Bug #20301
Updated by AMomchilov (Alexander Momchilov) 9 months ago
A common usage of `Set`s is to keep track of seen objects, and do something different whenever an object is seen for the first time, e.g.:
```ruby
SEEN_VALUES = Set.new
def receive_value(value)
if SEEN_VALUES.add?(value)
puts "Saw #{value} for the first time."
else
puts "Already seen #{value}, ignoring."
end
end
receive_value(1) # Saw 1 for the first time.
receive_value(2) # Saw 2 for the first time.
receive_value(3) # Saw 3 for the first time.
receive_value(1) # Already seen 1, ignoring.
```
Readers might reasonably assume that `add?` is only looking up into the set a single time, but it's actually doing two separate look-ups! ([source](https://github.com/ruby/ruby/blob/c976cb5/lib/set.rb#L517-L525))
```rb
class Set
def add?(o
# 1. `include?(o)` looks up into `@hash`
# 2. if the value isn't there, `add(o)` does a second look-up into `@hash`
add(o) unless include?(o)
end
end
```
This gets especially expensive if the values are large hash/arrays/objects, whose `#hash` is expensive to compute.
We can optimize this if it was possible to set a value in hash, *and* retrieve the value that was already there, in a single go. I propose adding `Hash#exchange_value` `Hash#update_value` to do exactly that. If that existed, we can re-implement `#add?` as:
```rb
class Set
def add?(o)
# Only requires a single look-up into `@hash`!
self unless @hash.exchange_value(o, @hash.update_value(o, true)
end
```
Here's a proof-of-concept implementation: https://github.com/ruby/ruby/pull/10093
# Theory
How much of a benefit this has depends on 2 factors:
1. How much `#hash` is called, which depends on how many **new** objects are added to the set.
* If every object is new, then `#hash` used to be called twice on every `#add?`.
* This is where this improvement makes the biggest (2x!) change.
* If every object has already been seen, then `#hash` was never being called twice before anyway, so there would be no improvement.
* It's important to not regress in this case, because many use cases of sets don't deal with many distinct objects, but just need to do quick checks against an existing set.
* Every other case lies somewhere in between those two, depending on the % of objects which are new.
2. How slow `#hash` is to compute for the key
* If the hash is slow to compute, this change will make a bigger improvement
* If the hash value is fast to compute, then it won't matter as much. Even if we called it half as much, it's a minority of the total time, so it won't have much net impact.
# Benchmark summary
| | All objects are new | All objects are preexisting |
|---------------------------|-------:|------:|
| objects with slow `#hash` | 100.0% | ~0.0% |
| objects with fast `#hash` | 24.5% | 4.6% |
As we see, this change makes a huge improvement the cases where it helps, and crucially, doesn't slow down the cases where it can't.
For the complete benchmark source code and results, see the PR: https://github.com/ruby/ruby/pull/10093