Add SpanWriter class for zero-allocation stream writing

Co-authored-by: stickies-v <stickies-v@protonmail.com>

src/streams.h

@@ -121,6 +121,38 @@ public:
     }
 };
 
+/** Minimal stream for writing to an existing span of bytes.
+ */
+class SpanWriter
+{
+private:
+    std::span<std::byte> m_dest;
+
+public:
+    explicit SpanWriter(std::span<std::byte> dest) : m_dest{dest} {}
+    template <typename... Args>
+    SpanWriter(std::span<std::byte> dest, Args&&... args) : SpanWriter{dest}
+    {
+        ::SerializeMany(*this, std::forward<Args>(args)...);
+    }
+
+    void write(std::span<const std::byte> src)
+    {
+        if (src.size() > m_dest.size()) {
+            throw std::ios_base::failure("SpanWriter::write(): exceeded buffer size");
+        }
+        memcpy(m_dest.data(), src.data(), src.size());
+        m_dest = m_dest.subspan(src.size());
+    }
+
+    template<typename T>
+    SpanWriter& operator<<(const T& obj)
+    {
+        ::Serialize(*this, obj);
+        return *this;
+    }
+};
+
 /** Double ended buffer combining vector and stream-like interfaces.
  *
  * >> and << read and write unformatted data using the above serialization templates.

src/test/streams_tests.cpp

@@ -207,6 +207,28 @@ BOOST_AUTO_TEST_CASE(streams_vector_writer)
     vch.clear();
 }
 
+BOOST_AUTO_TEST_CASE(streams_span_writer)
+{
+    unsigned char a(1);
+    unsigned char b(2);
+    unsigned char bytes[] = {3, 4, 5, 6};
+    std::array<std::byte, 8> arr{};
+
+    // Test operator<<
+    SpanWriter writer{arr};
+    writer << a << b;
+    BOOST_CHECK_EQUAL(HexStr(arr), "0102000000000000");
+
+    // Use variadic constructor and write to subspan.
+    SpanWriter{std::span{arr}.subspan(2), a, bytes, b};
+    BOOST_CHECK_EQUAL(HexStr(arr), "0102010304050602");
+
+    // Writing past the end throws
+    std::array<std::byte, 1> small{};
+    BOOST_CHECK_THROW(SpanWriter(std::span{small}, a, b), std::ios_base::failure);
+    BOOST_CHECK_THROW(SpanWriter(std::span{small}) << a << b, std::ios_base::failure);
+}
+
 BOOST_AUTO_TEST_CASE(streams_vector_reader)
 {
     std::vector<unsigned char> vch = {1, 255, 3, 4, 5, 6};