fixing #198

emcee/mh.py

@@ -115,10 +115,7 @@ def sample(self, p0, lnprob=None, randomstate=None, thin=1,
             diff = newlnprob - lnprob
 
             # M-H acceptance ratio
-            if diff < 0:
-                diff = np.exp(diff) - self._random.rand()
-
-            if diff > 0:
+            if diff >= 0.0 or diff >= np.log(self._random.rand()):
                 p = q
                 lnprob = newlnprob
                 self.naccepted += 1

emcee/tests.py

@@ -166,6 +166,13 @@ def test_mh(self):
                                  args=[self.icov])
         self.check_sampler(N=self.N * self.nwalkers, p0=self.p0[0])
 
+    def test_mh_unif(self):
+        f = lambda x: 0.0 if np.all((0.0 <= x) & (x <= 1.0)) else -np.inf
+        self.sampler = MHSampler(self.cov, self.ndim, f)
+        self.sampler.run_mcmc(np.random.rand(self.ndim), 100)
+        chain = self.sampler.chain
+        assert np.any(np.abs(np.diff(chain, axis=0)) > 0.0)
+
     def test_ensemble(self):
         self.sampler = EnsembleSampler(self.nwalkers, self.ndim,
                                        lnprob_gaussian, args=[self.icov])