From 2c7c727c1cbbf943c8d102610769b6d1fed80dca Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Mo=CC=88hrmann?= <martisch@uos.de>
Date: Fri, 9 Jan 2015 23:25:42 +0100
Subject: [PATCH] sort: reduce number of comparisons needed by medianOfThree
For some cases we can ensure the correct order of elements in two
instead of three comparisons. It is unnecessary to compare m0 and
m1 again if m2 and m1 are not swapped.
benchmark old ns/op new ns/op delta
BenchmarkSortString1K 302721 299590 -1.03%
BenchmarkSortInt1K 124055 123215 -0.68%
BenchmarkSortInt64K 12291522 12203402 -0.72%
BenchmarkSort1e2 58027 57111 -1.58%
BenchmarkSort1e4 12426805 12341761 -0.68%
BenchmarkSort1e6 1966250030 1960557883 -0.29%
Change-Id: I2b17ff8dee310ec9ab92a6f569a95932538768a9
Reviewed-on: https://go-review.googlesource.com/2614
Reviewed-by: Robert Griesemer <gri@golang.org>
---
src/sort/sort.go | 17 ++++++++---------
1 file changed, 8 insertions(+), 9 deletions(-)
diff --git a/src/sort/sort.go b/src/sort/sort.go
index 4ca027b6140..b52b54ed8f7 100644
--- a/src/sort/sort.go
+++ b/src/sort/sort.go
@@ -75,20 +75,19 @@ func heapSort(data Interface, a, b int) {
// Quicksort, following Bentley and McIlroy,
// ``Engineering a Sort Function,'' SP&E November 1993.
-// medianOfThree moves the median of the three values data[a], data[b], data[c] into data[a].
-func medianOfThree(data Interface, a, b, c int) {
- m0 := b
- m1 := a
- m2 := c
- // bubble sort on 3 elements
+// medianOfThree moves the median of the three values data[m0], data[m1], data[m2] into data[m1].
+func medianOfThree(data Interface, m1, m0, m2 int) {
+ // sort 3 elements
if data.Less(m1, m0) {
data.Swap(m1, m0)
}
+ // data[m0] <= data[m1]
if data.Less(m2, m1) {
data.Swap(m2, m1)
- }
- if data.Less(m1, m0) {
- data.Swap(m1, m0)
+ // data[m0] <= data[m2] && data[m1] < data[m2]
+ if data.Less(m1, m0) {
+ data.Swap(m1, m0)
+ }
}
// now data[m0] <= data[m1] <= data[m2]
}
--
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