Code

Model with a global interpretation

The first model is based on the global interpretation of the equation.

Model model = new Model("SEND+MORE=MONEY");
IntVar S = model.intVar("S", 1, 9, false);
IntVar E = model.intVar("E", 0, 9, false);
IntVar N = model.intVar("N", 0, 9, false);
IntVar D = model.intVar("D", 0, 9, false);
IntVar M = model.intVar("M", 1, 9, false);
IntVar O = model.intVar("0", 0, 9, false);
IntVar R = model.intVar("R", 0, 9, false);
IntVar Y = model.intVar("Y", 0, 9, false);

model.allDifferent(new IntVar[]{S, E, N, D, M, O, R, Y}).post();

IntVar[] ALL = new IntVar[]{
    S, E, N, D,
    M, O, R, E,
    M, O, N, E, Y};
int[] COEFFS = new int[]{
    1000, 100, 10, 1,
    1000, 100, 10, 1,
    -10000, -1000, -100, -10, -1};
model.scalar(ALL, COEFFS, "=", 0).post();

Solver solver = model.getSolver();
solver.showStatistics();
solver.showSolutions();
solver.findSolution();

The solution found is:

** Choco 3.3.3 (2015-12) : Constraint Programming Solver, Copyleft (c) 2010-2015
- Model[Model-0] features:
    Variables : 9
    Constraints : 4
    Default search strategy : no
    Completed search strategy : no
- Solution #1 found. Model[Model-0], 1 Solutions, Resolution time 0,004s, 3 Nodes (742,4 n/s), 1 Backtracks, 1 Fails, 0 Restarts
    S = 9 E = 5 N = 6 D = 7 M = 1 0 = 0 R = 8 Y = 2 .
- Complete search - 1 solution found.
    Model[Model-0]
    Solutions: 1
    Building time : 0,000s
    Resolution time : 0,008s
    Nodes: 3 (383,5 n/s)
    Backtracks: 1
    Fails: 1
    Restarts: 0

The alldifferent constraint (line 11) makes sure all letters are distinct.

Then, instead of using expressions, which tend to be too verbose, we directly declare a scalar product and post it (lines 13-21). Thus, the expression is not decomposed on posting (no additional variables are introduced) and it ensures a better filtering. Alternatively, the expression could have been post in extension, providing an even better filtering algorithm, but at high pre-process time.

Finally, we rely on the capacity of the solver to output data on console (lines 24-25). Basic search statistics (time, nodes, etc) are reported but also a pre-defined solution format: each variable’s name is followed a “=” sign and the value of the variable in the solution. This avoids getting a solution object and reads it.

Another alternative would be to call solver.solve() which looks for the first solution (if any) and stops on it, that is, on a state wherein each variable is assigned to a single value. Their value can then be read calling S.getValue():

model.getSolver().showStatistics();
if (model.getSolver().solve()) {
    System.out.printf("%s = %d\n", S.getName(), S.getValue());
    System.out.printf("%s = %d\n", E.getName(), E.getValue());
    // ...
}

Model with a local interpretation

The second model requires to introduces 3 additional variables.

// additional variables: the carries
BoolVar[] r = model.boolVarArray(3);
// declare local equations
D.add(E).eq(Y.add(r[0].mul(10))).post();
r[0].add(N).add(R).eq(E.add(r[1].mul(10))).post();
r[1].add(E).add(O).eq(N.add(r[2].mul(10))).post();
r[2].add(S).add(M).eq(O.add(M.mul(10))).post();

Doing so, we increase the number of variables and constraints and introduce some search noise. Indeed, without indication on how to explore the search, the solver will consider all variables as decisions ones, that is as search leader. But, we may want only to branch on letters and ignoring other ones (basically, the carries which are assigned by propagation.

To do so, we can precise the decision variables and how to branch on it:

solver.setSearch(Search.inputOrderLBSearch(S, E, N, D, M, O, R, Y));

Here, we consider the variables in the input order (S is selected first, then E, then N, …) and each of them are, in turn, assigned to their current lower bound.

Things to remember

• There exists multiple ways to declare expressions. Specific ones, like sum or scalar product, should be considered when modeling a problem.
• Extension constraints provide a powerful filtering algorithm but may come at a high pre-process time.
• The Solver enables having access to a solution by calling the solve() method. This is also an easy way to loop over all solutions.
• Defining a (dedicated) search strategy is a good habit to help the solver exploring the search space.