tests: Add amount compression/decompression fuzzing to existing fuzzing harness #17917

Small fuzzing improvement:

Add amount compression/decompression fuzzing to existing fuzzing harness: test compression round-trip (DecompressAmount(CompressAmount(…))).

Make the domain of CompressAmount(…) explicit.

Amount compression primer:

    Compact serialization for amounts

    Special serializer/deserializer for amount values. It is optimized for
    values which have few non-zero digits in decimal representation. Most
    amounts currently in the txout set take only 1 or 2 bytes to
    represent.

How to test this PR

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

ACK on the fuzzer. Not sure about the value of the comment.

@MarcoFalke Do you have any suggestion on how to phrase the precondition comment to make it more valuable?

@MarcoFalke

This is an example of a gotcha where the upper limit kicks in, an unsigned integer wraparound occurs and the DecompressAmount(CompressAmount(…)) roundtrip fails:

int main(void) {
  using CAmount = int64_t;
  CAmount before = -1;
  CAmount after = DecompressAmount(CompressAmount(before));

  std::cout << "before=" << before << std::endl;
  std::cout << "after=" << after << std::endl;
  assert(before == after);
}

-1 (int64_t) gets implicitly converted to std::numeric_limits<uint64_t>::max() which is above the mentioned upper limit around ~2^64 / 9.

Output:

comp.cpp:18:32: runtime error: unsigned integer overflow: 16602069666338596453 * 10 cannot be represented in type 'unsigned long'
comp.cpp:43:7: runtime error: unsigned integer overflow: 2049638230412172401 * 10 cannot be represented in type 'unsigned long'
before=-1
after=2049638230412172324
comp: comp.cpp:59: int main(): Assertion `before == after' failed.
Aborted (core dumped)

Rebased! :)

ACK. LGTM, just one nit. Let me know if you want to address it or just have this merged as is.