(parameters->num_parms==IPOPT_PARAMS);

    return 1;
}

static void ipopt_get_parameters(slv_system_t server, SlvClientToken asys
        , slv_parameters_t *parameters
){
    IpoptSystem *sys;
    UNUSED_PARAMETER(server);

    sys = SYS(asys);
    //if(check_system(sys)) return;
    mem_copy_cast(&(sys->p),parameters,sizeof(slv_parameters_t));
}


static void ipopt_set_parameters(slv_system_t server, SlvClientToken asys
        ,slv_parameters_t *parameters
){
    IpoptSystem *sys;
    UNUSED_PARAMETER(server);

    sys = SYS(asys);
    //if (check_system(sys)) return;
    mem_copy_cast(parameters,&(sys->p),sizeof(slv_parameters_t));
}

/*------------------------------------------------------------------------------
  EVALUATION AND RESULT HOOK FUNCTIONS
*/

/**
    update the model with new 'x' vector.
    @return 0 on success.
*/
int ipopt_update_model(IpoptSystem *sys, const double *x){
    ERROR_REPORTER_HERE(ASC_PROG_ERR,"Not implemented");
    return 1; /* error */
}

/** Function to evaluate the objective function f(x).
    @return 1 on success, 0 on failure

    @param n (in), the number of variables in the problem (dimension of 'x').
    @param x (in), the values for the primal variables, 'x' , at which 'f(x)' is to be evaluated.
    @param new_x (in), false if any evaluation method was previously called with the same values in 'x', true otherwise.
    @param obj_value (out) the value of the objective function ('f(x)').
*/
Bool ipopt_eval_f(Index n, Number *x, Bool new_x,  Number *obj_value, void *user_data){
    IpoptSystem *sys;
    sys = SYS(user_data);
    int res;

    asc_assert(n==sys->n);
    asc_assert(sys->obj!=NULL);

    if(new_x){
        res = ipopt_update_model(sys,x);
        if(res)return 0; /* fail model update */
    }

    sys->calc_ok = TRUE;

    *obj_value = relman_eval(sys->obj,&(sys->calc_ok),SLV_PARAM_BOOL(&(sys->p),IPOPT_PARAM_SAFEEVAL));

    return sys->calc_ok;
}

/**
    @return 1 on success
*/
Bool ipopt_eval_grad_f(Index n, Number* x, Bool new_x, Number* grad_f, void *user_data){
    IpoptSystem *sys;
    sys = SYS(user_data);
    int j, res, len;
    int count;
    double *derivatives;
    int *variables;
    static var_filter_t vfilter = {
        VAR_ACTIVE | VAR_INCIDENT | VAR_SVAR
        ,VAR_ACTIVE | VAR_INCIDENT | VAR_SVAR | VAR_FIXED
    };

    asc_assert(n==sys->n);
    asc_assert(sys->obj);

    if(new_x){
        res = ipopt_update_model(sys,x);
        if(res)return 0; /* fail model update */
    }


    /* evaluate grad_f(x) somehow */
    for(j=0; j<n; ++j){
        grad_f[j] = 0;
    }
    
    len = rel_n_incidences(sys->obj);
    variables = ASC_NEW_ARRAY(int,len);
    derivatives = ASC_NEW_ARRAY(double,len);

    relman_diff2(
        sys->obj,&vfilter,derivatives,variables
        , &count,SLV_PARAM_BOOL(&(sys->p),IPOPT_PARAM_SAFEEVAL)
    );

    for(j=0; j<len; ++j){
        grad_f[variables[j]] = derivatives[j];
    }

    ASC_FREE(variables);
    ASC_FREE(derivatives);

    return 1; /* success, presumably */
}

Bool ipopt_eval_g(Index n, Number* x, Bool new_x, Index m, Number *g, void *user_data){
    IpoptSystem *sys;
    sys = SYS(user_data);
    int i, res;

    asc_assert(n==sys->n);
    asc_assert(m==sys->m);

    if(new_x){
        res = ipopt_update_model(sys,x);
        if(res)return 0; /* fail model update */
    }

    for(i=0; i<m; ++i){
        g[i] = 0;
    }

    return 0; /* fail: not yet implemented */
}

Bool ipopt_eval_jac_g(Index n, Number* x, Bool new_x, Index m
        , Index nele_jac, Index* iRow, Index *jCol, Number* values
        , void *user_data
){
    IpoptSystem *sys;
    sys = SYS(user_data);
    int i,res;

    asc_assert(n==sys->n);
    asc_assert(nele_jac==sys->nnzJ);
    asc_assert(m==sys->m);

    if(new_x){
        res = ipopt_update_model(sys,x);
        if(res)return 0; /* fail model update */
    }

    /* loop through the constraints */
    for(i=0; i<m; ++i){
        /* get derivatives for constraint i */
        /* insert the derivatives into the matrix in row i, columns j */
    }

    return 0; /* fail: not yet implemented */
}

Bool ipopt_eval_h(Index n, Number* x, Bool new_x
        , Number obj_factor, Index m, Number* lambda
        , Bool new_lambda, Index nele_hess, Index* iRow
        , Index* jCol, Number* values
        , void *user_data
){
    if(iRow != NULL){
        asc_assert(jCol !=NULL);
        asc_assert(x==NULL); asc_assert(lambda==NULL); asc_assert(values==NULL);

        /* identify the sparsity structure of the Hessian (note: only the lower-
        left part is required by IPOPT , because the Hessian is symmetric) */

        /*
        for(i=0; i<nvars; ++i){
            for(j=i; j<nvars; ++j){
                if(relman_hess_expr(sys->obj, sys->vlist[i], sys->vlist[j]) != 0){
                    iRow[nele_hess] = i;
                    jCol[nele_hess] = j;
                    nele_hess++
                }
            }
        }
        */                              
                
    }else{
        asc_assert(jCol==NULL);
        asc_assert(x!=NULL); asc_assert(lambda!=NULL); asc_assert(values!=NULL);

        /* evaluate the Hessian matrix */
    }
    
    return 0; /* fail: not yet implemented */
}

/*------------------------------------------------------------------------------
  SOLVE ROUTINES
*/

static int ipopt_presolve(slv_system_t server, SlvClientToken asys){
    IpoptSystem *sys;
    int max, i;

    CONSOLE_DEBUG("PRESOLVE");

    sys = SYS(asys);
    ipopt_iteration_begins(sys);
    //check_system(sys);

    asc_assert(sys->vlist && sys->rlist);

    /** @TODO work out if matrix creation is not again needed */

    /** @TODO slv_sort_rels_and_vars(server,&(sys->m),&(sys->n)); */

    /* set all relations as being 'unsatisfied' to start with... */
    for(i=0; i < sys->rtot; ++i){
        rel_set_satisfied(sys->rlist[i],FALSE);
    }

    sys->obj = slv_get_obj_relation(server); /*may have changed objective*/
    if(!sys->obj){
        ERROR_REPORTER_HERE(ASC_PROG_ERR,"No objective function was specified");
        return -3;
    }

    CONSOLE_DEBUG("got objective rel %p",sys->obj);

    /** @TODO calculate nnz for hessian matrix */

    /* need to provide sparsity structure for hessian matrix? */

    max = relman_obj_direction(sys->obj);
    if(max==-1){
        CONSOLE_DEBUG("this is a MINIMIZE problem");
    }else{
        CONSOLE_DEBUG("this is a MAXIMIZE problem");
    }

    CONSOLE_DEBUG("got %d relations and %d vars in system", sys->rtot, sys->vtot);
    /* calculate number of non-zeros in the Jacobian matrix for the constraint equations */

    sys->nnzJ = relman_jacobian_count(sys->rlist, sys->rtot, &(sys->vfilt), &(sys->rfilt), &max);

    CONSOLE_DEBUG("got %d non-zeros in constraint Jacobian", sys->nnzJ);
    
    /* need to provide sparsity structure for jacobian? */


#if 0
    if(sys->presolved > 0) { /* system has been presolved before */
        if(!conopt_dof_changed(sys) /*no changes in fixed or included flags*/
                && sys->p.partition == sys->J.old_partition
                && sys->obj == sys->old_obj
        ){
            matrix_creation_needed = 0;
            CONOPT_CONSOLE_DEBUG("YOU JUST AVOIDED MATRIX DESTRUCTION/CREATION");
        }
    }
#endif

#if 0
    // check all this...

        sys->presolved = 1; /* full presolve recognized here */
        sys->resolve = 0;   /* initialize resolve flag */

        sys->J.old_partition = sys->p.partition;
        sys->old_obj = sys->obj;

        slv_sort_rels_and_vars(server,&(sys->con.m),&(sys->con.n));
        CONOPT_CONSOLE_DEBUG("FOUND %d CONSTRAINTS AND %d VARS",sys->con.m,sys->con.n);
        if (sys->obj != NULL) {
            CONOPT_CONSOLE_DEBUG("ADDING OBJECT AS A ROW");
            sys->con.m++; /* treat objective as a row */
        }

        cntvect = ASC_NEW_ARRAY(int,COIDEF_Size());
        COIDEF_Ini(cntvect);
        sys->con.cntvect = cntvect;
        CONOPT_CONSOLE_DEBUG("NUMBER OF CONSTRAINTS = %d",sys->con.m);
        COIDEF_NumVar(cntvect, &(sys->con.n));
        COIDEF_NumCon(cntvect, &(sys->con.m));
        sys->con.nz = num_jacobian_nonzeros(sys, &(sys->con.maxrow));
        COIDEF_NumNZ(cntvect, &(sys->con.nz));
        COIDEF_NumNlNz(cntvect, &(sys->con.nz));

        sys->con.base = 0;
        COIDEF_Base(cntvect,&(sys->con.base));
        COIDEF_ErrLim(cntvect, &(DOMLIM));
        COIDEF_ItLim(cntvect, &(ITER_LIMIT));

        if(sys->obj!=NULL){
            sys->con.optdir = relman_obj_direction(sys->obj);
            sys->con.objcon = sys->con.m - 1; /* objective will be last row */
            CONOPT_CONSOLE_DEBUG("SETTING OBJECTIVE CONSTRAINT TO BE %d",sys->con.objcon);
        }else{
            sys->con.optdir = 0;
            sys->con.objcon = 0;
        }
        COIDEF_OptDir(cntvect, &(sys->con.optdir));
        COIDEF_ObjCon(cntvect, &(sys->con.objcon));

        temp = 0;
        COIDEF_StdOut(cntvect, &temp);

        int debugfv = 1;
        COIDEF_DebugFV(cntvect, &debugfv);

        destroy_vectors(sys);
        destroy_matrices(sys);
        create_matrices(server,sys);
        create_vectors(sys);

        sys->s.block.current_reordered_block = -2;
    }

    //...

    if( matrix_creation_needed ) {
        destroy_array(sys->s.cost);
        sys->s.cost = create_zero_array(sys->s.costsize,struct slv_block_cost);
        for( ind = 0; ind < sys->s.costsize; ++ind ) {
            sys->s.cost[ind].reorder_method = -1;
        }
    } else {
        reset_cost(sys->s.cost,sys->s.costsize);
    }

#endif

    /* Reset status */
    sys->s.iteration = 0;
    sys->s.cpu_elapsed = 0.0;
    sys->s.converged = sys->s.diverged = sys->s.inconsistent = FALSE;
    sys->s.block.previous_total_size = 0;
    sys->s.costsize = 1+sys->s.block.number_of;


    /* set to go to first unconverged block */
    sys->s.block.current_block = -1;
    sys->s.block.current_size = 0;
    sys->s.calc_ok = TRUE;
    sys->s.block.iteration = 0;
    sys->obj_val =  MAXDOUBLE/2000.0;

    ipopt_iteration_ends(sys);

    CONSOLE_DEBUG("Reset status");

    /* sys->s.cost[sys->s.block.number_of].time=sys->s.cpu_elapsed; */

    ERROR_REPORTER_HERE(ASC_PROG_ERR,"presolve completed");
    return 0;
}


static int ipopt_solve(slv_system_t server, SlvClientToken asys){
    IpoptSystem *sys;
    UNUSED_PARAMETER(server);
    enum ApplicationReturnStatus status;
    int ret, i, j;
    struct var_variable *var;

    sys = SYS(asys);

    double *x, *x_L, *x_U, *g_L, *g_U, *mult_x_L, *mult_x_U;

    CONSOLE_DEBUG("SOLVING...");

    /* set the number of variables and allocate space for the bounds */
    x_L = ASC_NEW_ARRAY(Number,sys->n);
    x_U = ASC_NEW_ARRAY(Number,sys->n);

    /* @TODO set the values for the variable bounds */
    int jj = 0;
    for(j = 0; j < sys->vtot; j++){
        var = sys->vlist[j];
        if(var_apply_filter(var,&(sys->vfilt))){
            x_L[jj] = var_lower_bound(var);
            x_U[jj] = var_upper_bound(var);
            jj++;
        }
    }

    /** @TODO set bounds on the constraints? */

    /* create the IpoptProblem */
    sys->nlp = CreateIpoptProblem(sys->n, x_L, x_U, sys->m, g_L, g_U, sys->nnzJ, sys->nnzH, 0/*index style=C*/, 
        &ipopt_eval_f, &ipopt_eval_g, &ipopt_eval_grad_f, 
        &ipopt_eval_jac_g, &ipopt_eval_h
    );
  
    /* We can free the memory now - the values for the bounds have been
    copied internally in CreateIpoptProblem */
    ASC_FREE(x_L);
    ASC_FREE(x_U);
    ASC_FREE(g_L);
    ASC_FREE(g_U);

    /* set some options */
    AddIpoptNumOption(sys->nlp, "tol", SLV_PARAM_BOOL(&(sys->p),IPOPT_PARAM_TOL));
    AddIpoptStrOption(sys->nlp, "mu_strategy", SLV_PARAM_CHAR(&(sys->p),IPOPT_PARAM_MU_STRATEGY));

    /* initial values */
    x = ASC_NEW_ARRAY(Number, sys->n);

    /** @TODO get values of 'x' from the model */

    /* allocate space to store the bound multipliers at the solution */
    mult_x_L = ASC_NEW_ARRAY(Number, sys->n);
    mult_x_U = ASC_NEW_ARRAY(Number, sys->n);

    /* solve the problem */
    status = IpoptSolve(sys->nlp, x, NULL, &sys->obj_val, NULL, mult_x_L, mult_x_U, NULL);

    /** @TODO update the sys->s.xxxxx flags based on value of 'status' */

    if (status == Solve_Succeeded) {
        CONSOLE_DEBUG("Solution of the primal variables, x");
        for (i=0; i<sys->n; i++) {
            CONSOLE_DEBUG("   x[%d] = %e\n", i, x[i]); 
        }

        CONSOLE_DEBUG("Solution of the bound multipliers, z_L and z_U");
        for (i=0; i<sys->n; i++) {
            CONSOLE_DEBUG("   z_L[%d] = %e", i, mult_x_L[i]); 
        }
        for (i=0; i<sys->n; i++) {
            CONSOLE_DEBUG("    z_U[%d] = %e", i, mult_x_U[i]); 
        }

        CONSOLE_DEBUG("Objective value");
        CONSOLE_DEBUG("    f(x*) = %e", sys->obj_val); 

        ret = 0; /* success */
    }else{
        ERROR_REPORTER_HERE(ASC_PROG_ERR,"Failed solve, unknown status");
        ret = 1; /* failure */
    }
 
    /* free allocated memory */
    FreeIpoptProblem(sys->nlp);
    ASC_FREE(x);
    ASC_FREE(mult_x_L);
    ASC_FREE(mult_x_U);

    return ret;
}

/**
    Prepare sys for entering an iteration, increasing the iteration counts
    and starting the clock.
*/
static void ipopt_iteration_begins(IpoptSystem *sys){
    sys->clock = tm_cpu_time();
    ++(sys->s.block.iteration);
    ++(sys->s.iteration);
}


/*
    Prepare sys for exiting an iteration, stopping the clock and recording
    the cpu time.
*/
static void ipopt_iteration_ends(IpoptSystem *sys){
    double cpu_elapsed;   /* elapsed this iteration */

    cpu_elapsed = (double)(tm_cpu_time() - sys->clock);
    sys->s.block.cpu_elapsed += cpu_elapsed;
    sys->s.cpu_elapsed += cpu_elapsed;
}


static int ipopt_iterate(slv_system_t server, SlvClientToken asys){
    UNUSED_PARAMETER(server);
    CONSOLE_DEBUG("ITERATING...");
    ERROR_REPORTER_HERE(ASC_PROG_ERR,"Not implemented");
    return 1;
}

static int ipopt_resolve(slv_system_t server, SlvClientToken asys){
    UNUSED_PARAMETER(server);

    /* if implementing this, use the 'warm start' thing in IPOPT */

    /* provide initial values of the 'multipliers' */

    ERROR_REPORTER_HERE(ASC_PROG_ERR,"Not implemented");
    return 1;
}

static const SlvFunctionsT ipopt_internals = {
    67
    ,"IPOPT"
    ,ipopt_create
    ,ipopt_destroy
    ,ipopt_eligible_solver
    ,ipopt_get_default_parameters
    ,ipopt_get_parameters
    ,ipopt_set_parameters
    ,ipopt_get_status
    ,ipopt_solve
    ,ipopt_presolve
    ,ipopt_iterate
    ,ipopt_resolve
    ,NULL
    ,NULL
    ,NULL
};

int ipopt_register(void){
    return solver_register(&ipopt_internals);
}