From 4436c79821cfc899b2b9a001f2779208f680ce86 Mon Sep 17 00:00:00 2001
From: Richard Thier <magosit@outlook.hu>
Date: Sat, 1 Jul 2023 06:06:03 +0200
Subject: [PATCH] quicksort pivoting strategy changes when slowdown is
 recognized (works well against worst cases)

---
 thiersort.h | 37 ++++++++++++++++++++++++++-----------
 1 file changed, 26 insertions(+), 11 deletions(-)

diff --git a/thiersort.h b/thiersort.h
index de81338..4ff7758 100644
--- a/thiersort.h
+++ b/thiersort.h
@@ -234,6 +234,12 @@
  */
 
 #include <stdint.h>
+#ifndef TS_NO_RAND
+#include <stdlib.h> /* For rand() call for pivoting in quicksort */
+#define TS_RAND 1 /* For use in branches */
+#else
+#define TS_RAND 0 /* For use in branches */
+#endif /* TS_NO_RAND */
 
 #ifdef TS_SSE2
 #include <emmintrin.h>
@@ -539,7 +545,7 @@ static inline void ts_quicksort_inplace_impl(
 		const TSU32 bi,
 		void *reent_data),
 	void *reent_data,
-	TSBOOL const_check);
+	TSBOOL slowdown_detected);
 
 /** Simple inplace quicksort for tselems */
 static inline void ts_quicksort_inplace(
@@ -575,7 +581,7 @@ static inline void ts_quicksort_inplace_impl(
 			const TSU32 bi,
 			void *reent_data),
 		void *reent_data,
-		TSBOOL const_check) {
+		TSBOOL slowdown_detected) {
 	/* Must do this early exit! */
 	// TODO: TS_UNLIKELY
 	if(to == 0) return;
@@ -586,9 +592,18 @@ static inline void ts_quicksort_inplace_impl(
 
 	/* Pivoting */
 	TSU32 len = (to - from);
-	TSU32 mid = from + len / 2;
-	const union tskey pivotkey = arr[mid].key;
-	TSU32 pivoti = arr[mid].i;
+	TSU32 pivi;
+	/* Compiler should optimize this branch out compile time! */
+	if(!slowdown_detected || !TS_RAND) {
+		/* Also if TS_NO_RAND */
+		pivi = from + len / 2;
+	} else {
+		/* Only if there is no TS_NO_RAND and only when slowdown in progress */
+		/* This can help a bit to alleviate the worst case scenarios */
+		pivi = from + (rand() % len);
+	}
+	const union tskey pivotkey = arr[pivi].key;
+	TSU32 pivoti = arr[pivi].i;
 
 	/* Main loop */
 	TSU32 left = from;
@@ -609,7 +624,7 @@ static inline void ts_quicksort_inplace_impl(
 		TSU32 righti = arr[right].i;
 		while((left < right) && !lt(rightkey, pivotkey, righti, pivoti, reent_data)) {
 			/* Compiler should optimize this branch out compile time! */
-			if(const_check) {
+			if(slowdown_detected) {
 				/* (**): Check for stepping over everything because its all the same.. */
 				/* This with the above if gives two GE so an equality check */
 				if(!lt(pivotkey, rightkey, pivoti, righti, reent_data)) {
@@ -658,8 +673,8 @@ static inline void ts_quicksort_inplace_impl(
 
 		/* Swap */
 		const struct tselem tmp = arr[left];
-		arr[left] = arr[mid];
-		arr[mid] = tmp;
+		arr[left] = arr[pivi];
+		arr[pivi] = tmp;
 
 		/* XXX: Instead of (**) above, we could so some other sort here as a fallback (like a heapsort or merge) */
 		/* This ignores constantly the same arrays */
@@ -691,9 +706,9 @@ static inline void ts_quicksort_fromto(
 	if(from >= to) return;
 
 	TSU32 len = (to - from);
-	TSU32 mid = from + len / 2;
-	const union tskey pivotkey = src[mid].key;
-	TSU32 pivoti = src[mid].i;
+	TSU32 pivi = from + len / 2;
+	const union tskey pivotkey = src[pivi].key;
+	TSU32 pivoti = src[pivi].i;
 
 	TSU32 li = from;
 	TSU32 ri = to - 1;