diff --git a/lib/day7/day7.ex b/lib/day7/day7.ex
new file mode 100644
index 0000000..24ce3a1
--- /dev/null
+++ b/lib/day7/day7.ex
@@ -0,0 +1,113 @@
+defmodule Day7 do
+  
+  def get_requirement(str) do
+    [_, requirement, step] = Regex.run(~r/Step (\w) must be finished before step (\w) can begin./, str)
+    {step, requirement}
+  end
+
+  def all_nodes(requirements) do
+    requirements
+    |> Enum.reduce([], fn {step, req}, acc ->
+      [step | [req | acc]]
+    end)
+    |> Enum.uniq
+  end
+
+  def create_graph(requirements) do
+    Enum.reduce(requirements, %{}, fn {step, req}, acc ->
+      Map.update(acc, step, [req], &([req | &1]))
+    end)
+  end
+
+  @doc """
+  Topological sort implemented using Kahn's algorithm: https://en.wikipedia.org/wiki/Topological_sorting#Kahn's_algorithm
+  """
+  def kahn(_nodes, sorted, _graph, []), do: Enum.reverse(sorted)
+  def kahn(nodes, sorted, graph, no_incoming_edges) do
+    # sort no_incoming_edges lexicographically as per AoC problem
+    [n | rest_no_incoming_edges] = no_incoming_edges |> Enum.sort()
+    sorted = [n | sorted]
+    {graph, no_incoming_edges} =
+      nodes
+      |> Enum.filter(fn m -> n in Map.get(graph, m, []) end)
+      |> Enum.reduce({graph, rest_no_incoming_edges}, fn m, {graph, no_incoming_edges} ->
+        new_dependencies_of_m = graph |> Map.get(m) |> List.delete(n)
+        new_graph = graph |> Map.put(m, new_dependencies_of_m)
+        {
+          new_graph,
+          case new_dependencies_of_m do
+            [] -> [m | no_incoming_edges]
+            _ -> no_incoming_edges
+          end
+        }
+      end)
+    kahn(nodes, sorted, graph, no_incoming_edges)
+  end
+
+  def kahn(nodes, graph) do
+    no_incoming_edges = Enum.filter(nodes, fn node -> !Map.has_key?(graph, node) end)
+    kahn(nodes, [], graph, no_incoming_edges)
+  end
+
+  def step_time(nil), do: 0
+  def step_time(<<c::utf8, _rest::binary>>) do
+    # step - ?A + 1 + extra
+    c - ?A + 60
+  end
+
+  def tick(workers, nodes, graph, done \\ [], time \\ 0) do
+    {workers, {done, nodes}} =
+      Enum.map_reduce(workers, {done, nodes}, fn {current_step, remaining}, {done, nodes} ->
+        case remaining do
+          0 ->
+            new_done = case current_step do
+              nil -> done
+              _ -> [current_step | done]
+            end
+            next_step =
+              nodes
+              |> Enum.find(fn node ->
+                Enum.all?(Map.get(graph, node, []), fn dep -> dep in new_done end)
+              end)
+            new_nodes = List.delete(nodes, next_step)
+            {{next_step, step_time(next_step)}, {new_done, new_nodes}}
+
+          n when n > 0 ->
+            {{current_step, n - 1}, {done, nodes}}
+        end
+      end)
+
+    IO.inspect(nodes)
+    IO.inspect(done)
+    IO.inspect(workers)
+
+    if length(nodes) == 0 and Enum.all?(workers, fn {step, _} -> step == nil end) do
+      time - 1
+    else
+      tick(workers, nodes, graph, done, time + 1)
+    end
+  end
+
+  def parse_input() do
+    File.read!("lib/day7/input.txt")
+    |> String.split("\n", trim: true)
+    |> Enum.map(&get_requirement/1)
+  end
+
+  def part1() do
+    requirements = parse_input()
+    nodes = all_nodes(requirements)
+    graph = create_graph(requirements)
+    kahn(nodes, graph)
+    |> Enum.join()
+  end
+
+  def part2() do
+    requirements = parse_input()
+    nodes = all_nodes(requirements)
+    graph = create_graph(requirements)
+    workers = for _ <- 1..5, do: {nil, 1}
+    tick(workers, nodes, graph)
+  end
+
+end
\ No newline at end of file
diff --git a/lib/day7/input.txt b/lib/day7/input.txt
new file mode 100644
index 0000000..76d8d71
--- /dev/null
+++ b/lib/day7/input.txt
@@ -0,0 +1,101 @@
+Step T must be finished before step C can begin.
+Step V must be finished before step C can begin.
+Step Y must be finished before step H can begin.
+Step R must be finished before step U can begin.
+Step B must be finished before step J can begin.
+Step Q must be finished before step O can begin.
+Step W must be finished before step O can begin.
+Step S must be finished before step X can begin.
+Step I must be finished before step N can begin.
+Step X must be finished before step H can begin.
+Step M must be finished before step L can begin.
+Step A must be finished before step F can begin.
+Step G must be finished before step P can begin.
+Step U must be finished before step E can begin.
+Step Z must be finished before step E can begin.
+Step H must be finished before step L can begin.
+Step P must be finished before step C can begin.
+Step K must be finished before step F can begin.
+Step O must be finished before step C can begin.
+Step C must be finished before step F can begin.
+Step D must be finished before step L can begin.
+Step L must be finished before step F can begin.
+Step N must be finished before step E can begin.
+Step J must be finished before step F can begin.
+Step F must be finished before step E can begin.
+Step I must be finished before step A can begin.
+Step Z must be finished before step J can begin.
+Step I must be finished before step P can begin.
+Step T must be finished before step E can begin.
+Step R must be finished before step F can begin.
+Step U must be finished before step H can begin.
+Step K must be finished before step E can begin.
+Step D must be finished before step N can begin.
+Step U must be finished before step C can begin.
+Step D must be finished before step J can begin.
+Step N must be finished before step F can begin.
+Step C must be finished before step J can begin.
+Step U must be finished before step J can begin.
+Step A must be finished before step O can begin.
+Step H must be finished before step N can begin.
+Step P must be finished before step O can begin.
+Step I must be finished before step E can begin.
+Step G must be finished before step F can begin.
+Step O must be finished before step J can begin.
+Step Q must be finished before step F can begin.
+Step G must be finished before step J can begin.
+Step X must be finished before step E can begin.
+Step S must be finished before step D can begin.
+Step R must be finished before step P can begin.
+Step K must be finished before step L can begin.
+Step R must be finished before step Q can begin.
+Step L must be finished before step N can begin.
+Step Q must be finished before step C can begin.
+Step C must be finished before step D can begin.
+Step C must be finished before step N can begin.
+Step O must be finished before step E can begin.
+Step W must be finished before step F can begin.
+Step K must be finished before step D can begin.
+Step T must be finished before step H can begin.
+Step M must be finished before step D can begin.
+Step Y must be finished before step Z can begin.
+Step J must be finished before step E can begin.
+Step S must be finished before step F can begin.
+Step G must be finished before step U can begin.
+Step V must be finished before step S can begin.
+Step Y must be finished before step F can begin.
+Step G must be finished before step H can begin.
+Step T must be finished before step Q can begin.
+Step S must be finished before step U can begin.
+Step V must be finished before step D can begin.
+Step W must be finished before step M can begin.
+Step M must be finished before step E can begin.
+Step A must be finished before step H can begin.
+Step B must be finished before step F can begin.
+Step B must be finished before step N can begin.
+Step D must be finished before step F can begin.
+Step W must be finished before step K can begin.
+Step P must be finished before step E can begin.
+Step B must be finished before step X can begin.
+Step Q must be finished before step U can begin.
+Step Q must be finished before step X can begin.
+Step X must be finished before step N can begin.
+Step M must be finished before step Z can begin.
+Step G must be finished before step Z can begin.
+Step S must be finished before step G can begin.
+Step P must be finished before step F can begin.
+Step I must be finished before step O can begin.
+Step R must be finished before step A can begin.
+Step L must be finished before step J can begin.
+Step B must be finished before step I can begin.
+Step C must be finished before step E can begin.
+Step B must be finished before step W can begin.
+Step P must be finished before step N can begin.
+Step H must be finished before step C can begin.
+Step K must be finished before step J can begin.
+Step Y must be finished before step M can begin.
+Step Z must be finished before step P can begin.
+Step I must be finished before step K can begin.
+Step V must be finished before step E can begin.
+Step Y must be finished before step P can begin.
+Step T must be finished before step R can begin.
\ No newline at end of file
diff --git a/test/day7_test.exs b/test/day7_test.exs
new file mode 100644
index 0000000..6493791
--- /dev/null
+++ b/test/day7_test.exs
@@ -0,0 +1,4 @@
+defmodule Day7Test do
+  use ExUnit.Case
+  doctest Day7
+end