From: Tanaka Akira <akr@...>
Date: 2010-03-21T09:57:02+09:00
Subject: Re: Grouping elements of an array

2010/3/19 Steve Wilhelm <steve@studio831.com>:

> Example (second column is timestamp in seconds starting from zero).
>
> A 0
> B 15
> C 35
> D 100
> E 205
> F 215
> G 300
>
> would result in
>
> [[A, B, C], [D], [E, F], [G]]

I think this kind of problems which slices consecutive elements in an array
is not well supported in Ruby.

Enumerable#each_slice is not usable because it slices for each fixed number
of elements.

Ruby 1.9.2 has Enumerable#slice_before and it is usable but not so elegant
because it needs to maintain previous element.

% ruby -e '
a = [
  ["A", 0],
  ["B", 15],
  ["C", 35],
  ["D", 100],
  ["E", 205],
  ["F", 215],
  ["G", 300]
]
prev = nil
p a.slice_before {|s,t|
    tmp, prev = prev, t
    tmp && (t-tmp) > 30
  }.map {|es|
    es.map {|s,t| s }
  }
'
[["A", "B", "C"], ["D"], ["E", "F"], ["G"]]

We may need Enumerable#slice_between.

% ruby -e '
module Enumerable
  def slice_between(&b)
    Enumerator.new {|y|
      first = true
      buf = []
      prev = nil
      self.each {|elt|
        if first
          first = false
          buf << elt
          prev = elt
        else
          if b.call(prev, elt)
            y << buf
            buf = [elt]
          else
            buf << elt
          end
          prev = elt
        end
      }
      if !buf.empty?
        y << buf
      end
    }
  end
end
a = [
  ["A", 0],
  ["B", 15],
  ["C", 35],
  ["D", 100],
  ["E", 205],
  ["F", 215],
  ["G", 300]
]
p a.slice_between {|(s1,t1),(s2,t2)|
    (t2-t1) < 30
  }.map {|es|
    es.map {|s,t| s }
  }
'
[["A"], ["B"], ["C", "D", "E"], ["F", "G"]]
-- 
Tanaka Akira