# Generates the paces for each depth.
	#
	# Each entry of the returned array is a couple of the first pace
	# and the second one, they are alternated when deciphering a rail-encrypted string.
	#
	# Say we have the encrypted string "fgounbmtcieehkh" on 4 rails
	#
	# To find the distance between each character on the original railed
	# string, we need to compute the extremes.
	#
	# The extremes always have a distance of `depth - 1`, multiplied by 2, no pairing.
	#
	# In the example, that would be : [(4 - 1) * 2, (4 - 1) * 2] => [6,6]
	#
	# For every rail in-between, the first distance is the largest absolute value
	# of the difference between the current depth and the extremes, multiplied by 2.
	#
	# Its pair is the distance of maximum and the distance yielded by the previous
	# calculation.
	#
	# In our example, that would be :
	#
	# Maximums : (4 - 1) * 2 = 3 * 2 => [6,6]
	# In between : Distance for depth 2 : max(2 - 1, 4 - 2) => 2
	# The calculation yields the couple [(2 * 2), 6 - 4] => [4, 2]
	# The symmetric couple is reversed : [2, 4]
	#
	# In fine, the example yields the array : [[6,6], [4,2], [2,4], [6,6]]
	#
	# In the end, our string is read using the generated array
	#
	# SEE: `Text::unrail` for how the array is used
	private fun unrail_paces: Array[Couple[Int, Int]] do
		var ret = new Array[Couple[Int,Int]].with_capacity(self)
		var extremes = new Couple[Int, Int]((self - 1) * 2, (self - 1) * 2)
		for i in [0..self[ do
			ret.add extremes
		end
		var mid = ((self.to_f)/2.0).floor.to_i
		for i in [1 .. mid[ do
			var rd = i + 1
			var lodepth = self - rd
			var hidepth = (rd - self).abs
			var dd: Int
			if hidepth > lodepth then
				dd = hidepth * 2
			else
				dd = lodepth * 2
			end
			var cp = new Couple[Int, Int](dd, extremes.first-dd)
			var ccp = new Couple[Int, Int](extremes.first - dd, dd)

			ret[i] = cp
			ret[self - rd] = ccp
		end
		if not self.is_even then
			ret[mid] = new Couple[Int, Int](extremes.first/2, extremes.first/2)
		end
		return ret
	end