keep multiple un-ack'd packets in flight

hptp-send/src/main.rs

@@ -79,6 +79,8 @@ struct UnAcked {
     nacks: usize,
 }
 
+const IN_FLIGHT: usize = 30;
+
 impl<'i, IN> SegmentSource<'i, IN>
 where
     IN: AsyncRead + Unpin,
@@ -92,7 +94,15 @@ where
         }
     }
 
-    async fn read_segment(&mut self) -> Option<(SegIdx, &SegData)> {
+    fn retrans_seg_idx(&self) -> Option<SegIdx> {
+        self.unacked_segs.keys().skip(IN_FLIGHT).next().cloned()
+    }
+
+    fn any_unacked_seg_idx(&self) -> Option<SegIdx> {
+        self.unacked_segs.keys().next().cloned()
+    }
+
+    async fn fresh_seg_idx(&mut self) -> Option<SegIdx> {
         if self.eof {
             None
         } else {
@@ -100,21 +110,28 @@ where
             let payload = SegData::read(self.inp).await.expect("god help us");
             self.eof = payload.is_last_segment;
             self.unacked_upper_bound += 1;
-            let unack = self.unacked_segs.entry(seg_idx);
-            let unack = unack.or_insert(UnAcked { payload, nacks: 0 });
-            Some((seg_idx, &unack.payload))
+            self.unacked_segs
+                .insert(seg_idx, UnAcked { payload, nacks: 0 });
+            Some(seg_idx)
         }
     }
 
     async fn get_segment(&mut self) -> Option<(SegIdx, &SegData)> {
-        if self.unacked_segs.is_empty() {
-            self.read_segment().await
+        let seg_idx = {
+            if let Some(si) = self.retrans_seg_idx() {
+                si
+            } else if let Some(si) = self.fresh_seg_idx().await {
+                si
+            } else if let Some(si) = self.any_unacked_seg_idx() {
+                si
             } else {
-            self.unacked_segs
-                .iter()
-                .next() // get any entry, no ordering guarunteed or needed
-                .map(|(seg_idx, unack)| (*seg_idx, &unack.payload))
+                // early exit cus there's nothing left
+                return None;
             }
+        };
+        self.unacked_segs
+            .get(&seg_idx)
+            .map(|u| (seg_idx, &u.payload))
     }
 
     /// `seg_idxs` should be distinct and in increasing order.
@@ -149,7 +166,7 @@ where
 
     let mut src = SegmentSource::new(inp);
 
-    const DELAY_MS: u64 = 100;
+    const DELAY_MS: u64 = (1000 / IN_FLIGHT) as u64;
     let mut deadline = Instant::now();
     let mut to_send = vec![];