🌅

.formatter.exs

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+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

.gitignore

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+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+fsmx-*.tar
+

README.md

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+# Fsmx
+
+**TODO: Add description**
+
+## Installation
+
+If [available in Hex](https://hex.pm/docs/publish), the package can be installed
+by adding `fsmx` to your list of dependencies in `mix.exs`:
+
+```elixir
+def deps do
+  [
+    {:fsmx, "~> 0.1.0"}
+  ]
+end
+```
+
+Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
+and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
+be found at [https://hexdocs.pm/fsmx](https://hexdocs.pm/fsmx).
+

lib/fsmx.ex

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+defmodule Fsmx do
+  @type state_t :: binary
+
+  def transition(%mod{state: state} = struct, new_state) do
+    transitions = mod.__fsmx__(:transitions)
+
+    if valid_transition?(state, new_state, transitions) do
+      {:ok, %{struct | state: new_state}}
+    else
+      {:error, "invalid transition of #{mod} from #{state} to #{new_state}"}
+    end
+  end
+
+  defp valid_transition?(state, new_state, transitions) do
+    transitions
+    |> Map.get(state, [])
+    |> is_or_contains?(new_state)
+  end
+
+  defp is_or_contains?(state, state), do: true
+  defp is_or_contains?(states, state) when is_list(states), do: Enum.member?(states, state)
+  defp is_or_contains?(_, _), do: false
+end

lib/fsmx/struct.ex

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+defmodule Fsmx.Struct do
+  defmacro __using__(opts) do
+    quote do
+      def __fsmx__(:transitions), do: Keyword.fetch!(unquote(opts), :transitions)
+    end
+  end
+end

mix.exs

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+defmodule Fsmx.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :fsmx,
+      version: "0.1.0",
+      elixir: "~> 1.9",
+      elixirc_paths: elixirc_paths(Mix.env()),
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger]
+    ]
+  end
+
+  defp deps do
+    []
+  end
+
+  defp elixirc_paths(:test), do: ["lib", "test/support"]
+  defp elixirc_paths(_), do: ["lib"]
+end

test/fsmx_test.exs

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+defmodule FsmxTest do
+  use ExUnit.Case
+  doctest Fsmx
+
+  alias Fsmx.TestStructs.{Simple, WithCallbacks}
+
+  describe "transition/2" do
+    test "can do simple transitions" do
+      one = %Simple{state: "1"}
+
+      {:ok, two} = Fsmx.transition(one, "2")
+
+      assert %Simple{state: "2"} = two
+
+      {:ok, three} = Fsmx.transition(two, "3")
+
+      assert %Simple{state: "3"} = three
+    end
+
+    test "fails to perform invalid transitions" do
+      one = %Simple{state: "1"}
+
+      assert {:error, msg} = Fsmx.transition(one, "3")
+
+      assert msg == "invalid transition of Elixir.Fsmx.TestStructs.Simple from 1 to 3"
+    end
+  end
+
+  describe "transition/2 with callbacks" do
+    test "calls before_transition/2 on struct" do
+    end
+  end
+end

test/support/test_structs/simple.ex

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+defmodule Fsmx.TestStructs.Simple do
+  defstruct [:state]
+
+  use Fsmx.Struct, transitions: %{"1" => ["2"], "2" => ["3"]}
+end

test/support/test_structs/with_callbacks.ex

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+defmodule Fsmx.TestStructs.WithCallbacks do
+  defstruct [:state]
+
+  use Fsmx.Struct, transitions: %{"1" => ["2"], "2" => ["3"]}
+
+  def before_transition(struct, state) do
+    IO.inspect(struct, state)
+  end
+end

test/test_helper.exs

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+ExUnit.start()