classification
Title: Alternative algorithm for deque_remove()
Type: Stage: patch review
Components: Versions: Python 3.8
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: rhettinger Nosy List: rhettinger, serhiy.storchaka, taleinat
Priority: low Keywords: patch

Created on 2015-09-27 08:43 by rhettinger, last changed 2018-06-12 23:22 by pablogsal.

Files
File name Uploaded Description Edit
deque_better_remove.diff rhettinger, 2015-09-27 08:43 Alternate remove() review
Pull Requests
URL Status Linked Edit
PR 7671 closed pablogsal, 2018-06-12 23:22
Messages (3)
msg251691 - (view) Author: Raymond Hettinger (rhettinger) * (Python committer) Date: 2015-09-27 08:43
The current algorithm for remove() rotates the deque one-by-one until a match is found, pops it off the deque and does single mass rotate to undo the 1-step rotates.

An alternative approach is to use deque_index() to locate the value of interest and use deque_del_item() to remove it.  

If not value is found, the alternative is better because it never moves the data in the deque.  If the value is found, the alternative removes it using two mass rotations.  The advantage in that case is the mass rotates are faster than many 1-step rotates.  The disadvantage is that we go through the pointer chain twice (the first time visiting and comparing every element and the second time only following the chain of links).

If the deque mutates during the search, a RuntimeError is raised.  This is a behavior change, formerly it raised an IndexError.
msg261280 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2016-03-07 06:39
There is more optimal approach.

Find not just an index in a deque, but a block and an index in a block. After that move left or right part of a deque one position right or left. __delitem__() could be 2 times faster, remove() could be faster too. Helpers proposed in issue17394 allow to do this easily.
msg319399 - (view) Author: Tal Einat (taleinat) * (Python committer) Date: 2018-06-12 20:09
IMO both approaches sound better than the existing implementation.  Better to choose one than to do nothing.
History
Date User Action Args
2018-06-12 23:22:33pablogsalsetpull_requests: + pull_request7285
2018-06-12 20:09:10taleinatsetnosy: + taleinat
messages: + msg319399
2018-01-29 20:54:39rhettingersetpriority: normal -> low
versions: + Python 3.8, - Python 3.6
2016-03-07 06:39:16serhiy.storchakasetnosy: + serhiy.storchaka
messages: + msg261280
2015-09-27 08:43:41rhettingercreate