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

# include "tsp_greedy.h"

/******************************************************************************/

int *tsp_greedy ( int n, double *distance )

/******************************************************************************/
/*
  Purpose:

    tsp_greedy() seeks an optimal traveling salesperson path by the greedy method.

  Licensing:

    This code is distributed under the MIT license.

  Modified:

    02 June 2026

  Author:

    John Burkardt

  Input:

    int n: the number of cities.

    double *distance: the NxN table of pairwise distances between cities.

  Output:

    int *path: a list of the order in which the N cities should be visited.
*/
{
  double cost;
  double cost_best;
  int i;
  int *path;
  int *path_best;
  int start;

  printf ( "\n" );
  printf ( "tsp_greedy():\n" );
  printf ( "  For the traveling salesperson problem, find a greedy\n" );
  printf ( "  solution by choosing a starting city, and then constructing\n" );
  printf ( "  the tour by always moving to the nearest unvisited city.\n" );
/*
  Initialize:
    the best cost so far is Infinity;
    the best tour so far is 1, 2, ..., n.
*/
  cost_best = HUGE_VAL;
  path_best = ( int * ) malloc ( n * sizeof ( int ) );
  for ( i = 0; i < n; i++ )
  {
    path_best[i] = i;
  }
/*
  Try starting at city START and taking greedy steps.
*/
  for ( start = 0; start < n; start++ )
  {
    path = path_greedy ( n, distance, start );
    cost = path_cost ( n, distance, path );

    if ( cost < cost_best )
    {
      for ( i = 0; i < n; i++ )
      {
        path_best[i] = path[i];
      }
      cost_best = cost;
      printf ( "  Route starting at city %d has cost = %g\n", start, cost_best );
    } 
    free ( path );
  }

  return path_best;
}
/******************************************************************************/

double path_cost ( int n, double *distance, int *p )

/******************************************************************************/
/*
  Purpose:

    path_cost() evaluates the cost of a round trip.

  Licensing:

    This code is distributed under the MIT license.

  Modified:

    02 June 2026

  Author:

    John Burkardt

  Input:

    int n: the number of cities.

    double DISTANCE(N,N), the city to city distance table.

    int P(N), a permutation of 1:N, the route.

  Output:

    double COST, the cost of the route.
*/
{
  double cost;
  int from;
  int to;

  cost = 0.0;
  from = n - 1;
  for ( to = 0; to < n; to++ )
  {
    cost = cost + distance [ p[from] + p[to] * n ];
    from = to;
  }

  return cost;
}
/******************************************************************************/

int *path_greedy ( int n, double *distance, int start )

/******************************************************************************/
/*
  Purpose:

    path_greedy() finds a greedy route for a given start.

  Licensing:

    This code is distributed under the MIT license.

  Modified:

    02 June 2026

  Author:

    John Burkardt

  Input:

    int n: the number of cities.

    double DISTANCE(N,N), the city to city distance table.

    int start: the starting city.

  Output:

    int P[N]: the greedy route that starts at START.
*/
{
  double *d;
  double dmin;
  int from;
  int i;
  int j;
  int k;
  int *p;
  int to;
/*
  Make a copy of the distance table that we can modify.
*/
  d = ( double * ) malloc ( n * n * sizeof ( double ) );

  for ( i = 0; i < n; i++ )
  {
    for ( j = 0; j < n; j++ )
    {
      d[i+j*n] = distance[i+j*n];
    }
  }
  for ( i = 0; i < n; i++ )
  {
    d[i+i*n] = HUGE_VAL;
  }
/*
  At FROM, find nearest unvisited city TO.
*/
  p = ( int * ) malloc ( n * sizeof ( int ) );

  for ( j = 0; j < n; j++ )
  {
    if ( j == 0 )
    {
      to = start;
    }
    else
    {
      to = 0;
      dmin = d[from+to*n];
      for ( k = 1; k < n; k++ )
      {
        if ( d[from+k*n] < dmin )
        {
          to = k;
          dmin = d[from+to*n];
        }
      }
    }
/*
  Add TO to path, and move there.
*/
    p[j] = to;
    from = to;
/*
  Reset D so we can't revisit city TO.
*/
    for ( i = 0; i < n; i++ )
    {
      d[i+to*n] = HUGE_VAL;
    }
  }
  
  free ( d );

  return p;
}