From fe5bc1de7ccadd7da123c252791579f6b677edf0 Mon Sep 17 00:00:00 2001
From: Shadowfacts <me@shadowfacts.net>
Date: Fri, 6 Dec 2019 11:48:28 -0500
Subject: [PATCH] Intcode assembler: add _self label

---
 lib/intcode/assembler.ex        | 36 ++++++++++++++++++++++++++-------
 test/intcode/assembler_test.exs |  8 ++++++++
 2 files changed, 37 insertions(+), 7 deletions(-)

diff --git a/lib/intcode/assembler.ex b/lib/intcode/assembler.ex
index c51036a..c0116a0 100644
--- a/lib/intcode/assembler.ex
+++ b/lib/intcode/assembler.ex
@@ -61,14 +61,36 @@ defmodule Assembler do
         end
       end)
 
-    memory =
-      Enum.map(memory, fn
-        {:label, name} -> Map.fetch!(labels, name)
-        it -> it
-      end)
-
     # IO.inspect(labels)
-    Enum.reverse(memory)
+
+    memory
+    |> Enum.reverse()
+    |> Enum.with_index()
+    |> Enum.map(fn
+      {{:label, name}, index} -> get_label(name, index, labels)
+      {it, _} -> it
+    end)
+  end
+
+  @doc """
+  Get the address of a label.
+
+  `_self` is a special label that resolves to the address of where it's being inserted into the program.
+
+  ## Examples
+    iex> Assembler.get_label("test", 1, %{"test" => 14})
+    14
+    iex> Assembler.get_label("_self", 1, %{})
+    1
+  """
+  def get_label(name, index, labels)
+
+  def get_label("_self", index, _) do
+    index
+  end
+
+  def get_label(name, _, labels) do
+    Map.fetch!(labels, name)
   end
 
   @doc """
diff --git a/test/intcode/assembler_test.exs b/test/intcode/assembler_test.exs
index df526d9..509087e 100644
--- a/test/intcode/assembler_test.exs
+++ b/test/intcode/assembler_test.exs
@@ -68,4 +68,12 @@ defmodule AssemblerTest do
              0
            ]
   end
+
+  test "assembles a program with a _self label" do
+    program = """
+      add 1 2 _self
+    """
+
+    assert Assembler.assemble(program) == [1101, 1, 2, 3]
+  end
 end