LGS-164 belief propagation for polytrees, special case of OR cpds, refactored from LGSv0.0.2

1 files changed, 42 insertions, 0 deletions

diff --git a/beliefs/factors/bernoulli_or_cpd.py b/beliefs/factors/bernoulli_or_cpd.py
new file mode 100644
index 0000000..bfb3a95
--- /dev/null
+++ b/beliefs/factors/bernoulli_or_cpd.py
@@ -0,0 +1,42 @@
+import numpy as np
+
+from beliefs.factors.cpd import TabularCPD
+
+
+class BernoulliOrCPD(TabularCPD):
+    """CPD class for a Bernoulli random variable whose relationship to its
+    parents (also Bernoulli random variables) is described by OR logic.
+
+    If at least one of the variable's parents is True, then the variable
+    is True, and False otherwise.
+    """
+    def __init__(self, variable, parents=[]):
+        """
+        Args:
+          variable: int or string
+          parents: optional, list of int and/or strings
+        """
+        super().__init__(variable=variable,
+                         variable_card=2,
+                         parents=parents,
+                         parents_card=[2]*len(parents),
+                         values=[])
+        self._values = []
+
+    @property
+    def values(self):
+        if not any(self._values):
+            self._values = self._build_kwise_values_array(len(self.variables))
+            self._values = self._values.reshape(self.cardinality)
+        return self._values
+
+    @staticmethod
+    def _build_kwise_values_array(k):
+        # special case a completely independent factor, and
+        # return the uniform prior
+        if k == 1:
+            return np.array([0.5, 0.5])
+
+        return np.array(
+            [1.,] + [0.]*(2**(k-1)-1) + [0.,] + [1.]*(2**(k-1)-1)
+        )