[libFuzzer] Use std::discrete_distribution for input selection.

[morehouse]

Summary

Since we're casting from double to size_t during input selection, we
really want a discrete distribution over size_t rather than a piecewise
distribution over doubles.

[kcc]

LGTM

lib/fuzzer/FuzzerCorpus.h

public:
   InputInfo &ChooseUnitToMutate(Random &Rand) {
     InputInfo &II = *Inputs[ChooseUnitIdxToMutate(Rand)];
     assert(!II.U.empty());
     return II;
   };
 
   // Returns an index of random unit from the corpus to mutate.
   size_t ChooseUnitIdxToMutate(Random &Rand) {
-    size_t Idx = static_cast<size_t>(CorpusDistribution(Rand));
+    size_t Idx = CorpusDistribution(Rand);
     assert(Idx < Inputs.size());
     return Idx;
   }
 
   void PrintStats() {
     for (size_t i = 0; i < Inputs.size(); i++) {
       const auto &II = *Inputs[i];
       Printf("  [% 3zd %s] sz: % 5zd runs: % 5zd succ: % 5zd focus: %d\n", i,
private:
   // Must be called whenever the corpus or unit weights are changed.
   //
   // Hypothesis: units added to the corpus last are more interesting.
   //
   // Hypothesis: inputs with infrequent features are more interesting.
   void UpdateCorpusDistribution() {
     size_t N = Inputs.size();
     assert(N);
-    Intervals.resize(N + 1);
     Weights.resize(N);
-    std::iota(Intervals.begin(), Intervals.end(), 0);
     for (size_t i = 0; i < N; i++)
       Weights[i] = Inputs[i]->NumFeatures
                        ? (i + 1) * (Inputs[i]->HasFocusFunction ? 1000 : 1)
                        : 0.;
     if (FeatureDebug) {
       for (size_t i = 0; i < N; i++)
         Printf("%zd ", Inputs[i]->NumFeatures);
       Printf("SCORE\n");
       for (size_t i = 0; i < N; i++)
         Printf("%f ", Weights[i]);
       Printf("Weights\n");
     }
-    CorpusDistribution = std::piecewise_constant_distribution<double>(
-        Intervals.begin(), Intervals.end(), Weights.begin());
+    CorpusDistribution =
+        std::discrete_distribution<size_t>(Weights.begin(), Weights.end());
   }
-  std::piecewise_constant_distribution<double> CorpusDistribution;
+  std::discrete_distribution<size_t> CorpusDistribution;
 
-  Vector<double> Intervals;
   Vector<double> Weights;
 
   std::unordered_set<std::string> Hashes;
   Vector<InputInfo*> Inputs;
 
   size_t NumAddedFeatures = 0;
   size_t NumUpdatedFeatures = 0;
   uint32_t InputSizesPerFeature[kFeatureSetSize];
   uint32_t SmallestElementPerFeature[kFeatureSetSize];
 
   std::string OutputCorpus;
 };
 
 }  // namespace fuzzer
 
 #endif  // LLVM_FUZZER_CORPUS