http://twitter.com/yukihiro_matz/statuses/4079331380 経由で http://blog.bestinclass.dk/index.php/2009/09/scala-vs-clojure-round-2-concurrency/ より。
The problem
An old concurrency problem/exercise is ‘The sleeping barber’ and you can either read the full definition here or read my shorter version here:
We simulate a barber-shop with 1 barber, 3 waiting chairs and 1 chair for haircutting.
The barber sleeps between customers and is thus awoken when one enters the shop. If customers come in while he’s working they take a seat, if no seats are available they leave.
STM とか actor とか格好いいのではないですが、オーソドックスに SharedQueue を使って。
| queue seats debug shop barber watcher done | queue := SharedQueue new. done := SharedQueue new. seats := 3. debug := SharedQueue new. shop := [:a | debug nextPut: {'(s) entering shop'. a}. queue size < seats ifTrue: [queue nextPut: a] ifFalse: [debug nextPut: {'(s) turning away customer'. a}]]. barber := [ | next | (100 + 600 atRandom) milliSeconds asDelay wait. next := done nextPut: queue next. debug nextPut: {'(b) cutting hair of customer'. next}]. watcher := [[barber value] repeat] fork. 1 to: 20 do: [:customer | (100 + 200 atRandom) milliSeconds asDelay wait. [shop value: customer] fixTemps fork]. 2 seconds asDelay wait. watcher terminate. World findATranscript: nil. debug size timesRepeat: [ | next | next := debug next. Transcript cr; show: (next first forceTo: 31 paddingWith: $ ), next second printString]. Transcript cr; show: '(!) ', done size printString, ' customers got haircuts today'
(s) entering shop 1 (b) cutting hair of customer 1 (s) entering shop 2 (s) entering shop 3 (b) cutting hair of customer 2 (s) entering shop 4 (s) entering shop 5 (s) entering shop 6 (s) turning away customer 6 (b) cutting hair of customer 3 (s) entering shop 7 (b) cutting hair of customer 4 (s) entering shop 8 (s) entering shop 9 (s) turning away customer 9 (s) entering shop 10 (s) turning away customer 10 (b) cutting hair of customer 5 (s) entering shop 11 (b) cutting hair of customer 7 (s) entering shop 12 (s) entering shop 13 (s) turning away customer 13 (b) cutting hair of customer 8 (s) entering shop 14 (s) entering shop 15 (s) turning away customer 15 (b) cutting hair of customer 11 (s) entering shop 16 (s) entering shop 17 (s) turning away customer 17 (s) entering shop 18 (s) turning away customer 18 (b) cutting hair of customer 12 (s) entering shop 19 (b) cutting hair of customer 14 (s) entering shop 20 (b) cutting hair of customer 16 (b) cutting hair of customer 19 (b) cutting hair of customer 20 (!) 13 customers got haircuts today
Smalltalk でもこうして 30行程度で書けるので Clojure が Scala よりコンパクトに書けているのはＳ式だからと言うことでもないような…。
あー。SharedQueue だと、シートの空きを確認して座ろうとしたらもう他の誰かが座っていた的なことが起こりかねないのでやっぱりモニター版で書き直し。行数はさほど変わらない。
| queue seats debug shop barber watcher monitor tally | queue := OrderedCollection new. tally := 0. monitor := Monitor new. seats := 3. debug := OrderedCollection new. shop := [:a | monitor critical: [ debug add: {'(s) entering shop'. a}. queue size < seats ifTrue: [queue add: a] ifFalse: [debug add: {'(s) turning away customer'. a}]]]. barber := [ | next | (100 + 600 atRandom) milliSeconds asDelay wait. monitor critical: [ queue notEmpty ifTrue: [ next := queue removeFirst. tally := tally + 1. debug add: {'(b) cutting hair of customer'. next}]]]. watcher := [[barber value] repeat] fork. 1 to: 20 do: [:customer | (100 + 200 atRandom) milliSeconds asDelay wait. [shop value: customer] copy fixTemps fork]. 2 seconds asDelay wait. watcher terminate. World findATranscript: nil. debug do: [:each | Transcript cr; show: (each first forceTo: 31 paddingWith: $ ), each second printString]. Transcript cr; show: '(!) ', tally printString, ' customers got haircuts today'
