initial attempt at refactoring ed.MetropolisHastings' impl

edward/inferences/metropolis_hastings.py

@@ -12,6 +12,7 @@
 
 try:
   from edward.models import Uniform
+  from tensorflow.contrib.bayesflow.metropolis_hastings import evolve
 except Exception as e:
   raise ImportError("{0}. Your TensorFlow version is not supported.".format(e))
 
@@ -64,6 +65,19 @@ def __init__(self, latent_vars, proposal_vars, data=None):
 
   def initialize(self, *args, **kwargs):
     kwargs['auto_transform'] = False
+
+    # TODO In general, each latent variable has arbitrary shape and
+    # dtype. We cannot simply batch them into a single tf.Tensor with
+    # an extra dimension.
+    initial_sample = tf.stack([tf.gather(qz.params, 0)
+                               for qz in six.itervalues(self.latent_vars)])
+    self._state = tf.Variable(initial_sample, trainable=False, name="state")
+    self._state_log_density = tf.Variable(
+        self._log_joint(initial_sample),
+        trainable=False, name="state_log_density")
+    self._log_accept_ratio = tf.Variable(
+        tf.zeros_like(self._state_log_density.initialized_value()),
+        trainable=False, name="log_accept_ratio")
     return super(MetropolisHastings, self).initialize(*args, **kwargs)
 
   def build_update(self):
@@ -80,9 +94,75 @@ def build_update(self):
     The updates assume each Empirical random variable is directly
     parameterized by `tf.Variable`s.
     """
-    old_sample = {z: tf.gather(qz.params, tf.maximum(self.t - 1, 0))
-                  for z, qz in six.iteritems(self.latent_vars)}
-    old_sample = OrderedDict(old_sample)
+    old_state = self._state
+    forward_step = evolve(self._state,
+                          self._state_log_density,
+                          self._log_accept_ratio,
+                          self._log_density,
+                          self._proposal_fn,
+                          n_steps=1)
+    assign_ops = [forward_step]
+
+    with tf.control_dependencies([forward_step]):
+      # Update Empirical random variables.
+      for state, qz in zip(tf.unstack(self._state),
+                           six.itervalues(self.latent_vars)):
+        variable = qz.get_variables()[0]
+        assign_ops.append(tf.scatter_update(variable, self.t, state))
+
+      # Increment n_accept (if accepted).
+      # TODO old_state might always be same. It would be great if we
+      # could more naturally get the acceptance rate from ``evolve``.
+      is_proposal_accepted = tf.where(
+          tf.reduce_any(tf.not_equal(old_state, self._state)), 1, 0)
+      assign_ops.append(self.n_accept.assign_add(is_proposal_accepted))
+
+    return tf.group(*assign_ops)
+
+  def _log_joint(self, state):
+    """Utility function to calculate model's log joint density,
+    log p(x, z), for inputs z (and fixed data x).
+
+    Args:
+      state: tf.Tensor.
+    """
+    scope = self._scope + tf.get_default_graph().unique_name("sample")
+    # Form dictionary in order to replace conditioning on prior or
+    # observed variable with conditioning on a specific value.
+    # TODO verify ordering is preserved
+    dict_swap = {z: sample for z, sample in
+                 zip(six.iterkeys(self.latent_vars), state)}
+    for x, qx in six.iteritems(self.data):
+      if isinstance(x, RandomVariable):
+        if isinstance(qx, RandomVariable):
+          qx_copy = copy(qx, scope=scope)
+          dict_swap[x] = qx_copy.value()
+        else:
+          dict_swap[x] = qx
+
+    log_joint = 0.0
+    for z in six.iterkeys(self.latent_vars):
+      z_copy = copy(z, dict_swap, scope=scope)
+      log_joint += tf.reduce_sum(z_copy.log_prob(dict_swap[z]))
+
+    for x in six.iterkeys(self.data):
+      if isinstance(x, RandomVariable):
+        x_copy = copy(x, dict_swap, scope=scope)
+        log_joint += tf.reduce_sum(x_copy.log_prob(dict_swap[x]))
+
+    return log_joint
+
+  def proposal_fn(state):
+    """Utility function to propose new state,
+    znew ~ g(znew | zold) for inputs zold, and return the log density
+    ratio of log g(znew | zold) - log g(zold | znew).
+
+    Args:
+      state: tf.Tensor.
+    """
+    # TODO verify ordering is preserved
+    old_sample = {z: sample for z, sample in
+                  zip(six.iterkeys(self.latent_vars), state)}
 
     # Form dictionary in order to replace conditioning on prior or
     # observed variable with conditioning on a specific value.
@@ -99,7 +179,6 @@ def build_update(self):
     dict_swap_old.update(old_sample)
     base_scope = tf.get_default_graph().unique_name("inference") + '/'
     scope_old = base_scope + 'old'
-    scope_new = base_scope + 'new'
 
     # Draw proposed sample and calculate acceptance ratio.
     new_sample = old_sample.copy()  # copy to ensure same order
@@ -114,49 +193,14 @@ def build_update(self):
 
     dict_swap_new = dict_swap.copy()
     dict_swap_new.update(new_sample)
+    scope_new = base_scope + 'new'
 
     for z, proposal_z in six.iteritems(self.proposal_vars):
       # Build proposal g(zold | znew).
       proposal_zold = copy(proposal_z, dict_swap_new, scope=scope_new)
       # Increment ratio.
       ratio -= tf.reduce_sum(proposal_zold.log_prob(dict_swap_old[z]))
 
-    for z in six.iterkeys(self.latent_vars):
-      # Build priors p(znew) and p(zold).
-      znew = copy(z, dict_swap_new, scope=scope_new)
-      zold = copy(z, dict_swap_old, scope=scope_old)
-      # Increment ratio.
-      ratio += tf.reduce_sum(znew.log_prob(dict_swap_new[z]))
-      ratio -= tf.reduce_sum(zold.log_prob(dict_swap_old[z]))
-
-    for x in six.iterkeys(self.data):
-      if isinstance(x, RandomVariable):
-        # Build likelihoods p(x | znew) and p(x | zold).
-        x_znew = copy(x, dict_swap_new, scope=scope_new)
-        x_zold = copy(x, dict_swap_old, scope=scope_old)
-        # Increment ratio.
-        ratio += tf.reduce_sum(x_znew.log_prob(dict_swap[x]))
-        ratio -= tf.reduce_sum(x_zold.log_prob(dict_swap[x]))
-
-    # Accept or reject sample.
-    u = Uniform(low=tf.constant(0.0, dtype=ratio.dtype),
-                high=tf.constant(1.0, dtype=ratio.dtype)).sample()
-    accept = tf.log(u) < ratio
-    sample_values = tf.cond(accept, lambda: list(six.itervalues(new_sample)),
-                            lambda: list(six.itervalues(old_sample)))
-    if not isinstance(sample_values, list):
-      # `tf.cond` returns tf.Tensor if output is a list of size 1.
-      sample_values = [sample_values]
-
-    sample = {z: sample_value for z, sample_value in
-              zip(six.iterkeys(new_sample), sample_values)}
-
-    # Update Empirical random variables.
-    assign_ops = []
-    for z, qz in six.iteritems(self.latent_vars):
-      variable = qz.get_variables()[0]
-      assign_ops.append(tf.scatter_update(variable, self.t, sample[z]))
-
-    # Increment n_accept (if accepted).
-    assign_ops.append(self.n_accept.assign_add(tf.where(accept, 1, 0)))
-    return tf.group(*assign_ops)
+    # TODO verify ordering is preserved
+    new_sample = tf.stack(list(six.itervalues(new_sample)))
+    return (new_sample, ratio)