From 2a617d46f3ed39a71d8af8fb004fdfe8711160c2 Mon Sep 17 00:00:00 2001
From: Josh Bleecher Snyder <josharian@gmail.com>
Date: Thu, 18 Dec 2014 13:11:08 -0800
Subject: [PATCH] sort: reduce leaf calls in Stable
Move the symMerge recursion stopping condition
from the beginning of symMerge to the callers.
This halves the number of calls to symMerge
while running 'go test sort'.
benchmark old ns/op new ns/op delta
BenchmarkStable1e6 8358117060 7954143849 -4.83%
BenchmarkStable1e4 40116117 38583285 -3.82%
BenchmarkStableInt1K 119150 115182 -3.33%
BenchmarkStableInt64K 9799845 9515475 -2.90%
BenchmarkStableString1K 388901 393516 +1.19%
BenchmarkStable1e2 124917 123618 -1.04%
Change-Id: I7ba2ca277f213b076fe6830e1139edb47ac53800
Reviewed-on: https://go-review.googlesource.com/1820
Reviewed-by: Robert Griesemer <gri@golang.org>
---
src/sort/sort.go | 22 ++++++++++++++--------
1 file changed, 14 insertions(+), 8 deletions(-)
diff --git a/src/sort/sort.go b/src/sort/sort.go
index 63f8894a196..55134956c00 100644
--- a/src/sort/sort.go
+++ b/src/sort/sort.go
@@ -316,7 +316,7 @@ func StringsAreSorted(a []string) bool { return IsSorted(StringSlice(a)) }
// data.Less and O(n*log(n)*log(n)) calls to data.Swap.
func Stable(data Interface) {
n := data.Len()
- blockSize := 20
+ blockSize := 20 // must be > 0
a, b := 0, blockSize
for b <= n {
insertionSort(data, a, b)
@@ -332,7 +332,9 @@ func Stable(data Interface) {
a = b
b += 2 * blockSize
}
- symMerge(data, a, a+blockSize, n)
+ if m := a + blockSize; m < n {
+ symMerge(data, a, m, n)
+ }
blockSize *= 2
}
}
@@ -352,11 +354,11 @@ func Stable(data Interface) {
// rotation algorithm which uses O(M+N+gcd(M+N)) assignments. The argumentation
// in the paper carries through for Swap operations, especially as the block
// swapping rotate uses only O(M+N) Swaps.
+//
+// symMerge assumes non-degenerate arguments: a < m && m < b.
+// Having the caller check this condition eliminates many leaf recursion calls,
+// which improves performance.
func symMerge(data Interface, a, m, b int) {
- if a >= m || m >= b {
- return
- }
-
mid := a + (b-a)/2
n := mid + m
var start, r int
@@ -380,8 +382,12 @@ func symMerge(data Interface, a, m, b int) {
end := n - start
rotate(data, start, m, end)
- symMerge(data, a, start, mid)
- symMerge(data, mid, end, b)
+ if a < start && start < mid {
+ symMerge(data, a, start, mid)
+ }
+ if mid < end && end < b {
+ symMerge(data, mid, end, b)
+ }
}
// Rotate two consecutives blocks u = data[a:m] and v = data[m:b] in data:
--
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