/* Schelling’s 1970s racism model, as described in
 * <http://www.theatlantic.com/magazine/archive/2002/04/seeing-around-corners/302471/>.
 * I saw it in the article and thought I’d code it up quickly to see
 * if it really behaved like the article said.  It does!
 * Bizarrely, though, if the racists require more than 2 neighbors to
 * be the same (min_same), the clumping falls apart.
 * It’s kind of awesome that you can hack together something like this in 20 minutes.
 */

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

enum { width = 64, min_same = 2 };
#define LEN(x) ((sizeof(x))/sizeof(x[0]))
int main(int argc, char **argv)
{
     long generations = 512;
     if (argc > 1) generations = atol(argv[1]);
     if (argc > 2) srandom(atoi(argv[2]));

     enum { red, blue } people[width][width];
     struct { int x, y } dd[4] = { {0, 1}, {1, 0}, {0, -1}, {-1, 0} };

     // initialize randomly
     for (int ii = 0; ii < width; ii++) {
          for (int jj = 0; jj < width; jj++) {
               people[ii][jj] = (random() & 0x100) ? red : blue;
          }
     }

     // each generation, swap isolated racists to random positions
     for (int ii = 0; ii < generations; ii++) {
          for (int jj = 0; jj < width; jj++) {
               for (int kk = 0; kk < width; kk++) {
                    int same_neighbors = 0, 
                        me = people[jj][kk];

                    for (int nn = 0; nn < LEN(dd); nn++) {
                         if (me ==
                             people[(jj + dd[nn].x + width) % width]
                                   [(kk + dd[nn].y + width) % width]) {
                              same_neighbors++;
                         }
                    }

                    if (same_neighbors < min_same) {
                         int mm = random() % width,
                             nn = random() % width;
                         people[jj][kk] = people[mm][nn];
                         people[mm][nn] = me;
                    }
               }
          }
     }

     // at the end, dump out the entire system state
     for (int jj = 0; jj < width; jj++) {
          for (int kk = 0; kk < width; kk++) {
               printf("%s", people[jj][kk] ? "--" : "##");
          }
          printf("\n");
     }
     printf("\n");

     return 0;
}