rotate retransmitting packets

hptp-send/src/main.rs

@@ -77,6 +77,7 @@ struct SegmentSource<'i, IN> {
     // ordered by seg idx
     unacked_segs: Vec<(SegIdx, UnAcked)>,
     next_seg_idx: u32,
+    next_retrans_idx: usize, // NOTE: this is an index into *unacked_segs* array
     eof: bool,
 }
 
@@ -109,6 +110,7 @@ where
             is_cut: false,
             unacked_segs: Vec::new(),
             next_seg_idx: 0,
+            next_retrans_idx: 0,
             eof: false,
         }
     }
@@ -148,8 +150,13 @@ where
         }
     }
 
-    fn retrans_seg_idx(&self) -> Option<SegIdx> {
+    fn retrans_seg_idx(&mut self) -> Option<SegIdx> {
-        self.unacked_segs.iter().map(|(si, _)| *si).next()
+        if self.unacked_segs.is_empty() {
+            return None
+        }
+        let seg_idx = self.unacked_segs[self.next_retrans_idx].0;
+        self.next_retrans_idx = (self.next_retrans_idx + 1) % self.unacked_segs.len();
+        Some(seg_idx)
     }
 
     async fn fresh_seg_idx(&mut self) -> Option<SegIdx> {
@@ -195,6 +202,7 @@ where
             }
         }
         self.unacked_segs = new_unacked_segs;
+        self.next_retrans_idx = 0;
     }
 }