Define Cache and convert app to OTP

Define a simple in-memory key-value store for our cache.

TL;DR:
- Define `Cache` as a simple state-keeping `Agent`
- Define `Sup`, which starts our Cache process
- Define `App`, which starts our Supervisor process
- Whitelist `App` in `mix.exs`, so that it starts after calling `iex -S mix`

1 files changed, 27 insertions, 0 deletions

diff --git a/assessments/semiprimes/server/lib/cache.ex b/assessments/semiprimes/server/lib/cache.ex
new file mode 100644
index 000000000000..334bf901b6e6
--- /dev/null
+++ b/assessments/semiprimes/server/lib/cache.ex
@@ -0,0 +1,27 @@
+defmodule Cache do
+  @moduledoc """
+  Cache is an in-memory key-value store.
+  """
+  use Agent
+
+  @doc """
+  Inititalize the key-value store.
+  """
+  def start_link(_) do
+    Agent.start_link(fn -> %{} end, name: __MODULE__)
+  end
+
+  @doc """
+  Attempt to return the value stored at `key`
+  """
+  def get(key) do
+    Agent.get(__MODULE__, &Map.get(&1, key))
+  end
+
+  @doc """
+  Write the `value` under the `key`. Last writer wins.
+  """
+  def put(key, value) do
+    Agent.update(__MODULE__, &Map.put(&1, key, value))
+  end
+end