Feature #8707
closedHash#reverse_each
Description
Currently, {}.reverse_each
calls Enumerable#reverse_each
.
It will make array and its size can be large.
I made Hash#reverse_each
to avoid array creation and performance improvement.
benchmark:
require "benchmark"
Size = 10000
HASH = Hash[*Array.new(Size) {|i| [i, true] }.flatten]
Benchmark.bmbm do |x|
x.report("Hash#reverse_each") do
300.times do
HASH.reverse_each {|a, b|}
end
end
end
result:
trunk(r42256):
Rehearsal -----------------------------------------------------
Hash#reverse_each 1.210000 0.000000 1.210000 ( 1.207964)
-------------------------------------------- total: 1.210000sec
user system total real
Hash#reverse_each 0.950000 0.000000 0.950000 ( 0.951069)
proposal:
Rehearsal -----------------------------------------------------
Hash#reverse_each 0.600000 0.000000 0.600000 ( 0.600242)
-------------------------------------------- total: 0.600000sec
user system total real
Hash#reverse_each 0.450000 0.000000 0.450000 ( 0.459006)
Files
Updated by matz (Yukihiro Matsumoto) about 11 years ago
- Status changed from Open to Feedback
Do we really need it? What is use-cases?
Matz.
Updated by Anonymous about 11 years ago
Matz: This is quite a significant performance improvement and therefore I think it is worthwhile.
Updated by duerst (Martin Dürst) about 11 years ago
Hello Charlie,
On 2013/07/31 14:24, Charlie Somerville wrote:
Matz: This is quite a significant performance improvement and therefore I think it is worthwhile.
Only if the new method is actually used in practice.
That's why Matz is asking for use cases.
Regards, Martin.
Updated by ko1 (Koichi Sasada) about 11 years ago
- Assignee set to matz (Yukihiro Matsumoto)
Updated by funny_falcon (Yura Sokolov) about 11 years ago
Hash in Ruby1.9+ is hash table + double linked list - this is classic structure for LRU cache.
Simple LRU cache could be build with:
class LRU
def initialize
@hash = {}
end
def []=(k, v)
@hash.delete k
@hash[k] = v
end
def [](k)
if v = @hash.delete k
@hash[k] = v
end
v
end
def first
@hash.first
end
def shift
@hash.shift
end
# saves tail items until first stale item signaled
def drop_stale
save = true
stale = []
@hash.reverse_each{|k, v|
unless save && yield(k, v)
save = false
v = @hash.delete k
stale << [k, v]
end
}
stale
end
end
So that, reverse_each is very critical to be fast at this point
Updated by hsbt (Hiroshi SHIBATA) over 10 years ago
- Target version changed from 2.1.0 to 2.2.0
Updated by trans (Thomas Sawyer) over 10 years ago
Can #reverse
be an Enumerator?
Updated by bughit (bug hit) over 1 year ago
Do we really need it? What is use-cases?
When you have an ordered collection it seems self evident that you may need to iterate in reverse. This would for example back a performant last(n = 1)
method (#12165)
Would you demand to know why one may want the last n elements of an array? These are both ordered collections, if last(n = 1)
makes sense for one, it makes sense for the other.
Why is this closed with a status of "Feedback"?
It seems all feedback issues are labeled as closed which seems confusing.
Updated by bughit (bug hit) over 1 year ago
Another point is that that hash.reverse_each already exists via enumerable, but with a highly suboptimal array conversion. If it didn't exist you could perhaps debate whether it should be added, but that's moot at this point. The only question here is whether hash.reverse_each should be a hidden perf land-mine or have an optimal implementation for a Hash. Seems like a no-brainer.