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diff --git a/tools/fuzzing/libfuzzer/FuzzerDataFlowTrace.h b/tools/fuzzing/libfuzzer/FuzzerDataFlowTrace.h
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+//===- FuzzerDataFlowTrace.h - Internal header for the Fuzzer ---*- C++ -* ===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// fuzzer::DataFlowTrace; reads and handles a data-flow trace.
+//
+// A data flow trace is generated by e.g. dataflow/DataFlow.cpp
+// and is stored on disk in a separate directory.
+//
+// The trace dir contains a file 'functions.txt' which lists function names,
+// oner per line, e.g.
+// ==> functions.txt <==
+// Func2
+// LLVMFuzzerTestOneInput
+// Func1
+//
+// All other files in the dir are the traces, see dataflow/DataFlow.cpp.
+// The name of the file is sha1 of the input used to generate the trace.
+//
+// Current status:
+//   the data is parsed and the summary is printed, but the data is not yet
+//   used in any other way.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_DATA_FLOW_TRACE
+#define LLVM_FUZZER_DATA_FLOW_TRACE
+
+#include "FuzzerDefs.h"
+#include "FuzzerIO.h"
+
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+#include <string>
+
+namespace fuzzer {
+
+int CollectDataFlow(const std::string &DFTBinary, const std::string &DirPath,
+                    const Vector<SizedFile> &CorporaFiles);
+
+class BlockCoverage {
+ public:
+  bool AppendCoverage(std::istream &IN);
+  bool AppendCoverage(const std::string &S);
+
+  size_t NumCoveredFunctions() const { return Functions.size(); }
+
+  uint32_t GetCounter(size_t FunctionId, size_t BasicBlockId) {
+    auto It = Functions.find(FunctionId);
+    if (It == Functions.end()) return 0;
+    const auto &Counters = It->second;
+    if (BasicBlockId < Counters.size())
+      return Counters[BasicBlockId];
+    return 0;
+  }
+
+  uint32_t GetNumberOfBlocks(size_t FunctionId) {
+    auto It = Functions.find(FunctionId);
+    if (It == Functions.end()) return 0;
+    const auto &Counters = It->second;
+    return Counters.size();
+  }
+
+  uint32_t GetNumberOfCoveredBlocks(size_t FunctionId) {
+    auto It = Functions.find(FunctionId);
+    if (It == Functions.end()) return 0;
+    const auto &Counters = It->second;
+    uint32_t Result = 0;
+    for (auto Cnt: Counters)
+      if (Cnt)
+        Result++;
+    return Result;
+  }
+
+  Vector<double> FunctionWeights(size_t NumFunctions) const;
+  void clear() { Functions.clear(); }
+
+ private:
+
+  typedef Vector<uint32_t> CoverageVector;
+
+  uint32_t NumberOfCoveredBlocks(const CoverageVector &Counters) const {
+    uint32_t Res = 0;
+    for (auto Cnt : Counters)
+      if (Cnt)
+        Res++;
+    return Res;
+  }
+
+  uint32_t NumberOfUncoveredBlocks(const CoverageVector &Counters) const {
+    return Counters.size() - NumberOfCoveredBlocks(Counters);
+  }
+
+  uint32_t SmallestNonZeroCounter(const CoverageVector &Counters) const {
+    assert(!Counters.empty());
+    uint32_t Res = Counters[0];
+    for (auto Cnt : Counters)
+      if (Cnt)
+        Res = Min(Res, Cnt);
+    assert(Res);
+    return Res;
+  }
+
+  // Function ID => vector of counters.
+  // Each counter represents how many input files trigger the given basic block.
+  std::unordered_map<size_t, CoverageVector> Functions;
+  // Functions that have DFT entry.
+  std::unordered_set<size_t> FunctionsWithDFT;
+};
+
+class DataFlowTrace {
+ public:
+  int ReadCoverage(const std::string &DirPath);
+  int Init(const std::string &DirPath, std::string *FocusFunction,
+            Vector<SizedFile> &CorporaFiles, Random &Rand);
+  void Clear() { Traces.clear(); }
+  const Vector<uint8_t> *Get(const std::string &InputSha1) const {
+    auto It = Traces.find(InputSha1);
+    if (It != Traces.end())
+      return &It->second;
+    return nullptr;
+  }
+
+ private:
+  // Input's sha1 => DFT for the FocusFunction.
+  std::unordered_map<std::string, Vector<uint8_t> > Traces;
+  BlockCoverage Coverage;
+  std::unordered_set<std::string> CorporaHashes;
+};
+}  // namespace fuzzer
+
+#endif // LLVM_FUZZER_DATA_FLOW_TRACE