Automated contype/conaffinity search

Single point contacts plus zero sized gaps between adjacent meshes are the modelers nightmare. If you're lucky, the contype/conaffinity mechanism may get you out of this convex hell. But it is difficult to find a combination manually.

Here is a little program that performs an automated brute force search. It assumes that all contacts are allowed by default and only the exceptions are listed in the array no_contacts[].

Code:

#include <conio.h>
#include <iostream>
using namespace std;

const int BITS = 3; // increase this if it doesn't find a solution
const int MASK = (1 << BITS) - 1;

struct OBJECT { int id; char* name; } objects[] = {
  0, "everything else", // default at id 0
  1, "washer segment",
  2, "washer cylinder",
  3, "contact ball",
};
struct NO_COLLISION { int o1, o2; } no_collisions[] = {
  2, 0,
  3, 1,
};
const int NO = _countof(objects);
const int NNC = _countof(no_collisions);
bool forbidden[NO][NO];
unsigned int combi;

void verify_data() {
  for (int i = 0; i < NO; i++) {
    if (objects[i].id != i) throw "?ID SEQUENCE ERROR in objects[]";
  }
  for (int i = 0; i < NNC; i++) {
    if ((unsigned)no_collisions[i].o1 >= NO || (unsigned)no_collisions[i].o2 >= NO) throw "?OUT OF RANGE ERROR in no_collisions[]";
  }
  if (sizeof(combi) * 8 < NO * 2 * BITS) throw "sizeof(combi) too small, use __int64";
}

void init_forbidden() {
  cout << "No collisions between:" << endl;
  for (int i = 0; i < NNC; i++) {
    NO_COLLISION& nc = no_collisions[i];
    forbidden[nc.o1][nc.o2] = forbidden[nc.o2][nc.o1] = true;
    cout << "- " << objects[nc.o1].name << " and " << objects[nc.o2].name << endl;
  }
}

bool search_for_solution() {
  for (combi = 1 | (1 << BITS); combi < 1i64 << NO * 2 * BITS; combi += 1 << 2 * BITS) {
    for (int o1 = 0; o1 < NO; o1++) {
      int contype1 = int(combi >> o1 * 2 * BITS) & MASK;
      int conaffinity1 = int(combi >> o1 * 2 * BITS >> BITS) & MASK;
      for (int o2 = o1; o2 < NO; o2++) {
        int contype2 = int(combi >> o2 * 2 * BITS) & MASK;
        int conaffinity2 = int(combi >> o2 * 2 * BITS >> BITS) & MASK;
        bool collision = (contype1 & conaffinity2 || contype2 & conaffinity1);
        if (collision == forbidden[o1][o2]) goto next_combination;
      }
    }
    return true;
  next_combination:
    ;
  }
  return false;
}

void output_solution() {
  cout << "solution found" << endl;
  cout << "contype,conaffinity:" << endl;
  for (int o1 = 0; o1 < NO; o1++) {
    int contype = int(combi >> o1 * 2 * BITS) & MASK;
    int conaffinity = int(combi >> o1 * 2 * BITS >> BITS) & MASK;
    cout << "  " << contype << "," << conaffinity << " for " << objects[o1].name << endl;
  }
}

int _cdecl main(int argc, char* argv[]) {
  try {
    verify_data();
    init_forbidden();
    if (search_for_solution()) output_solution();
    else cout << "No solution found, increase BITS" << endl;
  } catch (const char* msg) {
    cerr << endl << msg;
  }
  cout << endl << "READY." << endl; do _getch(); while (_kbhit());
  return 0;
}