From 3789a59cc6a7da357468e5092bfa8b4d873c8c22 Mon Sep 17 00:00:00 2001
From: Tal Kedar <tal.kedar@k2dq.com>
Date: Thu, 30 Jul 2020 13:12:02 -0400
Subject: [PATCH] [NFC] base_hmc const accessor to _z

---
 src/stan/mcmc/hmc/base_hmc.hpp           | 23 +++++++++++++++++++++++
 src/test/unit/mcmc/hmc/base_hmc_test.cpp |  7 +++++++
 2 files changed, 30 insertions(+)

diff --git a/src/stan/mcmc/hmc/base_hmc.hpp b/src/stan/mcmc/hmc/base_hmc.hpp
index a4c5de62e81..7713486fd1d 100644
--- a/src/stan/mcmc/hmc/base_hmc.hpp
+++ b/src/stan/mcmc/hmc/base_hmc.hpp
@@ -15,6 +15,15 @@
 namespace stan {
 namespace mcmc {
 
+/**
+ * Base class for Hamiltonian samplers.
+ *
+ * @tparam Model The type of the Stan model.
+ * @tparam Hamiltonian The type of Hamiltonians over the (unconstrained)
+ * parameter space.
+ * @tparam Integrator The type of integrator (e.g. leapfrog).
+ * @tparam BaseRNG The type of random number generator.
+ */
 template <class Model, template <class, class> class Hamiltonian,
           template <class> class Integrator, class BaseRNG>
 class base_hmc : public base_mcmc {
@@ -132,8 +141,22 @@ class base_hmc : public base_mcmc {
     this->z_.ps_point::operator=(z_init);
   }
 
+  /**
+   * Gets the current point in the (unconstrained) parameter space.
+   *
+   * @return The current point in the (unconstrained) parameter space.
+   */
   typename Hamiltonian<Model, BaseRNG>::PointType& z() { return z_; }
 
+  /**
+   * Gets the current point in the (unconstrained) parameters space.
+   *
+   * @return The current point in the (unconstrained) parameters space.
+   */
+  const typename Hamiltonian<Model, BaseRNG>::PointType& z() const noexcept {
+    return z_;
+  }
+
   virtual void set_nominal_stepsize(double e) {
     if (e > 0)
       nom_epsilon_ = e;
diff --git a/src/test/unit/mcmc/hmc/base_hmc_test.cpp b/src/test/unit/mcmc/hmc/base_hmc_test.cpp
index abcf122aa30..00742532075 100644
--- a/src/test/unit/mcmc/hmc/base_hmc_test.cpp
+++ b/src/test/unit/mcmc/hmc/base_hmc_test.cpp
@@ -46,6 +46,13 @@ TEST(McmcBaseHMC, point_construction) {
   EXPECT_EQ(static_cast<int>(q.size()), sampler.z().g.size());
 }
 
+TEST(McmcBaseHMC, point_access_from_const_hmc) {
+  rng_t base_rng(0);
+  const stan::mcmc::mock_hmc sampler(stan::mcmc::mock_model(2), base_rng);
+  EXPECT_EQ(2, sampler.z().q.size());
+  EXPECT_EQ(2, sampler.z().g.size());
+}
+
 TEST(McmcBaseHMC, seed) {
   rng_t base_rng(0);