C++ Reference: constraint_solver
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Declaration of the core objects for the constraint solver.The literature around constraint programming is extremely dense but one can find some basic introductions in the following links:
- http://en.wikipedia.org/wiki/Constraint_programming
- http://kti.mff.cuni.cz/~bartak/constraints/index.html
Here is a very simple Constraint Programming problem:
If we see 56 legs and 20 heads, how many two-legged pheasants and four-legged rabbits are we looking at?
Here is some simple Constraint Programming code to find out:
void pheasant() {
Solver s("pheasant");
// Create integer variables to represent the number of pheasants and
// rabbits, with a minimum of 0 and a maximum of 20.
IntVar* const p = s.MakeIntVar(0, 20, "pheasant"));
IntVar* const r = s.MakeIntVar(0, 20, "rabbit"));
// The number of heads is the sum of pheasants and rabbits.
IntExpr* const heads = s.MakeSum(p, r);
// The number of legs is the sum of pheasants * 2 and rabbits * 4.
IntExpr* const legs = s.MakeSum(s.MakeProd(p, 2), s.MakeProd(r, 4));
// Constraints: the number of legs is 56 and heads is 20.
Constraint* const ct_legs = s.MakeEquality(legs, 56);
Constraint* const ct_heads = s.MakeEquality(heads, 20);
s.AddConstraint(ct_legs);
s.AddConstraint(ct_heads);
DecisionBuilder* const db = s.MakePhase(p, r,
Solver::CHOOSE_FIRST_UNBOUND,
Solver::ASSIGN_MIN_VALUE);
s.NewSearch(db);
CHECK(s.NextSolution());
LOG(INFO) << "rabbits -> " << r->Value() << ", pheasants -> "
<< p->Value();
LOG(INFO) << s.DebugString();
s.EndSearch();
}
which outputs:
rabbits -> 8, pheasants -> 12
Solver(name = "pheasant",
state = OUTSIDE_SEARCH,
branches = 0,
fails = 0,
decisions = 0
propagation loops = 11,
demons Run = 25,
Run time = 0 ms)