Optimize rollback hot paths (#53)

## Summary

Fixes #20.

- Reuse fighter animation scratch and output buffers across rollback frames.
- Select air-collision hits in one pass without candidate allocations or sorting.
- Index contiguous replay input frames directly and binary-search checksums.
- Add regression tests for buffer reuse and replay lookup/validation.

Reviewed-on: #53
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/replay.rs

@@ -152,6 +152,20 @@ fn validate(replay: &Replay) -> Result<(), String> {
     {
         return Err("replay frames must be strictly ordered".into());
     }
+    if replay
+        .frames
+        .windows(2)
+        .any(|frames| frames[0].frame.checked_add(1) != Some(frames[1].frame))
+    {
+        return Err("replay frames must be contiguous".into());
+    }
+    if replay
+        .checksums
+        .windows(2)
+        .any(|checksums| checksums[0].frame >= checksums[1].frame)
+    {
+        return Err("replay checksums must be strictly ordered".into());
+    }
     if replay
         .frames
         .iter()
@@ -173,6 +187,21 @@ impl Replay {
         handles.sort_unstable();
         handles != (0..handles.len()).collect::<Vec<_>>()
     }
+
+    fn frame(&self, frame: i32) -> Option<&ReplayFrame> {
+        let first_frame = self.frames.first()?.frame;
+        let index = frame.checked_sub(first_frame)? as usize;
+        self.frames
+            .get(index)
+            .filter(|recorded| recorded.frame == frame)
+    }
+
+    fn checksum(&self, frame: i32) -> Option<u128> {
+        self.checksums
+            .binary_search_by_key(&frame, |checksum| checksum.frame)
+            .ok()
+            .map(|index| self.checksums[index].checksum)
+    }
 }
 
 fn start_replay(mut commands: Commands, args: Res<Args>, loaded: Option<Res<LoadedReplay>>) {
@@ -210,11 +239,7 @@ fn replay_inputs(
                 .insert(frame.0, inputs.iter().map(|(input, _)| *input).collect());
         }
         ReplayMode::Playback { replay, mismatch } if !*mismatch => {
-            let Some(recorded) = replay
-                .frames
-                .iter()
-                .find(|recorded| recorded.frame == frame.0)
-            else {
+            let Some(recorded) = replay.frame(frame.0) else {
                 error!("Replay has no input for frame {}", frame.0);
                 *mismatch = true;
                 return;
@@ -235,21 +260,15 @@ fn record_checksum(
     let Some(mut runtime) = runtime else { return };
     if matches!(&runtime.mode, ReplayMode::Record { .. }) {
         runtime.checksums.insert(frame.0, checksum.0);
-    } else if let ReplayMode::Playback { replay, mismatch } = &mut runtime.mode {
-        if let Some(expected) = replay
-            .checksums
-            .iter()
-            .find(|value| value.frame == frame.0)
-            .map(|value| value.checksum)
-        {
-            if expected != checksum.0 {
-                error!(
-                    "Replay desync at frame {}: expected {:X}, got {:X}",
-                    frame.0, expected, checksum.0
-                );
-                *mismatch = true;
-            }
-        }
+    } else if let ReplayMode::Playback { replay, mismatch } = &mut runtime.mode
+        && let Some(expected) = replay.checksum(frame.0)
+        && expected != checksum.0
+    {
+        error!(
+            "Replay desync at frame {}: expected {:X}, got {:X}",
+            frame.0, expected, checksum.0
+        );
+        *mismatch = true;
     }
 }
 
@@ -407,4 +426,41 @@ mod tests {
             "unsupported replay version 1"
         );
     }
+
+    #[test]
+    fn playback_lookups_use_frame_offsets_and_support_sparse_checksums() {
+        let mut replay = sample();
+        replay.frames[0].frame = 10;
+        replay.frames.push(ReplayFrame {
+            frame: 11,
+            inputs: replay.frames[0].inputs.clone(),
+        });
+        replay.checksums.push(ReplayChecksum {
+            frame: 11,
+            checksum: 84,
+        });
+        replay.checksums[0].frame = 10;
+        validate(&replay).unwrap();
+
+        assert_eq!(replay.frame(10).unwrap().frame, 10);
+        assert_eq!(replay.frame(11).unwrap().frame, 11);
+        assert!(replay.frame(9).is_none());
+        assert_eq!(replay.checksum(10), Some(42));
+        assert_eq!(replay.checksum(11), Some(84));
+        assert_eq!(replay.checksum(12), None);
+    }
+
+    #[test]
+    fn noncontiguous_input_frames_are_rejected() {
+        let mut replay = sample();
+        replay.frames.push(ReplayFrame {
+            frame: 3,
+            inputs: replay.frames[0].inputs.clone(),
+        });
+
+        assert_eq!(
+            validate(&replay).unwrap_err(),
+            "replay frames must be contiguous"
+        );
+    }
 }