[szczepanskiNicolas]

Unit tests of the constraints Increasing and Decreasing of the section Constraints (Global) of the API (https://pycsp.org/documentation/api/constraints-global/#Increasing and https://pycsp.org/documentation/api/constraints-global/#Decreasing), written as for the previous global constraints (#66, #130), with the same harness.

This PR adds tests (pycsp3/tests/test_constraints_global_increasing_decreasing.py). The bugs found are reported in issues, and the tests concerned are marked as strict expected failures with the numbers of these issues. It also contains the fixes of #136 to #140 (8f8fcba, ad186a2, 289a264, 474d5ff, bb2b86a, one commit per issue), which remove the marks of these issues.

Tests

Increasing() and Decreasing() post an element <ordered> with the operator le, lt, ge or gt (with strict=True for lt and gt); according to XCSP3-core, ordered(X, L, ⊙) holds iff x[i] + l[i] ⊙ x[i+1] for each i. Most tests are run for the four orderings. The tests cover:

• the forms of the arguments (lists, tuples, generators, slices, nested lists, reversed arrays), sizes, matrices, different domains, negative values, arrays with holes, and the examples of the documentation;
• the parameter lengths: an integer, a list, a tuple, a range, zero or negative values, as many lengths as terms (the last one being ignored), variables, and integers mixed with variables;
• expressions (replaced by auxiliary variables), and the option -mini (decomposition into intensional constraints);
• groups of constraints (rows, sliding windows), and the constraints in logical expressions (~, |, &, imply(), either());
• terms given several times, the degenerated cases (less than two terms: the constraint always holds), the XCSP3 files generated, and invalid arguments.

Each constraint is solved with ACE, CHOCO and cosoco, and all the solutions are compared with the ones computed by brute force.

File	Commit	Tests	Passed	Expected failures (known bugs)	Skipped
test_constraints_global_increasing_decreasing.py	e912dd0 (tests only, before the fixes)	672	593	67	12
test_constraints_global_increasing_decreasing.py	bb2b86a (last commit)	686	653	21	12

Whole suite with the fixes (run locally, master not having moved since the creation of the branch): 3356 passed, 137 skipped and 224 expected failures, no failure. The 46 model files of PyCSP3-models using Increasing() or Decreasing() (58 compilations, with their variants and at most two data files each) give the same results before and after the fixes, except two XCSP3 files where an element <ordered> with a single variable is no longer generated (#138): ProductShelves (toy.json) and Blackhole_z (example.json).

Known bugs

Each issue has the labels bug, unit-tests, Constraints (Global) and the labels of the functions (Increasing, Decreasing).

Issue	Solvers (tests)	Tests	State
#136: Decreasing(strict=True) raises AttributeError in a logical expression (typo Strictly_DECREASING in ConstraintOrdered.to_list())	ACE (6), CHOCO (6), cosoco (6)	18	fixed in 8f8fcba, closed
#137: None is ignored, and lengths of a wrong size are reported by an assert without message	without solver	10	fixed in ad186a2, closed
#138: a single term generates an element <ordered> with a single variable (now no constraint; ConstraintDummyConstant.__or__ extended accordingly)	without solver	6	fixed in 289a264, closed
#139: lengths mixing integers and variables are accepted, and generate an invalid element <lengths> (now forbidden)	without solver	4	fixed in 474d5ff, closed
#140: a term given several times is accepted (now forbidden; on x[0] ⊙ x[0], ACE fails, CHOCO accepts x[0] < x[0], and cosoco reports its own solution as invalid)	without solver	8	fixed in bb2b86a, closed
chocoteam/choco-solver#1248 (section 6): ordered with lengths, CHOCO orders the whole sequence x[0], x[0] + l[0], x[1], ..., which loses solutions for some lengths	CHOCO (21)	21	open (the only expected failures at the last commit)

Decisions: integers among the terms are refused (TypeError: Wrong type for [x[0], 1, x[1]], accepted by the tests as an explicit error), and as many lengths as terms are tolerated (the last length being ignored).

Cases not handled by the solvers

The constraints on symbolic variables are skipped (ordered on symbolic variables is not part of XCSP3-core; 12 tests).