Add ref_rand helper function. Clip lower in logp

Author: Harivallabha

pymc3/distributions/discrete.py

@@ -913,7 +913,7 @@ def logp(self, value):
             - betaln(n - value + 1, bad - n + value + 1)
             - betaln(tot + 1, 1)
         )
-        lower = tt.switch(tt.gt(n - N + k, 0), n - N + k, 0)
+        lower = tt.clip(n - N + k, 0, n - N + k)
         upper = tt.switch(tt.lt(k, n), k, n)
         nonint_value = (value != intX(tt.floor(value)))
         return bound(result, lower <= value, value <= upper, nonint_value)

pymc3/tests/test_distributions_random.py

@@ -745,7 +745,16 @@ def test_geometric(self):
         pymc3_random_discrete(pm.Geometric, {"p": Unit}, size=500, fails=50, ref_rand=nr.geometric)
 
     def test_hypergeometric(self):
-        pymc3_random_discrete(pm.HyperGeometric, {"N": Nat, "n": Nat, "k": Nat}, size=500, fails=50, ref_rand=nr.hypergeometric)
+        def ref_rand(size, N, n, k):
+            return nr.hypergeometric(ngood=k, nbad=N-k, nsample=n, size=size)
+
+        pymc3_random_discrete(
+            pm.HyperGeometric, 
+            {"N": Nat, "n": Nat, "k": Nat}, 
+            size=500, 
+            fails=50, 
+            ref_rand=ref_rand,
+        )
 
     def test_discrete_uniform(self):
         def ref_rand(size, lower, upper):