133 lines
2.9 KiB
Elixir
133 lines
2.9 KiB
Elixir
defmodule Numbers do
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use GenServer
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def start_link do
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primes = [2, 3]
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max_n = 2
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GenServer.start_link(__MODULE__, {primes, max_n})
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end
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def is_prime?(pid, n) do
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GenServer.call(pid, {:is_prime?, n})
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end
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def primes_up_to(pid, n) do
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GenServer.call(pid, {:up_to, n})
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end
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@impl true
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def init(primes) do
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{:ok, primes}
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end
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@impl true
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def handle_call({:is_prime?, n}, _from, {primes, max_n}) do
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IO.inspect({:is_prime?, n, {primes, max_n}})
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{ans, {primes, max_n}} = check_prime(n, {primes, max_n})
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{:reply, ans, {primes, max_n}}
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end
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@impl true
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def handle_call({:up_to, n}, _from, {primes, max_n}) do
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{ans, {primes, max_n}} = gen_primes(n, {primes, max_n})
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{:reply, ans, {primes, max_n}}
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end
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defp check_prime(n, {primes, max_n}) do
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if n / 2 <= max_n do
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{Enum.member?(primes, n), {primes, max_n}}
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else
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possible_factors = primes ++ Enum.to_list((max_n + 1)..div(n, 2))
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has_factor = Enum.any?(possible_factors, &(rem(n, &1) == 0))
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{!has_factor, {primes, max_n}}
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end
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end
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defp gen_primes(n, {primes, max_n}) do
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if n <= max_n do
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{Enum.filter(primes, &(&1 <= n)), {primes, max_n}}
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else
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new_primes =
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Enum.reduce((max_n + 1)..n, primes, fn p, primes ->
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{is_prime, {primes, _}} = check_prime(p, {primes, p - 1})
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if is_prime do
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primes ++ [p]
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else
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primes
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end
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end)
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{new_primes, {new_primes, n}}
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end
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end
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end
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defmodule Day13 do
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defp read_input do
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Path.expand('input', Path.dirname(__ENV__.file))
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|> File.read!()
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end
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defp test_input do
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"""
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939
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7,13,x,x,59,x,31,19
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"""
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end
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defp parse_int(val, default) do
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try do
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String.to_integer(val)
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rescue
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ArgumentError -> default
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end
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end
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defp parse_schedule(text) do
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[line_1, line_2] = String.split(text, "\n", trim: true)
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timestamp = String.to_integer(line_1)
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bus_ids =
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String.split(line_2, ",")
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|> Enum.map(&parse_int(&1, nil))
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|> Enum.filter(& &1)
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{timestamp, bus_ids}
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end
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# defp lcm(a, b, numbers) do
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# bag_union(factors(a, numbers), factors(b, numbers))
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# end
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# defp factors(n, numbers) do
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# Numbers.primes_up_to(numbers, div(n, 2))
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# |> Enum.reduce(2..n, {n, %{}}, fn )
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# end
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# defp bag_union(b1, b2) do
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# Enum.reduce(b1, b2, fn {k, v}, bag ->
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# Map.put(bag, k, max(v, Map.get(bag, k, 0)))
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# end)
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# end
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defp first_multiple_after(timestamp, bus_id) do
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if rem(timestamp, bus_id) == 0 do
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timestamp
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else
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bus_id * (div(timestamp, bus_id) + 1)
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end
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end
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def part1 do
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{timestamp, bus_ids} = read_input() |> parse_schedule()
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{bus_id, time} =
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Enum.map(bus_ids, &{&1, first_multiple_after(timestamp, &1)})
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|> Enum.min_by(fn {_, time} -> time end)
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bus_id * (time - timestamp)
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end
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end
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Day13.part1() |> IO.inspect()
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