Try to fix MultinomialSampler (#102)

* Also refine unittest to multiple iteration to prevent luckily random number.

paddle/gserver/layers/MultinomialSampler.cpp

@@ -19,7 +19,7 @@ namespace paddle {
 
 MultinomialSampler::MultinomialSampler(const real* prob, int size)
     : rand_(0.0, size) {
-  intervals_.reserve(size + 1);
+  intervals_.resize(size + 1);
   double sum = 0;
   for (int i = 0; i < size; ++i) {
     sum += prob[i];
@@ -50,12 +50,13 @@ MultinomialSampler::MultinomialSampler(const real* prob, int size)
   int bigPos = nextBigPos(0);
 
   auto fillIntervals = [&]() {
-    while (bigPos < size && smallPos < size) {
+    while (bigPos < size) {
       while (intervals_[bigPos].thresh > 1 && smallPos < size) {
         intervals_[smallPos].otherId = bigPos;
         intervals_[bigPos].thresh -= 1 - intervals_[smallPos].thresh;
         smallPos = nextSmallPos(smallPos + 1);
       }
+      if (smallPos >= size) break;
       bigPos = nextBigPos(bigPos + 1);
       // If intervals_[bigPos].thresh < 1, it becomes a small interval
     }

paddle/gserver/tests/test_MultinomialSampler.cpp

@@ -41,39 +41,42 @@ public:
 TEST(MultinomialSampler, gen) {
   int numGrids = 1024 * 1024;
   int size = 1024 * 4;
-
   default_random_engine reng;
-  uniform_int_distribution<int> rand(1, numGrids / size * 1.8);
-  vector<real> prob;
-  int sum = 0;
-  for (int i = 0; i < size; ++i) {
-    prob.push_back(rand(reng));
-    sum += prob.back();
-  }
-  CHECK_LE(sum, numGrids);
-  prob.back() += numGrids - sum;
 
-  vector<int> counts(size);
-  MultinomialSamplerTester sampler(&prob[0], size);
-  counts.assign(size, 0);
-  {
-    double s = (double)size / (double)numGrids;
-    REGISTER_TIMER("MultinomialSampler");
-    for (double i = 0; i < numGrids; ++i) {
-      int ret = sampler.testGen([i, s]() { return s * i; });
-      if (ret < 0 || ret >= size) {
-        EXPECT_GE(ret, 0);
-        EXPECT_LT(ret, size);
-        break;
+  for (size_t iter=0; iter < 256; ++iter) {
+    uniform_int_distribution<int> rand(1, numGrids / size * 1.8);
+    vector<real> prob;
+    int sum = 0;
+    for (int i = 0; i < size; ++i) {
+      prob.push_back(rand(reng));
+      sum += prob.back();
+    }
+
+    CHECK_LE(sum, numGrids);
+    prob.back() += numGrids - sum;
+
+    vector<int> counts(size);
+    MultinomialSamplerTester sampler(&prob[0], size);
+    counts.assign(size, 0);
+    {
+      double s = (double)size / (double)numGrids;
+      REGISTER_TIMER("MultinomialSampler");
+      for (double i = 0; i < numGrids; ++i) {
+        int ret = sampler.testGen([i, s]() { return s * i; });
+        if (ret < 0 || ret >= size) {
+          EXPECT_GE(ret, 0);
+          EXPECT_LT(ret, size);
+          break;
+        }
+        ++counts[ret];
       }
-      ++counts[ret];
     }
-  }
-  for (int i = 0; i < size; ++i) {
-    if (prob[i] != counts[i]) {
-      EXPECT_EQ(prob[i], counts[i]);
-      LOG(INFO) << "i=" << i;
-      break;
+    for (int i = 0; i < size; ++i) {
+      if (prob[i] != counts[i]) {
+        EXPECT_EQ(prob[i], counts[i]);
+        LOG(INFO) << iter;
+        break;
+      }
     }
   }
 }
@@ -135,6 +138,7 @@ void benchmarkRandom() {
   LOG(INFO) << "sum1=" << sum1;
 }
 
+
 int main(int argc, char** argv) {
   initMain(argc, argv);
   testing::InitGoogleTest(&argc, argv);