diff --git a/server/src/mine.rs b/server/src/mine.rs
index d275f21..e20e11b 100644
--- a/server/src/mine.rs
+++ b/server/src/mine.rs
@@ -314,7 +314,7 @@ struct ChunkedTask {
     #[serde(skip_deserializing)]
     head: Arc<AtomicI32>, // highest active chunk
     #[serde(skip)]
-    in_flight: Arc<RwLock<Vec<i32>>>, // must remain sorted
+    canceled: Arc<RwLock<Vec<i32>>>, // must remain sorted
     max: i32,
 }
 
@@ -323,55 +323,49 @@ impl ChunkedTask {
         Self {
             confirmed: Default::default(),
             head: Default::default(),
-            in_flight: Default::default(),
+            canceled: Default::default(),
             max: parts,
         } 
     }
 
     fn done(&self) -> bool {
         let backstop = self.confirmed.load(Ordering::SeqCst);
-        backstop >= self.max
+        backstop + 1 >= self.max
     }
 
     fn allocated(&self) -> bool {
         let front = self.head.load(Ordering::SeqCst);
         front + 1 >= self.max
     }
+
     async fn next_chunk(&self) -> Option<i32> {
-        let mut in_flight = self.in_flight.clone().write_owned().await;
+        let mut cancelled = self.canceled.clone().write_owned().await;
 
-        tracing::trace!("running: {:?}", in_flight);
-
-        let backstop = self.confirmed.load(Ordering::SeqCst);
-
-        // we have a mutex anyway
-        if let Some(highest) = in_flight.last() {
-            self.head.store(*highest, Ordering::SeqCst);
+        if let Some(chunk) = cancelled.pop() {
+            return Some(chunk);
         }
 
-        for i in backstop..self.max {
-            if !in_flight.contains(&i) {
-                in_flight.push(i);
-                in_flight.sort_unstable();
-                info!("next: {i}");
-                return Some(i);
-            }
-        }
+        let head = self.head.fetch_add(1, Ordering::AcqRel);
 
-        return None;
+        if head < self.max {
+            Some(head)
+        } else {
+            None
+        }
     }
 
     async fn mark_done(&self, chunk: i32) {
-        let mut in_flight = self.in_flight.write().await;
+        let canceled = self.canceled.read().await;
 
-        let min = in_flight.iter().max() == Some(&chunk);
+        let min = match canceled.iter().min() {
+            None => true,
+            Some(minima) => chunk < *minima,
+        };
 
-        in_flight.retain(|c| c != &chunk);
-
-        if min { // make sure that head is no less than min
+        if min {
             loop {
                 let curr = self.confirmed.load(Ordering::SeqCst);
-                if let Ok(_) = self.confirmed.compare_exchange(curr, curr.max(chunk+1), Ordering::AcqRel, Ordering::SeqCst) {
+                if let Ok(_) = self.confirmed.compare_exchange(curr, curr.max(chunk), Ordering::AcqRel, Ordering::SeqCst) {
                     break;
                 }
             }
@@ -379,8 +373,15 @@ impl ChunkedTask {
     }
 
     async fn cancel(&self, chunk: i32) {
-        let mut in_flight = self.in_flight.write().await;
-        in_flight.retain(|c| c != &chunk);
+        let mut in_flight = self.canceled.write().await;
+        let max = self.head.load(Ordering::SeqCst);
+        if chunk < max {
+            in_flight.push(chunk);
+            in_flight.sort_unstable();
+        }
+        else {
+            error!("attempted to cancel a job that hasn't happened yet");
+        }
     }
 }
 
@@ -415,10 +416,25 @@ mod tests {
     async fn cancel_replay() {
         let tracker = ChunkedTask::new(5);
         assert_eq!(tracker.next_chunk().await, Some(0));
+        tracker.mark_done(0).await;
         assert_eq!(tracker.next_chunk().await, Some(1));
-        assert_eq!(tracker.next_chunk().await, Some(2));
+        tracker.mark_done(1).await;
         tracker.cancel(2).await;
         assert_eq!(tracker.next_chunk().await, Some(2));
+        assert_eq!(tracker.done(), false);
+        tracker.mark_done(2).await;
+        assert_eq!(tracker.next_chunk().await, Some(3));
+        tracker.mark_done(3).await;
+        tracker.cancel(2).await;
+        assert_eq!(tracker.next_chunk().await, Some(2));
+        assert_eq!(tracker.next_chunk().await, Some(4));
+        tracker.cancel(1).await;
+        assert_eq!(tracker.next_chunk().await, Some(1));
+        assert_eq!(tracker.done(), false);
+        assert_eq!(tracker.next_chunk().await, None);
+        assert_eq!(tracker.allocated(), true);
+        tracker.mark_done(4).await;
+        assert_eq!(tracker.done(), true);
     }
 
     #[tokio::test]