feat(client): add receive metadata scaffold

crates/irtt-client/src/metadata.rs

@@ -0,0 +1,90 @@
+use std::time::SystemTime;
+
+use crate::event::PacketMeta;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub(crate) struct ReceiveMeta {
+    pub(crate) traffic_class: Option<u8>,
+    pub(crate) kernel_rx_timestamp: Option<SystemTime>,
+}
+
+impl From<ReceiveMeta> for PacketMeta {
+    fn from(meta: ReceiveMeta) -> Self {
+        Self {
+            traffic_class: meta.traffic_class,
+            dscp: meta.traffic_class.map(|traffic_class| traffic_class >> 2),
+            ecn: meta.traffic_class.map(|traffic_class| traffic_class & 0b11),
+            kernel_rx_timestamp: meta.kernel_rx_timestamp,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::ReceiveMeta;
+    use crate::event::PacketMeta;
+
+    #[test]
+    fn metadata_default_is_unavailable() {
+        let meta = ReceiveMeta::default();
+        let packet_meta = PacketMeta::from(meta);
+
+        assert_eq!(packet_meta.traffic_class, None);
+        assert_eq!(packet_meta.dscp, None);
+        assert_eq!(packet_meta.ecn, None);
+        assert_eq!(packet_meta.kernel_rx_timestamp, None);
+    }
+
+    #[test]
+    fn metadata_observed_traffic_class_zero_preserves_observed_zero() {
+        let packet_meta = PacketMeta::from(ReceiveMeta {
+            traffic_class: Some(0),
+            kernel_rx_timestamp: None,
+        });
+
+        assert_eq!(packet_meta.traffic_class, Some(0));
+        assert_eq!(packet_meta.dscp, Some(0));
+        assert_eq!(packet_meta.ecn, Some(0));
+        assert_eq!(packet_meta.kernel_rx_timestamp, None);
+    }
+
+    #[test]
+    fn metadata_observed_traffic_class_derives_dscp_and_ecn() {
+        let packet_meta = PacketMeta::from(ReceiveMeta {
+            traffic_class: Some(184),
+            kernel_rx_timestamp: None,
+        });
+
+        assert_eq!(packet_meta.traffic_class, Some(184));
+        assert_eq!(packet_meta.dscp, Some(46));
+        assert_eq!(packet_meta.ecn, Some(0));
+        assert_eq!(packet_meta.kernel_rx_timestamp, None);
+
+        let packet_meta = PacketMeta::from(ReceiveMeta {
+            traffic_class: Some(186),
+            kernel_rx_timestamp: None,
+        });
+
+        assert_eq!(packet_meta.traffic_class, Some(186));
+        assert_eq!(packet_meta.dscp, Some(46));
+        assert_eq!(packet_meta.ecn, Some(2));
+        assert_eq!(packet_meta.kernel_rx_timestamp, None);
+    }
+
+    #[test]
+    fn metadata_mapping_preserves_none_vs_observed_zero() {
+        let unavailable = PacketMeta::from(ReceiveMeta::default());
+        let observed_zero = PacketMeta::from(ReceiveMeta {
+            traffic_class: Some(0),
+            kernel_rx_timestamp: None,
+        });
+
+        assert_eq!(unavailable.traffic_class, None);
+        assert_eq!(unavailable.dscp, None);
+        assert_eq!(unavailable.ecn, None);
+
+        assert_eq!(observed_zero.traffic_class, Some(0));
+        assert_eq!(observed_zero.dscp, Some(0));
+        assert_eq!(observed_zero.ecn, Some(0));
+    }
+}