From ec1ddbde815bffdf2f5e894246cd784012aad7c2 Mon Sep 17 00:00:00 2001
From: Robin Mueller <robin.mueller.m@pm.me>
Date: Thu, 18 Sep 2025 17:35:32 +0200
Subject: [PATCH] better error handling

---
 src/cfdp/pdu/nak.rs | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/src/cfdp/pdu/nak.rs b/src/cfdp/pdu/nak.rs
index f81e4ec..c66426c 100644
--- a/src/cfdp/pdu/nak.rs
+++ b/src/cfdp/pdu/nak.rs
@@ -52,13 +52,21 @@ fn write_start_and_end_of_scope(
     current_index
 }
 
+#[derive(Debug, PartialEq, Eq, Copy, Clone, thiserror::Error)]
+#[error("packet buffer too small for PDU header")]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
+pub struct TooSmallForPduHeaderError;
+
 /// This function can be used to retrieve the maximum amount of segment request given a PDU
 /// configuration to stay below a certain maximum packet size. This is useful to calculate how many
 /// NAK PDUs are required inside a NAK sequence.
+///
+/// Returns an error if the packet buffer can not even hold the PDU header.
 pub fn calculate_max_segment_requests(
     mut max_packet_size: usize,
     pdu_header: &PduHeader,
-) -> Result<usize, usize> {
+) -> Result<usize, TooSmallForPduHeaderError> {
     let mut decrement = pdu_header.header_len() + 1;
     if pdu_header.common_pdu_conf().crc_flag == CrcFlag::WithCrc {
         decrement += 2;
@@ -69,7 +77,7 @@ pub fn calculate_max_segment_requests(
         decrement += 16;
     }
     if max_packet_size < decrement {
-        return Err(max_packet_size);
+        return Err(TooSmallForPduHeaderError);
     }
     max_packet_size -= decrement;
     Ok(
@@ -332,7 +340,7 @@ impl<'buf> NakPduCreatorWithReservedSeqReqsBuf<'buf> {
     pub fn calculate_max_segment_requests(
         max_packet_size: usize,
         pdu_header: &PduHeader,
-    ) -> Result<usize, usize> {
+    ) -> Result<usize, TooSmallForPduHeaderError> {
         calculate_max_segment_requests(max_packet_size, pdu_header)
     }