Propagator for if-then-else construct.

propagator-if-then-else.c

// the propagated constraint is:
// Z = if B then X else Y endif

// propagator for Z:
if (lb(B) < ub(B)) { // B is not fixed
    setlb(Z, min(lb(X), lb(Y));
    setub(Z, max(ub(X), ub(Y));
} else if (lb(B) == 1) { // B is fixed and B = 1
    setlb(Z, lb(X));
    setub(Z, ub(X));
} else { // B is fixed and B = 0
    setlb(Z, lb(Y));
    setub(Z, ub(Y));
}

// propagator for X:
if (lb(B) == up(B) && lb(B) == 1) { // B is fixed and B = 1
    // B = 1, we set X according to Z
    setlb(X, lb(Z));
    setub(X, ub(Z));
} // else nothing is known

// propagator for Y:
if (lb(B) == up(B) && lb(B) == 0) { // B is fixed and B = 0
    // B = 0, we set Y according to Z
    setlb(Y, lb(Z));
    setub(Y, ub(Z));
} // else nothing is known

// propagator for B:
// this one is harder; based on bound of X, Y, Z we want to determine value of B
// these two booleans have meaning if such value 1/0 is possible
// they test if ranges of variables have non-empty intersection
b1 = lb(X) <= ub(Z) && lb(Z) <= ub(X); // X intersects Z
b0 = lb(Y) <= ub(Z) && lb(Z) <= ub(Y); // Y intersects Z
if (b1) {
    if(b0) { // both values possible
        setlb(B, 0);
        setub(B, 1);
    } else { // only B=1 possible
        setlb(B, 1);
        setub(B, 1);
    }
} else {
    if(b0) { // only B=0 possible
        setlb(B, 0);
        setub(B, 0);
    } else { // no value possible
        fail;
    }
}