On 02.08.2011, at 10:17, Nicolas Cellier wrote:

> I played a bit with RunArray, and found some un-optimized features.
> First, I don't know why RunArray is an ArrayedCollection. It cannot
> #add: but it can #addFirst: and #addLast:.
> It cannot #add:withOccurrences: but it can #addLast:times:. Why
> inventing new selectors for old behaviours ?

The RunArray selectors may in fact precede the more common ones. RunArray's purpose was to make Text operations efficient. I think that's still the only actual use of RunArrays. In any case, speeding up Text is a good idea, though you might want to check what methods it actually uses.

- Bert -

> These operations will cost a realloc it the last value is different,
> so the underlying runs/values could better be an OrderedCollection if
> these operations are used often.
> A RunArray cannot remove at all.
> Very weird collection species, I don't like the implementation too much.
> 
> Then, #do: loops could be far faster. They rely on ArrayedCollection
> which inlines do: loops with #to:do: and #at:
> But #at: is not that fast. Scanning the runs and counting elements
> would result in a n^2 cost.
> Fortunately there is a cache lastIndex,lastRun,lastOffset to keep a cost n.
> Nonetheless, all the tests cost, and the loop is suboptimal.
> Let use see:
> 
> version 1:
> RunArray>>fastDo: aBlock
> 	runs with: values do: [:r :v |
> 		r timesRepeat: [aBlock value: v]].
> 
> | tmp |
> tmp := ((Array new: 1000) collect: [:e | 4 atRandom]) as: RunArray.
> {
> [ tmp do: [:e |]] bench.
> [ tmp fastDo: [:e |]] bench.
> }
> #('3,220 per second.' '6,290 per second.')
> 
> But timesRepeat: is slow, it is unoptimized by the compiler and costs
> a message send.
> I think we should implement BlockClosure>>repeat: and optimize that
> call in Compiler.
> But let's not do it, and rather inline by ourself:
> 
> version 2:
> 	runs with: values do: [:r :v |
> 		1 to: r do: [:i | aBlock value: v]].
> 
> | tmp |
> tmp := ((Array new: 1000) collect: [:e | 4 atRandom]) as: RunArray.
> {
> [ tmp do: [:e |]] bench.
> [ tmp do2: [:e |]] bench.
> }
> #('3,070 per second.' '25,500 per second.')
> 
> We can even inline the with:do: loop itself:
> version 3:
> 	1 to: runs size do: [:i |
> 		| r v |
> 		v := values at: i.
> 		r := runs at: i.
> 		[( r := r - 1) >= 0]
> 			whileTrue: [aBlock value: v]].
> 
> | tmp |
> tmp := ((Array new: 1000) collect: [:e | 4 atRandom]) as: RunArray.
> {
> [ tmp do: [:e |]] bench.
> [ tmp do2: [:e |]] bench.
> }
> #('3,370 per second.' '32,200 per second.')
> 
> Now the operation I wanted to use was reverseDo: so I implemented:
> RunArray>>fastReverseDo: aBlock
> 	| i |
> 	i := runs size.
> 	[i > 0]
> 		whileTrue:
> 			[ | r v |
> 			v := values at: i.
> 			r := runs at: i.
> 			i := i - 1.
> 			[( r := r - 1) >= 0]
> 				whileTrue: [aBlock value: v]].
> | tmp |
> tmp := ((Array new: 1000) collect: [:e | 4 atRandom]) as: RunArray.
> {
> [ tmp reverseDo: [:e |]] bench.
> [ tmp reverseDo2: [:e |]] bench.
> }
> #('83.9 per second.' '32,600 per second.')
> 
> Ouch! The cache is missing a lot of indices, and our loop turns into a n^2 cost.
> I know, premature optimization bla bla bla, but a factor x400 is worth
> some inlining no?
> 
> I guess these features are never used.
> By now RunArray is kind of private utility for Text implementation.
> But it could / should be generic.
> 
> I also have proposals for count: / select: / collect:. etc...
> It would be to evaluate the block only once per group of values.
> For example
> RunArray>>collect: aBlock
> 	"Beware, the block will be evaluated only once per group of values."
> 	^(self class runs: (runs collect: aBlock) contents values: values
> copy) coalesce
> But that's controversial, it would make the RunArray behave
> differently if the block has side effects...
> 
> | i tmp tmp2 tmp3 |
> tmp := ((Array new: 1000) collect: [:e | 4 atRandom]).
> i := 0.
> tmp2 := tmp collect: [:e | i := i + 1].
> i := 0.
> tmp3 := (tmp as: RunArray) collect: [:e | i := i + 1].
> tmp2 = tmp3 asArray
> ==> false
> 
> Nicolas
>