reduce: reuse the output buffer allocation

This patch brings back the benefits of re-using the output buffer
allocation at the cost of some unsafe code reasoning.

Since the buffer is always cleared after each use we can simply store
its type erased pointer and temporarily bring it back to its Vec form
whenever there is some processing to do.

Signed-off-by: Petros Angelatos <petrosagg@gmail.com>

Author: petrosagg

src/operators/reduce.rs

@@ -683,6 +683,7 @@ where
 /// Implementation based on replaying historical and new updates together.
 mod history_replay {
 
+    use std::mem::ManuallyDrop;
     use ::difference::Semigroup;
     use lattice::Lattice;
     use trace::Cursor;
@@ -691,6 +692,13 @@ mod history_replay {
 
     use super::{PerKeyCompute, sort_dedup};
 
+    /// Clears and type erases a vector
+    fn vec_to_parts<T>(v: Vec<T>) -> (*mut (), usize) {
+        let mut v = ManuallyDrop::new(v);
+        v.clear();
+        (v.as_mut_ptr() as *mut (), v.capacity())
+    }
+
     /// The `HistoryReplayer` is a compute strategy based on moving through existing inputs, interesting times, etc in
     /// time order, maintaining consolidated representations of updates with respect to future interesting times.
     pub struct HistoryReplayer<'a, V1, V2, T, R1, R2>
@@ -705,6 +713,11 @@ mod history_replay {
         input_history: ValueHistory<'a, V1, T, R1>,
         output_history: ValueHistory<'a, V2, T, R2>,
         input_buffer: Vec<(&'a V1, R1)>,
+        // A type erased pointer and capacity for the temporary output buffer passed to `logic`.
+        // During `compute` the vector contains references to self which will get invalid as soon
+        // as compute returns. For this reason the temporary vector is always cleared before
+        // decomposing it back into its type erased parts
+        output_buffer_parts: (*mut (), usize),
         update_buffer: Vec<(V2, R2)>,
         output_produced: Vec<((V2, T), R2)>,
         synth_times: Vec<T>,
@@ -727,6 +740,7 @@ mod history_replay {
                 input_history: ValueHistory::new(),
                 output_history: ValueHistory::new(),
                 input_buffer: Vec::new(),
+                output_buffer_parts: vec_to_parts(Vec::<(&V2, R2)>::new()),
                 update_buffer: Vec::new(),
                 output_produced: Vec::new(),
                 synth_times: Vec::new(),
@@ -911,7 +925,16 @@ mod history_replay {
                         }
                         crate::consolidation::consolidate(&mut self.input_buffer);
 
-                        let mut output_buffer: Vec<(&V2, R2)> = Vec::new();
+                        let (ptr, cap) = self.output_buffer_parts;
+                        // SAFETY:
+                        //   * `ptr` is valid because is has been previously allocated by a Vec
+                        //      constructor parameterized with the same type argument
+                        //   * `len` and `cap` are valid because the vector is converted to parts
+                        //      only through `vec_to_parts` which clears the vector and gets its
+                        //      capacity
+                        let mut output_buffer = unsafe {
+                            Vec::from_raw_parts(ptr as *mut (&V2, R2), 0, cap)
+                        };
                         meet.as_ref().map(|meet| output_replay.advance_buffer_by(&meet));
                         for &((ref value, ref time), ref diff) in output_replay.buffer().iter() {
                             if time.less_equal(&next_time) {
@@ -936,7 +959,7 @@ mod history_replay {
                             logic(key, &self.input_buffer[..], &mut output_buffer, &mut self.update_buffer);
                             self.input_buffer.clear();
                         }
-                        drop(output_buffer);
+                        self.output_buffer_parts = vec_to_parts(output_buffer);
 
                         // output_replay.advance_buffer_by(&meet);
                         // for &((ref value, ref time), diff) in output_replay.buffer().iter() {