From bb602ade9c970272f281cd8a42b643cc09a9fa29 Mon Sep 17 00:00:00 2001
From: Patrick Simianer <p@simianer.de>
Date: Mon, 14 Nov 2011 01:20:50 +0100
Subject: l1 regularzation

---
 dtrain/dtrain.cc               | 51 +++++++++++++++++++++++++++++++++++++++++-
 dtrain/dtrain.h                |  6 +++++
 dtrain/test/example/dtrain.ini |  6 +++--
 3 files changed, 60 insertions(+), 3 deletions(-)

diff --git a/dtrain/dtrain.cc b/dtrain/dtrain.cc
index ca5f0c5e..448e639c 100644
--- a/dtrain/dtrain.cc
+++ b/dtrain/dtrain.cc
@@ -26,6 +26,8 @@ dtrain_init(int argc, char** argv, po::variables_map* cfg)
     ("select_weights", po::value<string>()->default_value("last"), "output 'best' or 'last' weights ('VOID' to throw away)")
     ("keep_w",         po::value<bool>()->zero_tokens(),                              "protocol weights for each iteration")
     ("unit_weight_vector", po::value<bool>()->zero_tokens(),                       "Rescale weight vector after each input")
+    ("l1_reg",         po::value<string>()->default_value("no"),         "apply l1 regularization as in Tsuroka et al 2010")
+    ("l1_reg_strength", po::value<weight_t>(),                                                 "l1 regularization strength")
 #ifdef DTRAIN_LOCAL
     ("refs,r",         po::value<string>(),                                                      "references in local mode")
 #endif
@@ -155,13 +157,25 @@ main(int argc, char** argv)
 
   // init weights
   vector<weight_t>& dense_weights = decoder.CurrentWeightVector();
-  SparseVector<weight_t> lambdas;
+  SparseVector<weight_t> lambdas, cumulative_penalties;
   if (cfg.count("input_weights")) Weights::InitFromFile(cfg["input_weights"].as<string>(), &dense_weights);
   Weights::InitSparseVector(dense_weights, &lambdas);
 
   // meta params for perceptron, SVM
   weight_t eta = cfg["learning_rate"].as<weight_t>();
   weight_t gamma = cfg["gamma"].as<weight_t>();
+  // l1 regularization
+  bool l1naive = false;
+  bool l1clip = false;
+  bool l1cumul = false;
+  weight_t l1_reg = 0;
+  if (cfg["l1_reg"].as<string>() != "no") {
+    string s = cfg["l1_reg"].as<string>();
+    if (s == "naive") l1naive = true;
+    else if (s == "clip") l1clip = true;
+    else if (s == "cumul") l1cumul = true;
+    l1_reg = cfg["l1_reg_strength"].as<weight_t>();
+  }
 
   // output
   string output_fn = cfg["output"].as<string>();
@@ -388,6 +402,41 @@ main(int argc, char** argv)
           lambdas.plus_eq_v_times_s(lambdas, -2*gamma*eta*(1./npairs));
         }
       }
+
+      // reset cumulative_penalties after 1 iter? 
+      // do this only once per INPUT (not per pair)
+      if (l1naive) {
+        for (unsigned d = 0; d < lambdas.size(); d++) {
+          weight_t v = lambdas.get(d);
+          lambdas.set_value(d, v - sign(v) * l1_reg);
+        }
+      } else if (l1clip) {
+        for (unsigned d = 0; d < lambdas.size(); d++) {
+          if (lambdas.nonzero(d)) {
+            weight_t v = lambdas.get(d);
+            if (v > 0) {
+              lambdas.set_value(d, max(0., v - l1_reg));
+            } else {
+              lambdas.set_value(d, min(0., v + l1_reg));
+            }
+          }
+        }
+      } else if (l1cumul) {
+        weight_t acc_penalty = (ii+1) * l1_reg; // ii is the index of the current input 
+        for (unsigned d = 0; d < lambdas.size(); d++) {
+          if (lambdas.nonzero(d)) {
+            weight_t v = lambdas.get(d);
+            weight_t penalty = 0;
+            if (v > 0) {
+              penalty = max(0., v-(acc_penalty + cumulative_penalties.get(d)));
+            } else {
+              penalty = min(0., v+(acc_penalty - cumulative_penalties.get(d)));
+            }
+            lambdas.set_value(d, penalty);
+            cumulative_penalties.set_value(d, cumulative_penalties.get(d)+penalty);
+          }
+        }
+      }
     }
 
     if (unit_weight_vector && sample_from == "forest") lambdas /= lambdas.l2norm();
diff --git a/dtrain/dtrain.h b/dtrain/dtrain.h
index 84f3f1f5..cfc3f460 100644
--- a/dtrain/dtrain.h
+++ b/dtrain/dtrain.h
@@ -85,5 +85,11 @@ inline void printWordIDVec(vector<WordID>& v)
   }
 }
 
+template<typename T>
+inline T sign(T z) {
+  if (z == 0) return 0;
+  return z < 0 ? -1 : +1;
+}
+
 #endif
 
diff --git a/dtrain/test/example/dtrain.ini b/dtrain/test/example/dtrain.ini
index 95eeb8e5..900878a5 100644
--- a/dtrain/test/example/dtrain.ini
+++ b/dtrain/test/example/dtrain.ini
@@ -6,11 +6,13 @@ epochs=20
 input=test/example/nc-1k-tabs.gz
 scorer=stupid_bleu
 output=weights.gz
-stop_after=10
+stop_after=100
 sample_from=forest
 pair_sampling=108010
 select_weights=VOID
 print_weights=Glue WordPenalty LanguageModel LanguageModel_OOV PhraseModel_0 PhraseModel_1 PhraseModel_2 PhraseModel_3 PhraseModel_4 PassThrough
 tmp=/tmp
-unit_weight_vector=true
+#unit_weight_vector=
 keep_w=true
+#l1_reg=clip
+#l1_reg_strength=0.00001
-- 
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