This is a reimplementation of the integer variable detection procedure.
The idea is to iteratively assume variables to be noninteger, and to
prove that this would lead to a false or erroneous result. If the proof
is successful, the variable is integer as a consequence.
With this change, domains detected for predicate parameters are properly
propagated to all occurences of the respective variables, enabling more
integer simplifications.
This adds a new syntax for declaring functions integer:
#external integer(<function name>(<arity)).
If a function is declared integer, it may enable some variables to be
detected as integer as well.
For clarity, this moves the Domain enum class to a separate header,
because it’s not just variable-specific but also applicable to
functions, for example.
This adds initial support for detecting integer variables in formulas.
The scope is somewhat limited in that variables matching predicate
parameters with known integer type aren’t propagated to be integer as
well yet. Also, the use of constants and functions prevents variables
from being detected as integer, because they have to be assumed to be
externally defined in such a way that they evaluate to general values
and not necessarily integers.
The Tristate class (representing truth values that are either true,
false, or unknown) is used at multiple ends. This moves it to a separate
header for reusing it properly.
This refactoring separates predicates from their declarations. The
purpose of this is to avoid duplicating properties specific to the
predicate declaration and not its occurrences in the program.
This implements a tableau containing simplification rules that can be
iteratively applied to input formulas until they remain unchanged.
First, this moves the rule “exists X (X = Y) === #true” to the tableau
as a reference implementation.
This adds support for declaring predicates as placeholders through the
“#external” directive in the input language of clingo.
Placeholders are not subject to completion. This prevents predicates
that represent instance-specific facts from being assumed as universally
false by default negation when translating an encoding.
This stretches clingo’s usual syntax a bit to make the implementation
lightweight. In order to declare a predicate with a specific arity as a
placeholder, the following statement needs to be added to the program:
#external <predicate name>(<arity>).
Multiple unit tests cover cases where placeholders are used or not as
well as a more complex graph coloring example.
With C++17, optionals, an experimental language feature, were moved to
the “std” namespace. This makes C++17 mandatory and drops the now
obsolete “experimental” namespace.
