From 0c4ffecf6ccad06b426463d8edc5e0c50935b9c9 Mon Sep 17 00:00:00 2001
From: Chris Dyer <cdyer@cs.cmu.edu>
Date: Sun, 4 Mar 2012 23:15:51 -0500
Subject: support full range of hyperparameter values for PYP (including
 strength <= 0)

---
 utils/ccrp.h | 68 ++++++++++++++++++++++++++++++++++++++----------------------
 1 file changed, 43 insertions(+), 25 deletions(-)

diff --git a/utils/ccrp.h b/utils/ccrp.h
index 68769635..c883c027 100644
--- a/utils/ccrp.h
+++ b/utils/ccrp.h
@@ -19,29 +19,44 @@ template <typename Dish, typename DishHash = boost::hash<Dish> >
 class CCRP {
  public:
   CCRP(double disc, double alpha) :
-    num_tables_(),
-    num_customers_(),
-    discount_(disc),
-    alpha_(alpha),
-    discount_prior_alpha_(std::numeric_limits<double>::quiet_NaN()),
-    discount_prior_beta_(std::numeric_limits<double>::quiet_NaN()),
-    alpha_prior_shape_(std::numeric_limits<double>::quiet_NaN()),
-    alpha_prior_rate_(std::numeric_limits<double>::quiet_NaN()) {}
+      num_tables_(),
+      num_customers_(),
+      discount_(disc),
+      alpha_(alpha),
+      discount_prior_alpha_(std::numeric_limits<double>::quiet_NaN()),
+      discount_prior_beta_(std::numeric_limits<double>::quiet_NaN()),
+      alpha_prior_shape_(std::numeric_limits<double>::quiet_NaN()),
+      alpha_prior_rate_(std::numeric_limits<double>::quiet_NaN()) {
+    check_hyperparameters();
+  }
 
   CCRP(double d_alpha, double d_beta, double c_shape, double c_rate, double d = 0.9, double c = 1.0) :
-    num_tables_(),
-    num_customers_(),
-    discount_(d),
-    alpha_(c),
-    discount_prior_alpha_(d_alpha),
-    discount_prior_beta_(d_beta),
-    alpha_prior_shape_(c_shape),
-    alpha_prior_rate_(c_rate) {}
+      num_tables_(),
+      num_customers_(),
+      discount_(d),
+      alpha_(c),
+      discount_prior_alpha_(d_alpha),
+      discount_prior_beta_(d_beta),
+      alpha_prior_shape_(c_shape),
+      alpha_prior_rate_(c_rate) {
+    check_hyperparameters();
+  }
+
+  void check_hyperparameters() {
+    if (discount_ < 0.0 || discount_ >= 1.0) {
+      std::cerr << "Bad discount: " << discount_ << std::endl;
+      abort();
+    }
+    if (alpha_ <= -discount_) {
+      std::cerr << "Bad strength: " << alpha_ << " (discount=" << discount_ << ")" << std::endl;
+      abort();
+    }
+  }
 
   double discount() const { return discount_; }
   double alpha() const { return alpha_; }
-  void set_discount(double d) { discount_ = d; }
-  void set_alpha(double a) { alpha_ = a; }
+  void set_discount(double d) { discount_ = d; check_hyperparameters(); }
+  void set_alpha(double a) { alpha_ = a; check_hyperparameters(); }
 
   bool has_discount_prior() const {
     return !std::isnan(discount_prior_alpha_);
@@ -215,14 +230,15 @@ class CCRP {
     if (has_discount_prior())
       lp = Md::log_beta_density(discount, discount_prior_alpha_, discount_prior_beta_);
     if (has_alpha_prior())
-      lp += Md::log_gamma_density(alpha, alpha_prior_shape_, alpha_prior_rate_);
+      lp += Md::log_gamma_density(alpha + discount, alpha_prior_shape_, alpha_prior_rate_);
     assert(lp <= 0.0);
     if (num_customers_) {
       if (discount > 0.0) {
         const double r = lgamma(1.0 - discount);
-        lp += lgamma(alpha) - lgamma(alpha + num_customers_)
-             + num_tables_ * log(discount) + lgamma(alpha / discount + num_tables_)
-             - lgamma(alpha / discount);
+        if (alpha)
+          lp += lgamma(alpha) - lgamma(alpha / discount);
+        lp += - lgamma(alpha + num_customers_)
+             + num_tables_ * log(discount) + lgamma(alpha / discount + num_tables_);
         assert(std::isfinite(lp));
         for (typename std::tr1::unordered_map<Dish, DishLocations, DishHash>::const_iterator it = dish_locs_.begin();
              it != dish_locs_.end(); ++it) {
@@ -245,15 +261,17 @@ class CCRP {
     StrengthResampler sr(*this);
     for (int iter = 0; iter < nloop; ++iter) {
       if (has_alpha_prior()) {
-        alpha_ = slice_sampler1d(sr, alpha_, *rng, 0.0,
+        alpha_ = slice_sampler1d(sr, alpha_, *rng, -discount_,
                                std::numeric_limits<double>::infinity(), 0.0, niterations, 100*niterations);
       }
       if (has_discount_prior()) {
-        discount_ = slice_sampler1d(dr, discount_, *rng, std::numeric_limits<double>::min(),
+        double min_discount = std::numeric_limits<double>::min();
+        if (alpha_ < 0.0) min_discount = -alpha_;
+        discount_ = slice_sampler1d(dr, discount_, *rng, min_discount,
                                1.0, 0.0, niterations, 100*niterations);
       }
     }
-    alpha_ = slice_sampler1d(sr, alpha_, *rng, 0.0,
+    alpha_ = slice_sampler1d(sr, alpha_, *rng, -discount_,
                              std::numeric_limits<double>::infinity(), 0.0, niterations, 100*niterations);
   }
 
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