1 files changed, 16 insertions, 12 deletions
diff --git a/pw_hdlc/public/pw_hdlc/decoder.h b/pw_hdlc/public/pw_hdlc/decoder.h
index 94caf17b0..0290a2ad8 100644
--- a/pw_hdlc/public/pw_hdlc/decoder.h
+++ b/pw_hdlc/public/pw_hdlc/decoder.h
@@ -22,6 +22,7 @@
 #include "pw_assert/assert.h"
 #include "pw_bytes/span.h"
 #include "pw_checksum/crc32.h"
+#include "pw_hdlc/internal/protocol.h"
 #include "pw_result/result.h"
 #include "pw_status/status.h"
 
@@ -30,19 +31,11 @@ namespace pw::hdlc {
 // Represents the contents of an HDLC frame -- the unescaped data between two
 // flag bytes. Instances of Frame are only created when a full, valid frame has
 // been read.
-//
-// For now, the Frame class assumes a single-byte control field and a 32-bit
-// frame check sequence (FCS).
 class Frame {
- private:
-  static constexpr size_t kMinimumAddressSize = 1;
-  static constexpr size_t kControlSize = 1;
-  static constexpr size_t kFcsSize = sizeof(uint32_t);
-
  public:
   // The minimum size of a frame, excluding control bytes (flag or escape).
-  static constexpr size_t kMinSizeBytes =
-      kMinimumAddressSize + kControlSize + kFcsSize;
+  static constexpr size_t kMinContentSizeBytes =
+      kMinAddressSize + kControlSize + kFcsSize;
 
   static Result<Frame> Parse(ConstByteSpan frame);
 
@@ -95,6 +88,17 @@ class Decoder {
   //
   Result<Frame> Process(std::byte new_byte);
 
+  // Returns the buffer space required for a `Decoder` to successfully decode a
+  // frame whose on-the-wire HDLC encoded size does not exceed `max_frame_size`.
+  static constexpr size_t RequiredBufferSizeForFrameSize(
+      size_t max_frame_size) {
+    // Flag bytes aren't stored in the internal buffer, so we can save a couple
+    // bytes.
+    return max_frame_size < Frame::kMinContentSizeBytes
+               ? Frame::kMinContentSizeBytes
+               : max_frame_size - 2;
+  }
+
   // Processes a span of data and calls the provided callback with each frame or
   // error.
   template <typename F, typename... Args>
@@ -115,7 +119,7 @@ class Decoder {
   void Clear() {
     state_ = State::kInterFrame;
     Reset();
-  };
+  }
 
  private:
   // State enum class is used to make the Decoder a finite state machine.
@@ -165,7 +169,7 @@ class DecoderBuffer : public Decoder {
   static constexpr size_t max_size() { return kSizeBytes; }
 
  private:
-  static_assert(kSizeBytes >= Frame::kMinSizeBytes);
+  static_assert(kSizeBytes >= Frame::kMinContentSizeBytes);
 
   std::array<std::byte, kSizeBytes> frame_buffer_;
 };