[cprudhom]

XCSP3: handle reified constraints

Closes #1248 (item 10, the last one: the other items were fixed by #1254 and #1255).

Problem

The attributes reifiedBy, hreifiedFrom and hreifiedTo were ignored: the constraint was posted alone. XParser reads them, but xcsp3-tools 2.5 does not pass them to any callback.

On the instance of the issue (element with a list and a value, reifiedBy="r"), this gave 38 assignments, 19 of them wrong, instead of the 27 expected ones.

How it works

XCSPParser overrides loadCtr. For a reified constraint:

1. The reification is stored in a field before the callback of the constraint is called.
2. A callback may deal with it itself, by calling reify(BoolVar) with the truth value of the constraint.
3. Otherwise, the constraints posted by the callback are unposted and reified (reifyWith, impliedBy or implies).

Step 3 is refused with a ParserException when it would be wrong: the callback introduced an auxiliary variable (views and constants are fine), added clauses, or posted nothing. So a reification is never silently ignored.

Semantics, as in the XCSP3 specifications: reifiedBy is r ⇔ c, hreifiedFrom is r ⇒ c, hreifiedTo is r ⇐ c.

Coverage

	Constraints
Dealt with by the callback	intension, sum, count, nValues, minimum, maximum, atLeast, atMost, with any kind of condition (relation with a constant or a variable, interval, set)
Reified from what the callback posts	allDifferent, allEqual, extension, ordered, lex, element, cardinality, channel, noOverlap (tested); other constraints as long as the guard above holds
Refused with an explicit message	regular, slide and logic meta-constraints (and, or, not...), and any callback that relies on auxiliary variables without calling reify

For the callbacks of the first row, the result of the function goes into a fresh variable, posted as usual, and the condition on this result is reified (resultVar).

Other change

element with a list and a condition on a constant is now stated on Boolean views ("at least one variable satisfies the condition"), instead of an element constraint with a free index variable. This makes it reifiable.

Tests

In XCSPParserTest, against brute force (same solutions, each found once):

• each reifiable constraint with the three kinds of reification;
• the refused cases, which must raise a ParserException;
• element with a condition, not reified;
• n-ary iff and xor in an intension, reified or not (parser-level test for Fix n-ary iff and xor expressions (#1248) #1255).

The xcsp group passes: 154 tests, no failure.

Limits

• The tests use three variables with domain 0..2.
• Reified slide and logic meta-constraints are refused, not handled.
• The status line printed on a refused instance is s UNKNOWN unless the log level is INFO, where it is s UNSUPPORTED.

[mergify]

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