extract msgpack into its own static library; move non-templated bodies to msgpack.cpp; introduce util INTERFACE lib for span_writer

util/CMakeLists.txt

@@ -0,0 +1,2 @@
+add_library(util INTERFACE)
+target_include_directories(util INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})

util/span_writer.h

@@ -0,0 +1,50 @@
+#pragma once
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <span>
+
+class span_writer {
+    uint8_t *m_data;
+    size_t m_capacity;
+    size_t m_size = 0;
+    bool m_overflow = false;
+
+public:
+    span_writer(uint8_t *data, size_t capacity) : m_data(data), m_capacity(capacity) {}
+    span_writer(std::span<uint8_t> buf) : m_data(buf.data()), m_capacity(buf.size()) {}
+
+    void push_back(uint8_t v) {
+        if (m_size < m_capacity) m_data[m_size++] = v;
+        else m_overflow = true;
+    }
+
+    template <class It>
+    void insert(uint8_t *, It first, It last) {
+        while (first != last) {
+            if (m_size < m_capacity) m_data[m_size++] = *first++;
+            else { m_overflow = true; return; }
+        }
+    }
+
+    size_t size() const { return m_size; }
+    size_t capacity() const { return m_capacity; }
+    bool full() const { return m_size >= m_capacity; }
+    bool overflow() const { return m_overflow; }
+
+    uint8_t *data() { return m_data; }
+    const uint8_t *data() const { return m_data; }
+
+    uint8_t *begin() { return m_data; }
+    uint8_t *end() { return m_data + m_size; }
+    const uint8_t *begin() const { return m_data; }
+    const uint8_t *end() const { return m_data + m_size; }
+
+    span_writer subspan(size_t offset) {
+        return span_writer(m_data + offset, m_capacity - offset);
+    }
+
+    span_writer subspan(size_t offset, size_t len) {
+        return span_writer(m_data + offset, len);
+    }
+};