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

@@ -41,8 +41,9 @@ public:
 TEST(MultinomialSampler, gen) {
   int numGrids = 1024 * 1024;
   int size = 1024 * 4;
-
   default_random_engine reng;
+
+  for (size_t iter=0; iter < 256; ++iter) {
     uniform_int_distribution<int> rand(1, numGrids / size * 1.8);
     vector<real> prob;
     int sum = 0;
@@ -50,6 +51,7 @@ TEST(MultinomialSampler, gen) {
       prob.push_back(rand(reng));
       sum += prob.back();
     }
+
     CHECK_LE(sum, numGrids);
     prob.back() += numGrids - sum;
 
@@ -72,11 +74,12 @@ TEST(MultinomialSampler, gen) {
     for (int i = 0; i < size; ++i) {
       if (prob[i] != counts[i]) {
         EXPECT_EQ(prob[i], counts[i]);
-      LOG(INFO) << "i=" << i;
+        LOG(INFO) << iter;
         break;
       }
     }
   }
+}
 
 void benchmarkRandom() {
   int n = 1024 * 1024;
@@ -135,6 +138,7 @@ void benchmarkRandom() {
   LOG(INFO) << "sum1=" << sum1;
 }
 
+
 int main(int argc, char** argv) {
   initMain(argc, argv);
   testing::InitGoogleTest(&argc, argv);