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day24.ex
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day24.ex
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defmodule Aoc201824 do
def run() do
IO.puts(part1())
IO.puts(part2())
end
defp part1(), do: groups() |> fight_until_finished() |> score()
defp part2() do
groups()
|> Stream.iterate(&boost_immune_system/1)
|> Stream.map(&fight_until_finished/1)
|> Enum.find(&(armies(&1) == ["Immune System"]))
|> score()
end
defp boost_immune_system(groups) do
{immune, infections} = Enum.split_with(Map.values(groups), &(&1.army == "Immune System"))
immune = Enum.map(immune, &%{&1 | damage: &1.damage + 1})
Map.new(Stream.map(immune ++ infections, &{group_id(&1), &1}))
end
defp fight_until_finished(groups), do: groups |> Stream.iterate(&fight/1) |> Enum.find(&finished?/1)
defp armies(groups), do: groups |> Map.values() |> Stream.map(& &1.army) |> Enum.uniq()
defp score(groups), do: groups |> Map.values() |> Stream.map(& &1.units) |> Enum.sum()
defp finished?(groups), do: Enum.empty?(attackers(groups)) or Enum.count(armies(groups)) < 2
defp fight(groups) do
groups
|> attackers()
|> Enum.to_list()
|> Enum.sort_by(fn {attacker_id, _defender_id} -> group(groups, attacker_id).initiative end, &>=/2)
|> Enum.reduce(groups, fn {attacker_id, defender_id}, groups -> attack(groups, attacker_id, defender_id) end)
end
defp attackers(groups) do
{_attackable, attack_pairs} =
groups
|> Map.values()
|> Enum.sort_by(&{effective_power(&1), &1.initiative}, &>=/2)
|> Enum.reduce({Enum.group_by(Map.values(groups), & &1.army), []}, &pick_target/2)
attack_pairs
end
defp pick_target(attacker, {attackable, attack_pairs}) do
enemy_army = enemy_army(attacker.army)
enemies = Map.get(attackable, enemy_army, [])
enemies
|> Stream.reject(&(inflicted_damage(attacker, &1) == 0))
|> Enum.max_by(&{inflicted_damage(attacker, &1), effective_power(&1), &1.initiative}, fn -> nil end)
|> case do
nil ->
{attackable, attack_pairs}
defender ->
attackable = Map.update!(attackable, enemy_army, &Enum.reject(&1, fn group -> group.id == defender.id end))
attack_pairs =
if div(inflicted_damage(attacker, defender), defender.hit_points) == 0,
do: attack_pairs,
else: [{group_id(attacker), group_id(defender)} | attack_pairs]
{attackable, attack_pairs}
end
end
defp attack(groups, attacker_id, defender_id) do
case Map.fetch(groups, attacker_id) do
{:ok, attacker} ->
defender = Map.fetch!(groups, defender_id)
case absorb_damage(defender, attacker) do
%{units: 0} -> Map.delete(groups, defender_id)
defender -> Map.put(groups, defender_id, defender)
end
:error ->
groups
end
end
defp absorb_damage(defender, attacker),
do: update_in(defender.units, &max(0, &1 - div(inflicted_damage(attacker, defender), defender.hit_points)))
defp inflicted_damage(attacker, defender) do
cond do
MapSet.member?(defender.immunities, attacker.damage_type) -> 0
MapSet.member?(defender.weaknesses, attacker.damage_type) -> effective_power(attacker) * 2
true -> effective_power(attacker)
end
end
defp enemy_army("Immune System"), do: "Infection"
defp enemy_army("Infection"), do: "Immune System"
defp group_id(group), do: {group.army, group.id}
defp group(groups, group_id), do: Map.fetch!(groups, group_id)
defp effective_power(group), do: group.units * group.damage
defp groups() do
Aoc.input_file(__MODULE__)
|> File.read!()
|> String.trim()
|> String.split("\n\n")
|> Stream.flat_map(&parse_groups/1)
|> Enum.map(&{group_id(&1), &1})
|> Map.new()
end
defp parse_groups(army) do
[name, groups] = String.split(army, ":\n")
groups
|> String.split("\n")
|> Stream.with_index(1)
|> Enum.map(fn {group, id} -> parse_group(group, id, name) end)
end
defp parse_group(group, id, army_name) do
captures =
[
~S/(?<units>\d+) units/,
~S/each with (?<hit_points>\d+) hit points(\s\((?<immunities_and_weaknesses>.+)\))?/,
~S/with an attack that does (?<damage>\d+) (?<damage_type>\w+) damage/,
~S/at initiative (?<initiative>\d+)/
]
|> Enum.join(" ")
|> Regex.compile!()
|> Regex.named_captures(group)
if captures == nil do
IO.puts(group)
System.halt()
end
immunities_and_weaknesses =
case Map.fetch!(captures, "immunities_and_weaknesses") do
"" ->
%{}
immunities_and_weaknesses ->
immunities_and_weaknesses
|> String.split(~r/;\s*/)
|> Enum.flat_map(&parse_immunities_and_weaknesses/1)
|> Enum.group_by(fn {kind, _action} -> kind end, fn {_kind, action} -> action end)
end
immunities = Map.get(immunities_and_weaknesses, "immune", [])
weaknesses = Map.get(immunities_and_weaknesses, "weak", [])
%{
id: id,
army: army_name,
units: String.to_integer(Map.fetch!(captures, "units")),
hit_points: String.to_integer(Map.fetch!(captures, "hit_points")),
initiative: String.to_integer(Map.fetch!(captures, "initiative")),
damage: String.to_integer(Map.fetch!(captures, "damage")),
damage_type: Map.fetch!(captures, "damage_type"),
immunities: MapSet.new(immunities),
weaknesses: MapSet.new(weaknesses)
}
end
defp parse_immunities_and_weaknesses(immunities_and_weaknesses) do
%{"kind" => kind, "actions" => actions} =
Regex.named_captures(~r/(?<kind>\w+) to (?<actions>.+)/, immunities_and_weaknesses)
actions
|> String.split(~r/,\s*/)
|> Enum.map(&{kind, &1})
end
end