A solution to the Dutch flag problem.

flag_sort.rb

def color(a, i)
  a[i]
end

def swap(a, i, j)
  a[i], a[j] = a[j], a[i]
end

def flag_sort(a)
  # Indices (not colors):
  l = 0
  m = 0
  r = a.size - 1

  while m <= r
    case color(a, m)
    when :red
      swap(a, l, m)
      l += 1
      m += 1
    when :white
      m += 1
    when :blue
      swap(a, m, r)
      r -= 1
    else
      fail "Wrong pebble color!"
    end
  end
end