util: improve streams.h:FindByte() performance

Avoid use of the expensive mod operator (%) when calculating the
buffer offset. No functional difference.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

src/bench/streams_findbyte.cpp

@@ -20,7 +20,7 @@ static void FindByte(benchmark::Bench& bench)
 
     bench.run([&] {
         bf.SetPos(0);
-        bf.FindByte(1);
+        bf.FindByte(std::byte(1));
     });
 
     // Cleanup

src/streams.h

@@ -756,15 +756,25 @@ public:
     }
 
     //! search for a given byte in the stream, and remain positioned on it
-    void FindByte(uint8_t ch)
+    void FindByte(std::byte byte)
     {
+        // For best performance, avoid mod operation within the loop.
+        size_t buf_offset{size_t(m_read_pos % uint64_t(vchBuf.size()))};
         while (true) {
-            if (m_read_pos == nSrcPos)
+            if (m_read_pos == nSrcPos) {
+                // No more bytes available; read from the file into the buffer,
+                // setting nSrcPos to one beyond the end of the new data.
+                // Throws exception if end-of-file reached.
                 Fill();
-            if (vchBuf[m_read_pos % vchBuf.size()] == std::byte{ch}) {
-                break;
             }
-            m_read_pos++;
+            const size_t len{std::min<size_t>(vchBuf.size() - buf_offset, nSrcPos - m_read_pos)};
+            const auto it_start{vchBuf.begin() + buf_offset};
+            const auto it_find{std::find(it_start, it_start + len, byte)};
+            const size_t inc{size_t(std::distance(it_start, it_find))};
+            m_read_pos += inc;
+            if (inc < len) break;
+            buf_offset += inc;
+            if (buf_offset >= vchBuf.size()) buf_offset = 0;
         }
     }
 };

src/test/fuzz/buffered_file.cpp

@@ -53,7 +53,7 @@ FUZZ_TARGET(buffered_file)
                         return;
                     }
                     try {
-                        opt_buffered_file->FindByte(fuzzed_data_provider.ConsumeIntegral<uint8_t>());
+                        opt_buffered_file->FindByte(std::byte(fuzzed_data_provider.ConsumeIntegral<uint8_t>()));
                     } catch (const std::ios_base::failure&) {
                     }
                 },

src/test/streams_tests.cpp

@@ -463,7 +463,7 @@ BOOST_AUTO_TEST_CASE(streams_buffered_file_rand)
                 size_t find = currentPos + InsecureRandRange(8);
                 if (find >= fileSize)
                     find = fileSize - 1;
-                bf.FindByte(uint8_t(find));
+                bf.FindByte(std::byte(find));
                 // The value at each offset is the offset.
                 BOOST_CHECK_EQUAL(bf.GetPos(), find);
                 currentPos = find;

src/validation.cpp

@@ -4565,7 +4565,7 @@ void Chainstate::LoadExternalBlockFile(
             try {
                 // locate a header
                 unsigned char buf[CMessageHeader::MESSAGE_START_SIZE];
-                blkdat.FindByte(params.MessageStart()[0]);
+                blkdat.FindByte(std::byte(params.MessageStart()[0]));
                 nRewind = blkdat.GetPos() + 1;
                 blkdat >> buf;
                 if (memcmp(buf, params.MessageStart(), CMessageHeader::MESSAGE_START_SIZE)) {