Change order of elements so labels precede their values

sort-1.rb

# test whether the given element is a month label
def is_label(o)
  String === o
end

# exchange values at positions
def swap(arr, i, j)
  # not using parrallel assignment since
  # we want to avoid all the fancy stuff
  x = arr[i]
  arr[i] = arr[j]
  arr[j] = x
end

# Input: array with Events followed by month
# Output: number of events without month at end
# Side effect: months precede their Events
def realign_months(arr)
  start = 0
  i = 0

  while i < arr.length
    if is_label(arr[i])
      # swap label and first
      swap(arr, i, start)

      # remember begin of next section
      start = i + 1
    end

    i += 1
  end

  # remaining elements without month
  return arr.length - start
end

# we add a field only for easier reading of output
Event = Struct.new :no

arr = [
  Event.new(1),
  Event.new(2),
  Event.new(3),
  "January",
  Event.new(4),
  "February",
  Event.new(5),
  Event.new(6),
  Event.new(7),
  "March",
  Event.new(8),
  Event.new(9),
  "April",
]

p arr

remainder = realign_months(arr)

p arr

if remainder > 0
  $stderr.puts "Last #{remainder} events are misaligned"
end

sort-2.rb

# test whether the given element is a month label
def is_label(o)
  String === o
end

# rotate the given sub section of the array
# by one to the right, inserting the last
# element at the beginning
def rotate_right(arr, first, last)
  if first > last
    # fix order, alternatively signal error
    x = first
    first = last
    last = x
  end

  x = arr[last]
  i = last

  while i > first
    arr[i] = arr[i - 1]
    i -= 1
  end

  arr[first] = x
end

# Input: array with Events followed by month
# Output: number of events without month at end
# Side effect: months precede their Events
def realign_months(arr)
  start = 0
  i = 0

  while i < arr.length
    if is_label(arr[i])
      rotate_right(arr, start, i)

      # remember begin of next section
      start = i + 1
    end

    i += 1
  end

  # remaining elements without month
  return arr.length - start
end

# we add a field only for easier reading of output
Event = Struct.new :no

arr = [
  Event.new(1),
  Event.new(2),
  Event.new(3),
  "January",
  Event.new(4),
  "February",
  Event.new(5),
  Event.new(6),
  Event.new(7),
  "March",
  Event.new(8),
  Event.new(9),
  "April",
]

p arr

remainder = realign_months(arr)

p arr

if remainder > 0
  $stderr.puts "Last #{remainder} events are misaligned"
end