precedence with covered: java.lang.Exception when the list has not more variables than values #21

The constraint precedence with covered="true" fails when the list has not more variables than values.

<instance format="XCSP3" type="CSP">
  <variables>
    <array id="x" size="[3]"> 0..3 </array>
  </variables>
  <constraints>
    <precedence>
      <list> x[] </list>
      <values covered="true"> 0 1 2 </values>
    </precedence>
  </constraints>
</instance>

Observed: ! ERROR: and no solution, and with -ev:

java.lang.Exception
	at constraints.global.Precedence.<init>(Precedence.java:101)
	at problem.Problem.precedence(Problem.java:2487)
	at problem.XCSP3.buildCtrPrecedence(XCSP3.java:524)

The constructor of Precedence controls (!covered || list.length > values.length) && values.length > 1 (this control is still in the current source, 3a6cd59, and ACE built from it fails too).

Expected: with as many variables as values, the single solution x = (0, 1, 2); with less variables than values, no solution (the instance is unsatisfiable, all the values having to be assigned). With more variables than values, ACE finds the expected solutions.

Results of the three solvers

with (ACE 2.6 and CHOCO (choco-parsers-4.10.15-beta), embedded in PyCSP3, and cosoco 2.6.1, from pip: the versions used by PyCSP3):

• ACE: java -jar ACE-2.6.jar model.xml -s=all -xe -xc=false
• CHOCO: java -cp choco-parsers-4.10.15-beta.jar org.chocosolver.parser.xcsp.ChocoXCSP model.xml -a -f -flt
• cosoco: cosoco model.xml -model=2 -nbsols=0

Tests

Found by the unit tests of PyCSP3 (branch tests-precedence, xcsp3team/pycsp3#148), where the following tests are marked as expected failures with ACE, in test_constraints_global_precedence.py:

• test_precedence_covered_with_not_more_variables_than_values (4 parameters: as many values as variables, 3 and 2, and more values than variables), whose model is, for instance,

x = VarArray(size=3, dom=range(4))

satisfy(
    Precedence(x, values=[0, 1, 2], covered=True)
)

• test_precedence_without_values_covered for shifted domains (4 variables, 5 values), once covered is taken into account by PyCSP3 (Precedence() ignores covered=True when values is not given, or when a single value is given pycsp3#150).

This issue was found by the unit tests of PyCSP3: xcsp3team/pycsp3#148