My friend Tim had a programming problem, described at his joint.
I thought about the problem for a while, started implementing it in ruby, thought “I could implement this better in haskell”, so I did:
group_seq :: (Int -> Bool) -> [Int] -> [[Int]] group_seq pred  =  group_seq pred (x:xs) | pred x = let (group, rest) = span pred (x:xs) in group:(group_seq pred rest) | otherwise = ([x]):(group_seq pred xs)
Then I implemented it in ruby:
class Array def group_sequential result =  i = 0 while i < length group = [self[i]] i += 1 if yield self[i-1] while i < length && (yield self[i]) group << self[i] i += 1 end end result << group end return result end end
And I thought, that would be much nicer if we had a enumerator interface with a “get next” and a “current value” method. Ruby doesn’t give us these toys, as far as I can see, but we can simulate them for a small dataset by doing Array.dup, then arr.shift is “move to next and return last” and “arr.first” is “current value”:
class Array def group_sequential arr = self.dup result =  while arr.first group = [arr.first] if yield arr.shift group << arr.shift while arr.first && (yield arr.first) end result << group end return result end end
And that one feels pretty good to me… It feels like it’d be nicer in a language with an iterator interface of the right sort. Hey, python has one of those, more or less, let’s try that.
def group_sequential(arr, pred): iterator = iter(arr) result =  group =  value = iterator.next() try: while True: group = [value] lastvalue, value = value, iterator.next() try: while pred(lastvalue) and pred(value): group.append(value) value = iterator.next() except: pass result.append(group) except: result.append(group) return result
And that one feels completely dreadful! Not being able to interrogate the iterator for the current value makes things a bit awkward.
I’m not entirely happy with any of them. Wanting to go back to the drawing board, I translated the functional one fairly directly into Ruby, hoping to get beautiful code at the cost of weird performance characteristics:
class Array def group_sequential(&block) return  if empty? return [self] if length == 1 rest = self[1..-1].group_sequential(&block) if (yield self.first) && (yield rest.first.first) rest.first.unshift(self.first) else rest.unshift([self.first]) end rest end end
Which is possibly the best yet, although it’s very strange-looking for ruby code. But I don’t like any of them, really.
In conclusion, I Don’t Know.