brachistochrone.c

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/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
/*                                                                           */
/*                  This file is part of the program and library             */
/*         SCIP --- Solving Constraint Integer Programs                      */
/*                                                                           */
/*    Copyright (C) 2002-2020 Konrad-Zuse-Zentrum                            */
/*                            fuer Informationstechnik Berlin                */
/*                                                                           */
/*  SCIP is distributed under the terms of the ZIB Academic License.         */
/*                                                                           */
/*  You should have received a copy of the ZIB Academic License              */
/*  along with SCIP; see the file COPYING. If not visit scipopt.org.         */
/*                                                                           */
/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */

/**@file   brachistochrone.c
 * @brief  Computing a minimum-time trajectory for a particle to move from point A to B under gravity only
 * @author Anass Meskini
 * @author Stefan Vigerske
 *
 * This is an example that uses expressions and expression trees to set up non-linear constraints in SCIP when used as
 * a callable library. This example implements a discretized model to obtain the trajectory associated with the shortest
 * time to go from point A to B for a particle under gravity only.
 *
 * The model:
 *
 * Given \f$N\f$ number of points for the discretisation of the trajectory, we can approximate the time to go from
 * \f$(x_0,y_0)\f$ to \f$ (x_N,y_N)\f$ for a given trajectory by \f[T = \sqrt{\frac{2}{g}}
 * \sum_{0}^{N-1} \frac{\sqrt{(y_{i+1} - y_i)^2 + (x_{i+1} - x_i)^2}}{\sqrt{1-y_{i+1}} + \sqrt{1 - y_i}}.\f]
 * We seek to minimize \f$T\f$.
 * A more detailed description of the model can be found in the brachistochrone directory of http://scipopt.org/workshop2018/pyscipopt-exercises.tgz
 *
 * Passing this equation as it is to SCIP does not lead to satisfying results, though, so we reformulate a bit.
 * Let \f$t_i \geq \frac{\sqrt{(y_{i+1} - y_i)^2 + (x_{i+1} - x_i)^2}}{\sqrt{1-y_{i+1}} + \sqrt{1 - y_i}}\f$.
 * To avoid a potential division by zero, we rewrite this as
 * \f$t_i (\sqrt{1-y_{i+1}} + \sqrt{1 - y_i}) \geq \sqrt{(y_{i+1} - y_i)^2 + (x_{i+1} - x_i)^2}, t_i\geq 0\f$.
 * Further, introduce \f$v_i \geq 0\f$ such that \f$v_i \geq \sqrt{(y_{i+1} - y_i)^2 + (x_{i+1} - x_i)^2}\f$.
 * Then we can state the optimization problem as
 * \f{align}{ \min \;& \sqrt{\frac{2}{g}} \sum_{i=0}^{N-1} t_i \\
 *   \text{s.t.} \; & t_i (\sqrt{1-y_{i+1}} + \sqrt{1 - y_i}) \geq v_i \\
 *     & v_i^2 \geq (y_{i+1} - y_i)^2 + (x_{i+1} - x_i)^2 \\
 *     & t_i \geq 0,\; v_i \geq 0 \\
 *     & i = 0, \ldots, N-1
 * \f}
 *
 * Further, we can require that the particle moves only in direction horizontally, that is
 * \f$x_i \leq x_{i+1}\f$ if \f$x_0 \leq x_N\f$, and \f$x_{i+1} \leq x_{i}\f$, otherwise,
 * and that it will not move higher than the start-coordinate.
 */

/*--+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----8----+----9----+----0----+----1----+----2*/

#include <stdio.h>
#include <stdlib.h>

#include "scip/pub_misc.h"
#include "scip/scip.h"
#include "scip/scipdefplugins.h"

/* default start and end points */
#define Y_START  1.0
#define Y_END    0.0
#define X_START  0.0
#define X_END   10.0

/** sets up problem */
static
SCIP_RETCODE setupProblem(
   SCIP*                 scip,               /**< SCIP data structure */
   unsigned int          n,                  /**< number of points for discretization */
   SCIP_Real*            coord,              /**< array containing [y(0), y(N), x(0), x(N)] */
   SCIP_VAR***           xvars,              /**< buffer to store pointer to x variables array */
   SCIP_VAR***           yvars               /**< buffer to store pointer to y variables array */
   )
{
   /* variables:
    * t[i] i=0..N-1, such that: value function=sum t[i]
    * v[i] i=0..N-1, such that: v_i = ||(x_{i+1},y_{i+1})-(x_i,y_i)||_2
    * y[i] i=0..N, projection of trajectory on y-axis
    * x[i] i=0..N, projection of trajectory on x-axis
    */
   SCIP_VAR** t;
   SCIP_VAR** v;
   SCIP_VAR** x;
   SCIP_VAR** y;

   char namet[SCIP_MAXSTRLEN];
   char namev[SCIP_MAXSTRLEN];
   char namey[SCIP_MAXSTRLEN];
   char namex[SCIP_MAXSTRLEN];

   SCIP_Real ylb;
   SCIP_Real yub;
   SCIP_Real xlb;
   SCIP_Real xub;

   /* an upper bound for v */
   SCIP_Real maxdistance = 10.0 * sqrt(SQR(coord[1]-coord[0]) + SQR(coord[3]-coord[2]));
   unsigned int i;

   /* create empty problem */
   SCIP_CALL( SCIPcreateProbBasic(scip, "brachistochrone") );

   /* allocate arrays of SCIP_VAR* */
   SCIP_CALL( SCIPallocBufferArray(scip, &t, (size_t) n ) );
   SCIP_CALL( SCIPallocBufferArray(scip, &v, (size_t) n ) );
   SCIP_CALL( SCIPallocMemoryArray(scip, &y, (size_t) n + 1) );
   SCIP_CALL( SCIPallocMemoryArray(scip, &x, (size_t) n + 1) );
   *xvars = x;
   *yvars = y;

   /* create and add variables to the problem and set the initial and final point constraints through upper and lower
    * bounds
    */
   for( i = 0; i < n+1; ++i )
   {
      /* setting up the names of the variables */
      if( i < n )
      {
         SCIPsnprintf(namet, SCIP_MAXSTRLEN, "t(%d)", i);
         SCIPsnprintf(namev, SCIP_MAXSTRLEN, "v(%d)", i);
      }
      SCIPsnprintf(namey, SCIP_MAXSTRLEN, "y(%d)", i);
      SCIPsnprintf(namex, SCIP_MAXSTRLEN, "x(%d)", i);

      /* fixing y(0), y(N), x(0), x(N) through lower and upper bounds */
      if( i == 0 )
      {
         ylb = coord[0];
         yub = coord[0];
         xlb = coord[2];
         xub = coord[2];
      }
      else if( i == n )
      {
         ylb = coord[1];
         yub = coord[1];
         xlb = coord[3];
         xub = coord[3];
      }
      else
      {
         /* constraint the other variables to speed up solving */
         ylb = -SCIPinfinity(scip);
         yub = coord[0];
         xlb = MIN(coord[2], coord[3]);
         xub = MAX(coord[2], coord[3]);
      }

      if( i < n )
      {
         SCIP_CALL( SCIPcreateVarBasic(scip, &t[i], namet, 0.0, SCIPinfinity(scip), sqrt(2.0/9.80665), SCIP_VARTYPE_CONTINUOUS) );
         SCIP_CALL( SCIPcreateVarBasic(scip, &v[i], namev, 0.0, maxdistance, 0.0, SCIP_VARTYPE_CONTINUOUS) );
      }
      SCIP_CALL( SCIPcreateVarBasic(scip, &y[i], namey, ylb, yub, 0.0, SCIP_VARTYPE_CONTINUOUS) );
      SCIP_CALL( SCIPcreateVarBasic(scip, &x[i], namex, xlb, xub, 0.0, SCIP_VARTYPE_CONTINUOUS) );

      if( i < n )
      {
         SCIP_CALL( SCIPaddVar(scip, t[i]) );
         SCIP_CALL( SCIPaddVar(scip, v[i]) );
      }
      SCIP_CALL( SCIPaddVar(scip, y[i]) );
      SCIP_CALL( SCIPaddVar(scip, x[i]) );
   }

   /* add constraints
    *
    * t(i) * sqrt(1 - y(i+1)) + t(i) * sqrt(1 - y(i)) >= v(i)
    * v(i)^2 >= (y(i+1) - y(i))^2 + (x(i+1) - x(i))^2
    * in the loop, create the i-th constraint
    */
   {
      SCIP_CONS* cons;
      char consname[SCIP_MAXSTRLEN];

      /* child expressions:
       * yplusexpr: expression for y[i+1]
       * yexpr: expression for y[i]
       * texpr, texpr2: expression for t[i]
       */
      SCIP_EXPR* yplusexpr;
      SCIP_EXPR* yexpr;
      SCIP_EXPR* texpr;
      SCIP_EXPR* texpr2;

      /* intermediary expressions */
      SCIP_EXPR* expr1;
      SCIP_EXPR* expr2;
      SCIP_EXPR* expr3;
      SCIP_EXPR* expr4;
      SCIP_EXPR* expr5;
      SCIP_EXPR* expr6;

      /* trees to hold the non-linear parts of the constraint */
      SCIP_EXPRTREE* exprtree[2];

      SCIP_Real minusone = -1.0;

      /* at each iteration create an expression for the non-linear part of the i-th constraint and add it the problem */
      for( i = 0; i < n; ++i )
      {
         /* vars to be added to the exprtree in this step of the loop */
         SCIP_VAR* varstoadd[3] = { y[i+1], y[i], t[i] };

         /* create expressions meant to be child expressions in the tree. give different indexes to the expressions to
          * assign the correct variables to them later
          */
         SCIP_CALL( SCIPexprCreate(SCIPblkmem(scip), &yplusexpr, SCIP_EXPR_VARIDX, 0) );
         SCIP_CALL( SCIPexprCreate(SCIPblkmem(scip), &yexpr, SCIP_EXPR_VARIDX, 1) );
         SCIP_CALL( SCIPexprCreate(SCIPblkmem(scip), &texpr, SCIP_EXPR_VARIDX, 2) );
         SCIP_CALL( SCIPexprCreate(SCIPblkmem(scip), &texpr2, SCIP_EXPR_VARIDX, 2) );

         /* set up the i-th constraint
          * expr1: 1 - y[i+1]
          * expr2: 1 - y[i]
          * expr3: sqrt(1 - y[i+1])
          * expr4: sqrt(1 - y[i])
          * expr5: t[i] * sqrt(1 - y[i+1])
          * expr6: t[i] * sqrt(1 - y[i])
          */
         SCIP_CALL( SCIPexprCreateLinear(SCIPblkmem(scip), &expr1, 1, &yplusexpr, &minusone, 1.0) );
         SCIP_CALL( SCIPexprCreateLinear(SCIPblkmem(scip), &expr2, 1, &yexpr, &minusone, 1.0) );
         SCIP_CALL( SCIPexprCreate(SCIPblkmem(scip), &expr3, SCIP_EXPR_SQRT, expr1) );
         SCIP_CALL( SCIPexprCreate(SCIPblkmem(scip), &expr4, SCIP_EXPR_SQRT, expr2) );
         SCIP_CALL( SCIPexprCreate(SCIPblkmem(scip), &expr5, SCIP_EXPR_MUL, expr3, texpr) );
         SCIP_CALL( SCIPexprCreate(SCIPblkmem(scip), &expr6, SCIP_EXPR_MUL, expr4, texpr2) );

         /* create the expression trees with expr5 and expr6 as root */
         SCIP_CALL( SCIPexprtreeCreate(SCIPblkmem(scip), &exprtree[0], expr5, 3, 0, NULL) );
         SCIP_CALL( SCIPexprtreeCreate(SCIPblkmem(scip), &exprtree[1], expr6, 3, 0, NULL) );
         SCIP_CALL( SCIPexprtreeSetVars(exprtree[0], 3, varstoadd) );
         SCIP_CALL( SCIPexprtreeSetVars(exprtree[1], 3, varstoadd) );

         /* use the tree and a linear term to add the constraint exprtree0 + exprtree1 - v_i >= 0 */
         SCIPsnprintf(consname, SCIP_MAXSTRLEN, "timestep(%d)", i);
         SCIP_CALL( SCIPcreateConsBasicNonlinear(scip, &cons, consname, 1, &v[i],
                                                 &minusone, 2, exprtree, NULL, 0.0, SCIPinfinity(scip)) );

         /* add the constraint to the problem, release the constraint and free the trees */
         SCIP_CALL( SCIPaddCons(scip, cons) );
         SCIP_CALL( SCIPreleaseCons(scip, &cons) );
         SCIP_CALL( SCIPexprtreeFree(&exprtree[0]) );
         SCIP_CALL( SCIPexprtreeFree(&exprtree[1]) );

         /* add constraint v_i^2 >= (y_{i+1}^2 - 2*y_{i+1}y_i + y_i^2) + (x_{i+1}^2 - 2*x_{i+1}x_i + x_i^2)
          * SCIP should recognize that this can be formulated as SOC and do this reformulation
          */
         SCIPsnprintf(consname, SCIP_MAXSTRLEN, "steplength(%d)", i);
         SCIP_CALL( SCIPcreateConsBasicQuadratic(scip, &cons, consname, 0, NULL, NULL, 0, NULL, NULL, NULL, -SCIPinfinity(scip), 0.0) );
         SCIP_CALL( SCIPaddSquareCoefQuadratic(scip, cons, y[i], 1.0) );
         SCIP_CALL( SCIPaddSquareCoefQuadratic(scip, cons, y[i+1], 1.0) );
         SCIP_CALL( SCIPaddSquareCoefQuadratic(scip, cons, x[i], 1.0) );
         SCIP_CALL( SCIPaddSquareCoefQuadratic(scip, cons, x[i+1], 1.0) );
         SCIP_CALL( SCIPaddBilinTermQuadratic(scip, cons, y[i], y[i+1], -2.0) );
         SCIP_CALL( SCIPaddBilinTermQuadratic(scip, cons, x[i], x[i+1], -2.0) );
         SCIP_CALL( SCIPaddSquareCoefQuadratic(scip, cons, v[i], -1.0) );

         /* add the constraint to the problem and forget it */
         SCIP_CALL( SCIPaddCons(scip, cons) );
         SCIP_CALL( SCIPreleaseCons(scip, &cons) );

         /* add constraint x[i] <= x[i+1], if x[0] < x[N], otherwise add x[i+1] <= x[i] */
         SCIPsnprintf(consname, SCIP_MAXSTRLEN, "xorder(%d)", i);
         SCIP_CALL( SCIPcreateConsBasicLinear(scip, &cons, consname, 0, NULL, NULL, -SCIPinfinity(scip), 0.0) );
         SCIP_CALL( SCIPaddCoefLinear(scip, cons, x[i],   coord[2] < coord[3] ?  1.0 : -1.0) );
         SCIP_CALL( SCIPaddCoefLinear(scip, cons, x[i+1], coord[2] < coord[3] ? -1.0 :  1.0) );

         SCIP_CALL( SCIPaddCons(scip, cons) );
         SCIP_CALL( SCIPreleaseCons(scip, &cons) );
      }
   }

   /* release intermediate variables */
   for( i = 0; i < n; ++i )
   {
      SCIP_CALL( SCIPreleaseVar(scip, &t[i]) );
      SCIP_CALL( SCIPreleaseVar(scip, &v[i]) );
   }

   /* free arrays allocated */
   SCIPfreeBufferArray(scip, &t);
   SCIPfreeBufferArray(scip, &v);

   return SCIP_OKAY;
}

/** plots solution by use of gnuplot */
static
void visualizeSolutionGnuplot(
   SCIP*                 scip,               /**< SCIP data structure */
   SCIP_SOL*             sol,                /**< solution to plot */
   unsigned int          n,                  /**< number of points for discretization */
   SCIP_VAR**            x,                  /**< x coordinates */
   SCIP_VAR**            y                   /**< y coordinates */
   )
{
#if _POSIX_C_SOURCE < 2
   SCIPinfoMessage(scip, NULL, "No POSIX version 2. Try http://distrowatch.com/.");
#else
   FILE* stream;
   unsigned int i;

   /* -p (persist) to keep the plot open after gnuplot terminates (if terminal is not dumb) */
   stream = popen("gnuplot -p", "w");
   if( stream == NULL )
   {
      SCIPerrorMessage("Could not open pipe to gnuplot.\n");
      return;
   }
   /* take out this line to get a non-ascii plot */
   fputs("set terminal dumb\n", stream);

   fprintf(stream, "plot '-' smooth csplines title \"Time = %.4fs\"\n", SCIPgetSolOrigObj(scip, sol));
   for( i = 0; i < n+1; ++i )
      fprintf(stream, "%g %g\n", SCIPgetSolVal(scip, sol, x[i]), SCIPgetSolVal(scip, sol, y[i]));
   fputs("e\n", stream);

   pclose(stream);
#endif
}

/** runs the brachistochrone example*/
static
SCIP_RETCODE runBrachistochrone(
   unsigned int          n,                  /**< number of points for discretization */
   SCIP_Real*            coord               /**< array containing [y(0), y(N), x(0), x(N)] */
   )
{
   SCIP* scip;
   SCIP_VAR** y;
   SCIP_VAR** x;
   unsigned int i;

   assert(n >= 2);

   SCIP_CALL( SCIPcreate(&scip) );
   SCIP_CALL( SCIPincludeDefaultPlugins(scip) );

   SCIPinfoMessage(scip, NULL, "\n");
   SCIPinfoMessage(scip, NULL, "**********************************************\n");
   SCIPinfoMessage(scip, NULL, "* Running Brachistochrone Problem            *\n");
   SCIPinfoMessage(scip, NULL, "* between A=(%g,%g) and B=(%g,%g) with %d points *\n", coord[2], coord[0], coord[3], coord[1], n);
   SCIPinfoMessage(scip, NULL, "**********************************************\n");
   SCIPinfoMessage(scip, NULL, "\n");

   /* set gap at which SCIP will stop */
   SCIP_CALL( SCIPsetRealParam(scip, "limits/gap", 0.05) );

   SCIP_CALL( setupProblem(scip, n, coord, &x, &y) );

   SCIPinfoMessage(scip, NULL, "Original problem:\n");
   SCIP_CALL( SCIPprintOrigProblem(scip, NULL, "cip", FALSE) );

   SCIPinfoMessage(scip, NULL, "\nSolving...\n");
   SCIP_CALL( SCIPsolve(scip) );

   if( SCIPgetNSols(scip) > 0 )
   {
      SCIPinfoMessage(scip, NULL, "\nSolution:\n");
      SCIP_CALL( SCIPprintSol(scip, SCIPgetBestSol(scip), NULL, FALSE) );

      visualizeSolutionGnuplot(scip, SCIPgetBestSol(scip), n, x, y);
   }

   /* release problem variables */
   for( i = 0; i < n+1; ++i )
   {
      SCIP_CALL( SCIPreleaseVar(scip, &y[i]) );
      SCIP_CALL( SCIPreleaseVar(scip, &x[i]) );
   }

   /* free arrays allocated */
   SCIPfreeMemoryArray(scip, &x);
   SCIPfreeMemoryArray(scip, &y);

   SCIP_CALL( SCIPfree(&scip) );

   return SCIP_OKAY;
}

/** main method starting SCIP */
int main(
   int                   argc,               /**< number of arguments from the shell */
   char**                argv                /**< arguments: number of points and end coordinates y(N), x(N)*/
   )
{
   SCIP_RETCODE retcode;

   /* setting up default problem parameters */
   unsigned int n = 3;
   SCIP_Real coord[4] = { Y_START, Y_END, X_START, X_END };

   /* change some parameters if given as arguments */
   if( argc == 4 || argc == 2  )
   {
       char *end1 = NULL;
       char *end2 = NULL;
       char *end3 = NULL;

       n = strtol(argv[1], &end1, 10);
       if( argc == 4 )
       {
          coord[1] = strtof(argv[2], &end2);
          coord[3] = strtof(argv[3], &end3);
       }

       if( end1 == argv[1] || ( argc == 4 && ( end2 == argv[2] || end3 == argv[3] ) ) )
       {
          fprintf(stderr, "Error: expected real values as arguments.\n");
          return EXIT_FAILURE;
       }
   }
   else if( argc != 1 )
   {
       fprintf(stderr, "Usage: %s [<N> [<y(N)> <x(N)>]]\n", argv[0]);
       return EXIT_FAILURE;
   }

   /* check that y(0) > y(N) */
   if( coord[0] <= coord[1] )
   {
      fprintf(stderr, "Error: expected y(N) < 1.0\n");
      return EXIT_FAILURE;
   }

   retcode = runBrachistochrone(n, coord);

   /* evaluate return code of the SCIP process */
   if( retcode != SCIP_OKAY )
   {
      /* write error back trace */
      SCIPprintError(retcode);
      return EXIT_FAILURE;
   }

   return EXIT_SUCCESS;
}