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

7e9c711 compressor: Make the domain of CompressAmount(...) explicit (practicalswift)
4a7fd7a tests: Add amount compression/decompression fuzzing to existing fuzzing harness: test compression round-trip (practicalswift)

Pull request description:

  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
  …
  ```

Top commit has no ACKs.

Tree-SHA512: 0f7c05b97012ccd5cd05a96c209e6b4d7d2fa73138bac9615cf531baa3f614f9003e29a198015bcc083af9f5bdc752bb52615b82c5df3c519b1a064bd4fc6664

Author: MarcoFalke

src/test/fuzz/integer.cpp

@@ -2,6 +2,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <amount.h>
 #include <arith_uint256.h>
 #include <compressor.h>
 #include <consensus/merkle.h>
@@ -56,7 +57,14 @@ void test_one_input(const std::vector<uint8_t>& buffer)
 
     const Consensus::Params& consensus_params = Params().GetConsensus();
     (void)CheckProofOfWork(u256, u32, consensus_params);
-    (void)CompressAmount(u64);
+    if (u64 <= MAX_MONEY) {
+        const uint64_t compressed_money_amount = CompressAmount(u64);
+        assert(u64 == DecompressAmount(compressed_money_amount));
+        static const uint64_t compressed_money_amount_max = CompressAmount(MAX_MONEY - 1);
+        assert(compressed_money_amount <= compressed_money_amount_max);
+    } else {
+        (void)CompressAmount(u64);
+    }
     static const uint256 u256_min(uint256S("0000000000000000000000000000000000000000000000000000000000000000"));
     static const uint256 u256_max(uint256S("ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"));
     const std::vector<uint256> v256{u256, u256_min, u256_max};