From 6f1835dce02b592b28a4ea9bb9a77dbc990198a2 Mon Sep 17 00:00:00 2001
From: Stefan Nilsson <snilsson@nada.kth.se>
Date: Thu, 2 Dec 2010 09:18:20 -0800
Subject: [PATCH] Sort: reduced stack depth to lg(n) in quickSort
Doing the tail recursion elimination explicitly
seems safer than leaving it to the compiler;
the code is still clean and easy to understand.
R=r, r2, gri
CC=golang-dev
https://golang.org/cl/3373041
---
src/pkg/sort/sort.go | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/src/pkg/sort/sort.go b/src/pkg/sort/sort.go
index 2abe22d5c75..02e647fca9a 100644
--- a/src/pkg/sort/sort.go
+++ b/src/pkg/sort/sort.go
@@ -122,11 +122,19 @@ func doPivot(data Interface, lo, hi int) (midlo, midhi int) {
}
func quickSort(data Interface, a, b int) {
- if b-a > 7 {
+ for b-a > 7 {
mlo, mhi := doPivot(data, a, b)
- quickSort(data, a, mlo)
- quickSort(data, mhi, b)
- } else if b-a > 1 {
+ // Avoiding recursion on the larger subproblem guarantees
+ // a stack depth of at most lg(b-a).
+ if mlo-a < b-mhi {
+ quickSort(data, a, mlo)
+ a = mhi // i.e., quickSort(data, mhi, b)
+ } else {
+ quickSort(data, mhi, b)
+ b = mlo // i.e., quickSort(data, a, mlo)
+ }
+ }
+ if b-a > 1 {
insertionSort(data, a, b)
}
}
--
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