Code katas - small toy problems to practice technique

turnstile.exs

# At a building there are two queues of people: entering and leaving.
# Only one person can go through the turnstile at a time.
# A person leaving and a person entering can approach the turnstile at the same time.
# Given two integer arrays of times and directions of people, figure out who goes when.
#
# If no conflict at time t, first come first served
# Otherwise,
#   if gate was not used in previous second,       out has priority
#   if gate was used in previous second to go out, out has priority
#   if gate was used in previous second to go in,   in has priority

# person 0 approaches turnstile at time 0 and wants to exit
# person 1 approaches turnstile at time 1 and wants to enter
# person 2 approaches turnstile at time 1 and wants to exit
# person 3 approaches turnstile at time 3 and wants to exit
# person 4 approaches turnstile at time 3 and wants to enter
times = [0, 1, 1, 3, 3]
directions = [0, 1, 0, 0, 1]

# person 0 uses turnstile at time 0
# person 1 uses turnstile at time 2
# person 2 uses turnstile at time 1
# person 3 uses turnstile at time 4
# person 4 uses turnstile at time 3
expected = [0, 2, 1, 4, 3]

defmodule Person do
  defstruct [:id, :approach_time, :direction, :pass_time]

  def new(id, times, directions) do
    struct(__MODULE__,
      id: id,
      approach_time: Enum.at(times, id),
      direction: if(Enum.at(directions, id) == 0, do: :out, else: :in)
    )
  end
end

n = length(times)
range = 0..(n - 1)

people = Enum.map(range, &Person.new(&1, times, directions))

# [
#   %Person{approach_time: 0, direction: :out, id: 0, pass_time: nil},
#   %Person{approach_time: 1, direction: :in, id: 1, pass_time: nil},
#   %Person{approach_time: 1, direction: :out, id: 2, pass_time: nil},
#   %Person{approach_time: 3, direction: :out, id: 3, pass_time: nil},
#   %Person{approach_time: 3, direction: :in, id: 4, pass_time: nil}
# ]

timing_conflicts = Enum.group_by(people, & &1.approach_time)

# %{
#   0 => [
#     %Person{approach_time: 0, direction: :out, id: 0, pass_time: nil}
#   ],
#   1 => [
#     %Person{approach_time: 1, direction: :in, id: 1, pass_time: nil},
#     %Person{approach_time: 1, direction: :out, id: 2, pass_time: nil}
#   ],
#   3 => [
#     %Person{approach_time: 3, direction: :out, id: 3, pass_time: nil},
#     %Person{approach_time: 3, direction: :in, id: 4, pass_time: nil}
#   ]
# }

person_priority = fn
  [%{direction: dir} = a, b], dir -> {a, b}
  [a, %{direction: dir} = b], dir -> {b, a}
end

Enum.reduce(timing_conflicts, [], fn
  # No conflict, person can enter/leave right away.
  {t, [solo_person]}, pqueue ->
    [
      %{solo_person | pass_time: t}
      | pqueue
    ]

  # Two people are conflicting on enter/leave time, resolve the priority.
  {t, person_pair}, pqueue ->
    case Enum.find(pqueue, &(&1.pass_time == t - 1)) do
      # Gate not used in previous second, out has priority
      nil ->
        {leaving, entering} = person_priority.(person_pair, :out)

        [
          %{leaving | pass_time: t},
          %{entering | pass_time: t + 1}
          | pqueue
        ]

      # Gate used in previous second to leave, so leave has priority.
      %{direction: :out} ->
        {leaving, entering} = person_priority.(person_pair, :out)

        [
          %{leaving | pass_time: t},
          %{entering | pass_time: t + 1}
          | pqueue
        ]

      # Gate used in previous second to enter, so enter has priority.
      %{direction: :in} ->
        {entering, leaving} = person_priority.(person_pair, :in)

        [
          %{entering | pass_time: t},
          %{leaving | pass_time: t + 1}
          | pqueue
        ]
    end
end)
|> Enum.sort_by(& &1.id)
|> Enum.map(& &1.pass_time)
|> IO.inspect()
|> Kernel.==(expected)
|> IO.inspect()

Shouldn't the expected be

[0, 2, 1, 3, 4]

?

Since at 2 second there's no use of the turnstile 3 - exit gets priority?