Merge #18190: tests: Add fuzzing harness for Golomb-Rice coding (GolombRiceEncode/GolombRiceDecode)

69749fbe6a tests: Add fuzzing harness for Golomb-Rice coding (GolombRiceEncode/GolombRiceDecode) (practicalswift)

Pull request description:

  Add fuzzing harness for Golomb-Rice coding (`GolombRiceEncode`/`GolombRiceDecode`).

  Test this PR using:

  ```
  $ make distclean
  $ ./autogen.sh
  $ CC=clang CXX=clang++ ./configure --enable-fuzz \
        --with-sanitizers=address,fuzzer,undefined
  $ make
  $ src/test/fuzz/golomb_rice
  …
  ```

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Tree-SHA512: 1b26512301b8c22ab3b804d9b9e4baf933f26f8c05e462d583863badcec7e694548a34849a0d7c4ff7d58b19f6338b51819976ecf642bc4659b04ef71182d748

src/util/golombrice.h

@@ -0,0 +1,43 @@
+// Copyright (c) 2018-2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_GOLOMBRICE_H
+#define BITCOIN_UTIL_GOLOMBRICE_H
+
+#include <streams.h>
+
+#include <cstdint>
+
+template <typename OStream>
+void GolombRiceEncode(BitStreamWriter<OStream>& bitwriter, uint8_t P, uint64_t x)
+{
+    // Write quotient as unary-encoded: q 1's followed by one 0.
+    uint64_t q = x >> P;
+    while (q > 0) {
+        int nbits = q <= 64 ? static_cast<int>(q) : 64;
+        bitwriter.Write(~0ULL, nbits);
+        q -= nbits;
+    }
+    bitwriter.Write(0, 1);
+
+    // Write the remainder in P bits. Since the remainder is just the bottom
+    // P bits of x, there is no need to mask first.
+    bitwriter.Write(x, P);
+}
+
+template <typename IStream>
+uint64_t GolombRiceDecode(BitStreamReader<IStream>& bitreader, uint8_t P)
+{
+    // Read unary-encoded quotient: q 1's followed by one 0.
+    uint64_t q = 0;
+    while (bitreader.Read(1) == 1) {
+        ++q;
+    }
+
+    uint64_t r = bitreader.Read(P);
+
+    return (q << P) + r;
+}
+
+#endif // BITCOIN_UTIL_GOLOMBRICE_H