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 ? 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