Split into main/net/tusb_config, extract ring_buffer and usb_cdc headers

include/ring_buffer.h

@@ -0,0 +1,44 @@
+#pragma once
+#include <array>
+#include <cstdint>
+#include <span>
+
+template <uint16_t N>
+struct ring_buffer {
+    std::array<uint8_t, N> data = {};
+    uint16_t head = 0;
+    uint16_t tail = 0;
+
+    uint16_t used() const { return tail - head; }
+    uint16_t free() const { return N - used(); }
+    bool empty() const { return head == tail; }
+
+    void push(std::span<const uint8_t> src) {
+        if (src.size() > free()) return;
+        for (auto b : src)
+            data[(tail++) % N] = b;
+    }
+
+    uint16_t peek(std::span<uint8_t> dst) const {
+        uint16_t len = dst.size() < used() ? dst.size() : used();
+        for (uint16_t i = 0; i < len; i++)
+            dst[i] = data[(head + i) % N];
+        return len;
+    }
+
+    void consume(uint16_t len) {
+        head += len;
+        if (head >= N) {
+            head -= N;
+            tail -= N;
+        }
+    }
+
+    std::span<const uint8_t> read_contiguous() const {
+        uint16_t offset = head % N;
+        uint16_t contig = N - offset;
+        uint16_t pending = used();
+        uint16_t len = pending < contig ? pending : contig;
+        return {data.data() + offset, len};
+    }
+};