Merge pull request #39 from arcondello/projection

Add projection method to Constraint

docs/reference/constraint.rst

@@ -42,10 +42,11 @@ Transformations
    Constraint.fix_variable
    Constraint.flip_variable
 
-Copy
-----
+Copies and projections
+----------------------
 
 .. autosummary::
    :toctree: generated/
 
    Constraint.copy
+   Constraint.projection

dwavebinarycsp/core/constraint.py

@@ -470,7 +470,7 @@ def func(*args):
         self.name = '{} ({} flipped)'.format(self.name, v)
 
     #
-    # copy
+    # copies and projections
     #
 
     def copy(self):
@@ -493,3 +493,40 @@ def copy(self):
         """
         # each object is itself immutable (except the function)
         return self.__class__(self.func, self.configurations, self.variables, self.vartype, name=self.name)
+
+    def projection(self, variables):
+        """Create a new constraint that is the projection onto a subset of the variables.
+
+        Args:
+            variables (iterable):
+                Subset of the constraint's variables.
+
+        Returns:
+            :obj:`.Constraint`: A new constraint over a subset of the variables.
+
+        Examples:
+
+            >>> import dwavebinarycsp
+            ...
+            >>> const = dwavebinarycsp.Constraint.from_configurations([(0, 0), (0, 1)],
+            ...                                                       ['a', 'b'],
+            ...                                                       dwavebinarycsp.BINARY)
+            >>> proj = const.projection(['a'])
+            >>> proj.variables
+            ['a']
+            >>> proj.configurations
+            {(0,)}
+
+        """
+        # resolve iterables or mutability problems by casting the variables to a set
+        variables = set(variables)
+
+        if not variables.issubset(self.variables):
+            raise ValueError("Cannot project to variables not in the constraint.")
+
+        idxs = [i for i, v in enumerate(self.variables) if v in variables]
+
+        configurations = frozenset(tuple(config[i] for i in idxs) for config in self.configurations)
+        variables = tuple(self.variables[i] for i in idxs)
+
+        return self.from_configurations(configurations, variables, self.vartype)

tests/test_constraint.py

@@ -198,3 +198,27 @@ def test_copy(self):
         new_const = const.copy()
 
         self.assertEqual(const, new_const)
+        self.assertIsNot(const, new_const)
+
+    def test_projection_identity(self):
+        const = dwavebinarycsp.Constraint.from_func(operator.eq, ['a', 'b'], dwavebinarycsp.SPIN)
+
+        proj = const.projection(['a', 'b'])
+
+        self.assertEqual(const, proj)
+        self.assertIsNot(const, proj)
+
+    def test_projection_unknown_variables(self):
+        const = dwavebinarycsp.Constraint.from_func(operator.eq, ['a', 'b'], dwavebinarycsp.SPIN)
+
+        with self.assertRaises(ValueError):
+            proj = const.projection(['a', 'c'])
+
+    def test_projection_reducible(self):
+        const = dwavebinarycsp.Constraint.from_configurations([(0, 0), (0, 1)], ['a', 'b'], dwavebinarycsp.BINARY)
+
+        a = dwavebinarycsp.Constraint.from_configurations([(0,)], ['a'], dwavebinarycsp.BINARY)
+        b = dwavebinarycsp.Constraint.from_configurations([(0,), (1,)], ['b'], dwavebinarycsp.BINARY)
+
+        self.assertEqual(const.projection(['b']), b)
+        self.assertEqual(const.projection(['a']), a)