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Solve 2018/7
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‎lib/2018/day_07.ex‎

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defmodule AdventOfCode.Y2018.Day07 do
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@moduledoc """
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--- Day 7: The Sum of Its Parts ---
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Problem Link: https://adventofcode.com/2018/day/7
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Difficulty: m
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Tags: graph topological-sort simulation
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"""
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alias AdventOfCode.Helpers.{InputReader, Transformers}
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alias Yog.Builder.Labeled
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alias Yog.Traversal
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def input, do: InputReader.read_from_file(2018, 7)
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def run(input \\ input()) do
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# Build the directed graph of dependencies.
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graph = parse(input)
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{run_1(graph), run_2(graph)}
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end
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defp run_1(graph) do
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# Part 1: Order of steps.
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# We must pick the alphabetically first step among all available ones.
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# Yog's lexicographical_topological_sort is built for exactly this!
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{:ok, ids} =
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Traversal.lexicographical_topological_sort(graph, fn a, b ->
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cond do
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a < b -> :lt
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a > b -> :gt
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true -> :eq
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end
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end)
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ids
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|> Enum.map(&Yog.Model.node(graph, &1))
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|> Enum.join("")
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end
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defp run_2(graph) do
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# Part 2: Time to complete steps with 5 workers.
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# Each step duration: 60 + letter_value ('A'=1, etc.)
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all_nodes = Yog.Model.all_nodes(graph)
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# Initial in-degrees for dependency tracking
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in_degrees =
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all_nodes
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|> Map.new(fn id -> {id, length(Yog.Model.predecessors(graph, id))} end)
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# Initial available nodes (no dependencies)
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available =
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all_nodes
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|> Enum.filter(&(in_degrees[&1] == 0))
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|> Enum.sort_by(&Yog.Model.node(graph, &1))
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simulate(0, [], available, in_degrees, graph)
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end
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# Simulation loop: advances time and manages workers.
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defp simulate(time, [], [], _in_degrees, _graph), do: time
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defp simulate(time, workers, available, in_degrees, graph) do
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# Assign free workers (up to 5 total)
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num_free = 5 - length(workers)
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{to_assign, remaining_available} = Enum.split(available, num_free)
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new_workers =
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Enum.reduce(to_assign, workers, fn id, acc ->
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label = Yog.Model.node(graph, id)
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duration = 60 + hd(String.to_charlist(label)) - ?A + 1
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[{id, duration} | acc]
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end)
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# Find the nearest event (finish of a worker)
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dt = Enum.min_by(new_workers, fn {_, t} -> t end) |> elem(1)
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# Advance time!
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# Update worker timers and separate finishing ones
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{finishing, busy} =
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new_workers
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|> Enum.map(fn {id, t} -> {id, t - dt} end)
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|> Enum.split_with(fn {_, t} -> t == 0 end)
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# When a step finishes, we decrease the in-degree of its successors
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{new_in_degrees, newly_available} =
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Enum.reduce(finishing, {in_degrees, []}, fn {id, _}, {degs, acc_new} ->
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successors = Yog.Model.successors(graph, id) |> Enum.map(&elem(&1, 0))
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Enum.reduce(successors, {degs, acc_new}, fn succ, {d_acc, a_acc} ->
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new_deg = d_acc[succ] - 1
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new_d_acc = Map.put(d_acc, succ, new_deg)
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new_a_acc = if new_deg == 0, do: [succ | a_acc], else: a_acc
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{new_d_acc, new_a_acc}
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end)
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end)
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# Finalize the next iteration's available nodes (must be sorted alphabetically)
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final_available =
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(remaining_available ++ newly_available)
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|> Enum.sort_by(&Yog.Model.node(graph, &1))
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simulate(time + dt, busy, final_available, new_in_degrees, graph)
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end
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def parse(data \\ input()) do
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# Build a directed graph from the dependency strings.
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Transformers.lines(data)
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|> Enum.reduce(Labeled.directed(), fn line, builder ->
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# Example: "Step C must be finished before step P can begin."
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words = String.split(line, " ")
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prereq = Enum.at(words, 1)
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step = Enum.at(words, 7)
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Labeled.add_unweighted_edge(builder, prereq, step)
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end)
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|> Labeled.to_graph()
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end
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end

‎priv/input_files/2018_7.txt‎

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Step C must be finished before step P can begin.
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Step V must be finished before step Q can begin.
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Step T must be finished before step X can begin.
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Step B must be finished before step U can begin.
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Step Z must be finished before step O can begin.
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Step P must be finished before step I can begin.
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Step D must be finished before step G can begin.
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Step A must be finished before step Y can begin.
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Step R must be finished before step O can begin.
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Step J must be finished before step E can begin.
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Step N must be finished before step S can begin.
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Step X must be finished before step H can begin.
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Step F must be finished before step L can begin.
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Step S must be finished before step I can begin.
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Step W must be finished before step Q can begin.
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Step H must be finished before step K can begin.
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Step K must be finished before step Q can begin.
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Step E must be finished before step L can begin.
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Step Q must be finished before step O can begin.
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Step U must be finished before step G can begin.
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Step L must be finished before step O can begin.
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Step Y must be finished before step G can begin.
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Step G must be finished before step I can begin.
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Step M must be finished before step I can begin.
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Step I must be finished before step O can begin.
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Step A must be finished before step N can begin.
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Step H must be finished before step O can begin.
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Step T must be finished before step O can begin.
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Step H must be finished before step U can begin.
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Step A must be finished before step I can begin.
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Step B must be finished before step R can begin.
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Step V must be finished before step T can begin.
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Step H must be finished before step M can begin.
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Step C must be finished before step A can begin.
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Step B must be finished before step G can begin.
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Step L must be finished before step Y can begin.
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Step T must be finished before step J can begin.
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Step A must be finished before step R can begin.
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Step X must be finished before step L can begin.
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Step B must be finished before step L can begin.
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Step A must be finished before step F can begin.
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Step K must be finished before step O can begin.
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Step W must be finished before step M can begin.
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Step Z must be finished before step N can begin.
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Step Z must be finished before step S can begin.
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Step R must be finished before step K can begin.
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Step Q must be finished before step L can begin.
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Step G must be finished before step O can begin.
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Step F must be finished before step Y can begin.
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Step V must be finished before step H can begin.
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Step E must be finished before step I can begin.
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Step W must be finished before step Y can begin.
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Step U must be finished before step I can begin.
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Step F must be finished before step K can begin.
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Step M must be finished before step O can begin.
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Step Z must be finished before step H can begin.
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Step X must be finished before step S can begin.
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Step J must be finished before step O can begin.
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Step B must be finished before step I can begin.
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Step F must be finished before step H can begin.
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Step D must be finished before step U can begin.
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Step E must be finished before step M can begin.
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Step Z must be finished before step X can begin.
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Step P must be finished before step L can begin.
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Step W must be finished before step H can begin.
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Step C must be finished before step D can begin.
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Step A must be finished before step X can begin.
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Step Q must be finished before step I can begin.
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Step R must be finished before step Y can begin.
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Step B must be finished before step A can begin.
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Step N must be finished before step L can begin.
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Step H must be finished before step G can begin.
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Step Y must be finished before step M can begin.
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Step L must be finished before step G can begin.
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Step G must be finished before step M can begin.
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Step Z must be finished before step R can begin.
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Step S must be finished before step Q can begin.
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Step P must be finished before step J can begin.
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Step V must be finished before step J can begin.
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Step J must be finished before step I can begin.
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Step J must be finished before step X can begin.
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Step W must be finished before step O can begin.
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Step B must be finished before step F can begin.
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Step R must be finished before step M can begin.
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Step V must be finished before step S can begin.
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Step H must be finished before step E can begin.
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Step E must be finished before step U can begin.
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Step R must be finished before step W can begin.
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Step X must be finished before step Q can begin.
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Step N must be finished before step G can begin.
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Step T must be finished before step I can begin.
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Step L must be finished before step M can begin.
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Step H must be finished before step I can begin.
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Step U must be finished before step M can begin.
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Step C must be finished before step H can begin.
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Step P must be finished before step H can begin.
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Step J must be finished before step F can begin.
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Step A must be finished before step O can begin.
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Step X must be finished before step M can begin.
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Step H must be finished before step L can begin.
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Step W must be finished before step K can begin.

‎test/2018/day_07_test.exs‎

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defmodule AdventOfCode.Y2018.Day07Test do
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@moduledoc false
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use ExUnit.Case, async: true
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@moduletag :y1807
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alias AdventOfCode.Y2018.Day07, as: Solution
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test "Year 2018, Day 7 run/1" do
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assert Solution.run() == {"BCADPVTJFZNRWXHEKSQLUYGMIO", 973}
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end
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end

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