nValues: java.lang.Exception with the condition (gt,1) or (ge,2) on two variables (notAllEqual) #18

The constraint nValues with the condition (gt,1) or (ge,2) (that is to say, notAllEqual) fails when its list contains only two variables.

<instance format="XCSP3" type="CSP">
  <variables>
    <array id="x" size="[2]"> 0..2 </array>
  </variables>
  <constraints>
    <nValues>
      <list> x[0] x[1] </list>
      <condition> (gt,1) </condition>
    </nValues>
  </constraints>
</instance>

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

java.lang.Exception
	at utility.Kit.control(Kit.java:173)
	at utility.Kit.control(Kit.java:183)
	at constraints.global.NotAllEqual.<init>(NotAllEqual.java:49)
	at constraints.global.NValues$NValuesCst.buildFrom(NValues.java:134)
	at problem.Problem.nValues(Problem.java:2948)
	at problem.XCSP3.buildCtrNValues(XCSP3.java:613)
	at problem.XCSP3.buildCtrNotAllEqual(XCSP3.java:628)

The constructor of NotAllEqual controls scp.length > 2 (this control is still in the current source), whereas NValuesCst.buildFrom() builds a NotAllEqual for the conditions (gt,1) and (ge,2) whatever the number of variables.

Expected: the 6 solutions where x[0] != x[1]. With the condition (eq,2), or with three variables, 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-notallequal, xcsp3team/pycsp3#130), where the following tests are marked as expected failures with ACE, in test_constraints_global_notallequal.py:

• test_notallequal_sizes with two variables (2 parameters), whose model is

x = VarArray(size=2, dom=range(2))

satisfy(
    NotAllEqual(x)
)

• test_notallequal_on_rows (NotAllEqual(row) on rows of two variables);
• test_notallequal_with_integers for NotAllEqual(x[0], 5) (the integer is replaced by an auxiliary variable);
• test_notallequal_on_expressions and test_notallequal_on_expressions_with_option_mini for NotAllEqual(Sum(x[0], x[1]), x[2] + x[3]) (two terms).

This issue was found by the unit tests of PyCSP3: xcsp3team/pycsp3#130