Fix nearly sorted rejection for small inputs

Author: Voultapher

src/drift.rs

@@ -3,7 +3,6 @@ use core::intrinsics;
 use core::mem::MaybeUninit;
 
 use crate::merge::merge;
-use crate::smallsort::SmallSortTypeImpl;
 
 pub fn sort<T, F: FnMut(&T, &T) -> bool>(
     v: &mut [T],
@@ -19,16 +18,16 @@ pub fn sort<T, F: FnMut(&T, &T) -> bool>(
 
     // It's important to have a relatively high entry barrier for pre-sorted runs, as the presence
     // of a single such run will force on average several merge operations and shrink the maximum
-    // quicksort size a lot. For that reason we use sqrt(len) as our pre-sorted run threshold, with
-    // SMALL_SORT_THRESHOLD as the lower limit. When eagerly sorting we also use
-    // SMALL_SORT_THRESHOLD as our threshold, as we will call small_sort on any runs smaller than
-    // this.
-    let min_good_run_len =
-        if eager_sort || len <= (T::SMALL_SORT_THRESHOLD * T::SMALL_SORT_THRESHOLD) {
-            T::SMALL_SORT_THRESHOLD
-        } else {
-            sqrt_approx(len)
-        };
+    // quicksort size a lot. For that reason we use sqrt(len) as our pre-sorted run threshold.
+    const MIN_SQRT_RUN_LEN: usize = 64;
+
+    let min_good_run_len = if len <= (MIN_SQRT_RUN_LEN * MIN_SQRT_RUN_LEN) {
+        // For small input length `MIN_SQRT_RUN_LEN` would break pattern detection of full or nearly
+        // sorted inputs.
+        cmp::min(len - len / 2, MIN_SQRT_RUN_LEN)
+    } else {
+        sqrt_approx(len)
+    };
 
     // (stack_len, runs, desired_depths) together form a stack maintaining run
     // information for the powersort heuristic. desired_depths[i] is the desired
@@ -221,6 +220,8 @@ fn create_run<T, F: FnMut(&T, &T) -> bool>(
     eager_sort: bool,
     is_less: &mut F,
 ) -> DriftsortRun {
+    use crate::smallsort::SmallSortTypeImpl;
+
     let (run_len, was_reversed) = find_existing_run(v, is_less);
 
     let len = v.len();