Index: llvm/trunk/lib/Fuzzer/FuzzerInterface.h
===================================================================
--- llvm/trunk/lib/Fuzzer/FuzzerInterface.h
+++ llvm/trunk/lib/Fuzzer/FuzzerInterface.h
@@ -66,18 +66,6 @@
   // Return a random number in range [0,n).
   size_t operator()(size_t n) { return n ? Rand() % n : 0; }
   bool RandBool() { return Rand() % 2; }
-
-  // The methods below is to satisfy UniformRandomNumberGenerator:
-  // http://en.cppreference.com/w/cpp/concept/UniformRandomNumberGenerator\
-
-  // Returns a random number between 0 and RAND_MAX inclusive.
-  double operator()() { return operator()(RAND_MAX); }
-
-  // Returns the smallest value that operator() may return.
-  double min() { return 0; }
-
-  // Returns the largest value that operator() may return.
-  double max() { return RAND_MAX; }
 };
 
 // Using libc's stand/rand.
Index: llvm/trunk/lib/Fuzzer/FuzzerInternal.h
===================================================================
--- llvm/trunk/lib/Fuzzer/FuzzerInternal.h
+++ llvm/trunk/lib/Fuzzer/FuzzerInternal.h
@@ -17,7 +17,6 @@
 #include <chrono>
 #include <cstddef>
 #include <cstdlib>
-#include <random>
 #include <string>
 #include <string.h>
 #include <vector>
@@ -201,7 +200,7 @@
     bool PrintNewCovPcs = false;
   };
   Fuzzer(UserSuppliedFuzzer &USF, FuzzingOptions Options);
-  void AddToCorpus(const Unit &U) { Corpus.push_back(U); UpdateCorpusDistribution(); }
+  void AddToCorpus(const Unit &U) { Corpus.push_back(U); }
   size_t ChooseUnitIdxToMutate();
   const Unit &ChooseUnitToMutate() { return Corpus[ChooseUnitIdxToMutate()]; };
   void Loop();
@@ -242,9 +241,6 @@
   void WriteUnitToFileWithPrefix(const Unit &U, const char *Prefix);
   void PrintStats(const char *Where, const char *End = "\n");
   void PrintStatusForNewUnit(const Unit &U);
-  // Updates the probability distribution for the units in the corpus.
-  // Must be called whenever the corpus or unit weights are changed.
-  void UpdateCorpusDistribution();
 
   void SyncCorpus();
 
@@ -284,7 +280,6 @@
     return Res;
   }
 
-  std::piecewise_constant_distribution<double> CorpusDistribution;
   UserSuppliedFuzzer &USF;
   FuzzingOptions Options;
   system_clock::time_point ProcessStartTime = system_clock::now();
Index: llvm/trunk/lib/Fuzzer/FuzzerLoop.cpp
===================================================================
--- llvm/trunk/lib/Fuzzer/FuzzerLoop.cpp
+++ llvm/trunk/lib/Fuzzer/FuzzerLoop.cpp
@@ -163,7 +163,6 @@
     if (UnitHashesAddedToCorpus.insert(Hash(X)).second) {
       if (RunOne(X)) {
         Corpus.push_back(X);
-        UpdateCorpusDistribution();
         PrintStats("RELOAD");
       }
     }
@@ -201,7 +200,6 @@
     }
   }
   Corpus = NewCorpus;
-  UpdateCorpusDistribution();
   for (auto &X : Corpus)
     UnitHashesAddedToCorpus.insert(Hash(X));
   PrintStats("INITED");
@@ -349,7 +347,6 @@
 
 void Fuzzer::ReportNewCoverage(const Unit &U) {
   Corpus.push_back(U);
-  UpdateCorpusDistribution();
   UnitHashesAddedToCorpus.insert(Hash(U));
   USF.GetMD().RecordSuccessfulMutationSequence();
   PrintStatusForNewUnit(U);
@@ -412,11 +409,22 @@
 
 // Returns an index of random unit from the corpus to mutate.
 // Hypothesis: units added to the corpus last are more likely to be interesting.
-// This function gives more weight to the more recent units.
+// This function gives more wieght to the more recent units.
 size_t Fuzzer::ChooseUnitIdxToMutate() {
-    size_t Idx = static_cast<size_t>(CorpusDistribution(USF.GetRand()));
-    assert(Idx < Corpus.size());
-    return Idx;
+    size_t N = Corpus.size();
+    size_t Total = (N + 1) * N / 2;
+    size_t R = USF.GetRand()(Total);
+    size_t IdxBeg = 0, IdxEnd = N;
+    // Binary search.
+    while (IdxEnd - IdxBeg >= 2) {
+      size_t Idx = IdxBeg + (IdxEnd - IdxBeg) / 2;
+      if (R > (Idx + 1) * Idx / 2)
+        IdxBeg = Idx;
+      else
+        IdxEnd = Idx;
+    }
+    assert(IdxBeg < N);
+    return IdxBeg;
 }
 
 // Experimental search heuristic: drilling.
@@ -439,7 +447,6 @@
   std::vector<Unit> SavedCorpus;
   SavedCorpus.swap(Corpus);
   Corpus.push_back(U);
-  UpdateCorpusDistribution();
   assert(Corpus.size() == 1);
   RunOne(U);
   PrintStats("DRILL ");
@@ -503,14 +510,4 @@
   ExecuteCommand(Options.SyncCommand + " " + Options.OutputCorpus);
 }
 
-void Fuzzer::UpdateCorpusDistribution() {
-  size_t N = Corpus.size();
-  std::vector<double> Intervals(N+1);
-  std::vector<double> Weights(N);
-  std::iota(Intervals.begin(), Intervals.end(), 0);
-  std::iota(Weights.begin(), Weights.end(), 1);
-  CorpusDistribution = std::piecewise_constant_distribution<double>(
-      Intervals.begin(), Intervals.end(), Weights.begin());
-}
-
 }  // namespace fuzzer
Index: llvm/trunk/lib/Fuzzer/test/FuzzerUnittest.cpp
===================================================================
--- llvm/trunk/lib/Fuzzer/test/FuzzerUnittest.cpp
+++ llvm/trunk/lib/Fuzzer/test/FuzzerUnittest.cpp
@@ -6,7 +6,7 @@
 
 // For now, have LLVMFuzzerTestOneInput just to make it link.
 // Later we may want to make unittests that actually call LLVMFuzzerTestOneInput.
-extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+extern "C" void LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
   abort();
 }
 
@@ -400,23 +400,3 @@
   EXPECT_EQ("YWJjeHk=", Base64({'a', 'b', 'c', 'x', 'y'}));
   EXPECT_EQ("YWJjeHl6", Base64({'a', 'b', 'c', 'x', 'y', 'z'}));
 }
-
-TEST(Corpus, Distribution) {
-  FuzzerRandomLibc Rand(0);
-  SimpleUserSuppliedFuzzer USF(&Rand, LLVMFuzzerTestOneInput);
-  Fuzzer::FuzzingOptions Options;
-  Fuzzer Fuzz(USF, Options);
-  size_t N = 10;
-  size_t TriesPerUnit = 1<<20;
-  for (size_t i = 0; i < N; i++) {
-    Fuzz.AddToCorpus(Unit{ static_cast<uint8_t>(i) });
-  }
-  std::vector<size_t> Hist(N);
-  for (size_t i = 0; i < N * TriesPerUnit; i++) {
-    Hist[Fuzz.ChooseUnitIdxToMutate()]++;
-  }
-  for (size_t i = 0; i < N; i++) {
-    // A weak sanity check that every unit gets invoked.
-    EXPECT_GT(Hist[i], TriesPerUnit / N / 3);
-  }
-}