Overflow detection, span-based signatures, flatten control flow

firmware/include/span_writer.h

@@ -8,6 +8,7 @@ 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) {}
@@ -15,17 +16,21 @@ public:
 
     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 && m_size < m_capacity)
-            m_data[m_size++] = *first++;
+        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; }