ループを排除して、心持ち、すっきりとさせてみました。心持ち。ｗ
SequenceableCollection >> combinations: digits atATimeAt: target
| size sum ranges cursor digit |
size _ self size.
(size < digits or: [digits < 1]) ifTrue: [^ self species new].
ranges _ OrderedCollection new.
(size - 1 to: digits - 1 by: -1)
inject: 0 into: [: lastIndex : n | ranges add: lastIndex + (n take: digits - 1)].
cursor _ ranges findFirst: [: lastIndex | lastIndex >= target].
sum _ cursor > 1 ifTrue: [ranges at: cursor - 1] ifFalse: [0].
digit _ self species with: (self at: cursor).
^ digit, ((self allButFirst: cursor) combinations: digits - 1 atATimeAt: target - sum)
