perf: use binary search in ForwardInMemoryCursor (#21049)

crates/trie/trie/src/forward_cursor.rs

@@ -53,9 +53,13 @@ impl<'a, K, V> ForwardInMemoryCursor<'a, K, V> {
     }
 }
 
+/// Threshold for remaining entries above which binary search is used instead of linear scan.
+/// For small slices, linear scan has better cache locality and lower overhead.
+const BINARY_SEARCH_THRESHOLD: usize = 64;
+
 impl<K, V> ForwardInMemoryCursor<'_, K, V>
 where
-    K: PartialOrd + Clone,
+    K: Ord + Clone,
     V: Clone,
 {
     /// Returns the first entry from the current cursor position that's greater or equal to the
@@ -73,19 +77,22 @@ where
     /// Advances the cursor forward while `predicate` returns `true` or until the collection is
     /// exhausted.
     ///
+    /// Uses binary search for large remaining slices (>= 64 entries), linear scan for small ones.
+    ///
     /// Returns the first entry for which `predicate` returns `false` or `None`. The cursor will
     /// point to the returned entry.
     fn advance_while(&mut self, predicate: impl Fn(&K) -> bool) -> Option<(K, V)> {
-        let mut entry;
-        loop {
-            entry = self.current();
-            if entry.is_some_and(|(k, _)| predicate(k)) {
+        let remaining = self.entries.len().saturating_sub(self.idx);
+        if remaining >= BINARY_SEARCH_THRESHOLD {
+            let slice = &self.entries[self.idx..];
+            let pos = slice.partition_point(|(k, _)| predicate(k));
+            self.idx += pos;
+        } else {
+            while self.current().is_some_and(|(k, _)| predicate(k)) {
                 self.next();
-            } else {
-                break;
             }
         }
-        entry.cloned()
+        self.current().cloned()
     }
 }
 
@@ -94,7 +101,7 @@ mod tests {
     use super::*;
 
     #[test]
-    fn test_cursor() {
+    fn test_cursor_small() {
         let mut cursor = ForwardInMemoryCursor::new(&[(1, ()), (2, ()), (3, ()), (4, ()), (5, ())]);
         assert_eq!(cursor.current(), Some(&(1, ())));
 
@@ -113,4 +120,72 @@ mod tests {
         assert_eq!(cursor.seek(&6), None);
         assert_eq!(cursor.current(), None);
     }
+
+    #[test]
+    fn test_cursor_large_binary_search() {
+        // Create a large enough collection to trigger binary search
+        let entries: Vec<(i32, ())> = (0..200).map(|i| (i * 2, ())).collect();
+        let mut cursor = ForwardInMemoryCursor::new(&entries);
+
+        // Seek to beginning
+        assert_eq!(cursor.seek(&0), Some((0, ())));
+        assert_eq!(cursor.idx, 0);
+
+        // Seek to middle (should use binary search)
+        assert_eq!(cursor.seek(&100), Some((100, ())));
+        assert_eq!(cursor.idx, 50);
+
+        // Seek to non-existent key (should find next greater)
+        assert_eq!(cursor.seek(&101), Some((102, ())));
+        assert_eq!(cursor.idx, 51);
+
+        // Seek to end
+        assert_eq!(cursor.seek(&398), Some((398, ())));
+        assert_eq!(cursor.idx, 199);
+
+        // Seek past end
+        assert_eq!(cursor.seek(&1000), None);
+    }
+
+    #[test]
+    fn test_first_after_large() {
+        let entries: Vec<(i32, ())> = (0..200).map(|i| (i * 2, ())).collect();
+        let mut cursor = ForwardInMemoryCursor::new(&entries);
+
+        // first_after should find strictly greater
+        assert_eq!(cursor.first_after(&0), Some((2, ())));
+        assert_eq!(cursor.idx, 1);
+
+        // Reset and test from beginning
+        cursor.reset();
+        assert_eq!(cursor.first_after(&99), Some((100, ())));
+
+        // first_after on exact match
+        cursor.reset();
+        assert_eq!(cursor.first_after(&100), Some((102, ())));
+    }
+
+    #[test]
+    fn test_cursor_consistency() {
+        // Verify binary search and linear scan produce same results
+        let entries: Vec<(i32, ())> = (0..200).map(|i| (i * 3, ())).collect();
+
+        for search_key in [0, 1, 3, 50, 150, 299, 300, 597, 598, 599, 1000] {
+            // Test with fresh cursor (binary search path)
+            let mut cursor1 = ForwardInMemoryCursor::new(&entries);
+            let result1 = cursor1.seek(&search_key);
+
+            // Manually advance to trigger linear path by getting close first
+            let mut cursor2 = ForwardInMemoryCursor::new(&entries);
+            if search_key > 100 {
+                cursor2.seek(&(search_key - 50));
+            }
+            let result2 = cursor2.seek(&search_key);
+
+            assert_eq!(
+                result1, result2,
+                "Mismatch for key {search_key}: binary={result1:?}, linear={result2:?}"
+            );
+        }
+    }
 }