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Posted to issues@commons.apache.org by "ASF GitHub Bot (JIRA)" <ji...@apache.org> on 2016/05/19 19:55:12 UTC
[jira] [Commented] (LANG-1176) Improve ArrayUtils removeElements
time complexity to O(n)
[ https://issues.apache.org/jira/browse/LANG-1176?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15292023#comment-15292023 ]
ASF GitHub Bot commented on LANG-1176:
--------------------------------------
GitHub user PascalSchumacher opened a pull request:
https://github.com/apache/commons-lang/pull/144
LANG-1176: Improve ArrayUtils removeElements time complexity to O(n)
patch submitted by Jeffery Yuan
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/PascalSchumacher/commons-lang LANG-1176
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/commons-lang/pull/144.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #144
----
commit 0386f0e1db7894fceb70653b35ed3072a4d146ed
Author: pascalschumacher <pa...@gmx.net>
Date: 2016-05-19T19:52:51Z
LANG-1176: Improve ArrayUtils removeElements time complexity to O(n)
patch submitted by Jeffery Yuan
----
> Improve ArrayUtils removeElements time complexity to O(n)
> ---------------------------------------------------------
>
> Key: LANG-1176
> URL: https://issues.apache.org/jira/browse/LANG-1176
> Project: Commons Lang
> Issue Type: Improvement
> Components: lang.*
> Affects Versions: 3.4
> Reporter: jefferyyuan
> Priority: Minor
> Fix For: 3.5
>
> Original Estimate: 24h
> Remaining Estimate: 24h
>
> The current ArrayUtils removeElements implementation is not efficient when try to get all indexes that needed to be removed.
> It's O(m*n): n is the length of origin array, m is the unique count of toBeRemoved.
> {code:java}
> final BitSet toRemove = new BitSet();
> for (final Map.Entry<Byte, MutableInt> e : occurrences.entrySet()) {
> final Byte v = e.getKey();
> int found = 0;
> for (int i = 0, ct = e.getValue().intValue(); i < ct; i++) {
> found = indexOf(array, v.byteValue(), found);
> if (found < 0) {
> break;
> }
> toRemove.set(found++);
> }
> }
> {code}
> We can just scan the origin array once to get all indexes that needed to be removed.
> {code:java}
> final BitSet toRemove = new BitSet();
> // the only change here, time complexity changed to O(n)
> for (int i = 0; i < array.length; i++) {
> final int key = array[i];
> final MutableInt count = occurrences.get(key);
> if (count != null) {
> count.decrement();
> if (count.intValue() == 0) {
> occurrences.remove(key);
> }
> toRemove.set(i);
> }
> }
> {code}
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