Kaprekar routine

# title: Kaprekar routine
# source: https://rosettacode.org/wiki/Kaprekar_numbers
# category: Rosetta Code
# difficulty: Medium
# summary: Apply the Kaprekar routine to a 4-digit number and watch it converge to 6174.
# tags: popular, math, sequences, sorting
# vibe: 0.2

def digits_of(n, pad)
  d = []
  remaining = n
  while remaining > 0
    d = d.push(remaining % 10)
    remaining = remaining / 10
  end
  while d.size < pad
    d = d.push(0)
  end
  d
end

def sort_digits(digits)
  arr = []
  i = 0
  while i < digits.size
    arr = arr.push(digits[i])
    i = i + 1
  end

  i = 0
  while i < arr.size - 1
    j = i + 1
    while j < arr.size
      if arr[j] < arr[i]
        temp = arr[i]
        arr[i] = arr[j]
        arr[j] = temp
      end
      j = j + 1
    end
    i = i + 1
  end
  arr
end

def from_digits(digits)
  result = 0
  i = 0
  while i < digits.size
    result = result * 10 + digits[i]
    i = i + 1
  end
  result
end

def reverse_array(arr)
  result = []
  i = arr.size - 1
  while i >= 0
    result = result.push(arr[i])
    i = i - 1
  end
  result
end

def kaprekar_step(n)
  d = digits_of(n, 4)
  ascending = sort_digits(d)
  descending = reverse_array(ascending)
  from_digits(descending) - from_digits(ascending)
end

def kaprekar_sequence(start)
  steps = [start]
  current = start
  while current != 6174 && steps.size < 10
    current = kaprekar_step(current)
    steps = steps.push(current)
  end
  steps
end

def run
  {
    from_3524: kaprekar_sequence(3524),
    from_1234: kaprekar_sequence(1234),
    from_9998: kaprekar_sequence(9998),
    from_1000: kaprekar_sequence(1000)
  }
end