change ack msg to store as array

hptp/src/msg.rs

@@ -1,7 +1,6 @@
 use super::seg::SegIdx;
 pub use super::seg::{DOWN_HEADER_SIZE, MAX_TOTAL_PACKET_SIZE, UP_HEADER_SIZE};
 use byteorder::ByteOrder;
-use std::collections::HashSet;
 
 #[derive(Clone, Debug)]
 pub enum UpMsg {
@@ -14,7 +13,8 @@ pub enum UpMsg {
 
 #[derive(Clone, Debug)]
 pub enum DownMsg {
-    Ack { idxs: HashSet<SegIdx> },
+    /// `idxs` must be distinct and in increasing order.
+    Ack { idxs: Vec<SegIdx> },
 }
 
 #[derive(Error, Debug)]
@@ -56,19 +56,29 @@ impl SerDes for UpMsg {
 
 impl SerDes for DownMsg {
     fn des(buf: &[u8]) -> Result<Self, DesError> {
-        let mut idxs = HashSet::new();
+        let mut idxs = vec![];
         for (i, b) in buf.iter().cloned().enumerate() {
             for j in 0..8 {
                 if b & (1 << j) != 0 {
-                    idxs.insert((i * 8 + j) as SegIdx);
+                    idxs.push((i * 8 + j) as SegIdx);
                 }
             }
         }
         Ok(DownMsg::Ack { idxs })
     }
 
-    fn ser_to(&self, _buf: &mut [u8]) -> usize {
-        // TODO: implement this!
-        0
+    fn ser_to(&self, buf: &mut [u8]) -> usize {
+        match self {
+            DownMsg::Ack { idxs } => {
+                let mut max_i = 0;
+                for seg_idx in idxs.iter().cloned() {
+                    let i = seg_idx as usize / 8;
+                    let j = (seg_idx % 8) as u8;
+                    buf[i] |= 1 << j;
+                    max_i = std::cmp::max(i, max_i);
+                }
+                max_i + 1
+            }
+        }
     }
 }