1 files changed, 65 insertions, 0 deletions
diff --git a/aoc-2019-elixir/lib/day02.ex b/aoc-2019-elixir/lib/day02.ex
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+++ b/aoc-2019-elixir/lib/day02.ex
@@ -0,0 +1,65 @@
+defmodule Day02 do
+  @add_opcode 1
+  @mul_opcode 2
+  @ret_opcode 99
+  @noun_address 1
+  @verb_address 2
+
+  @spec part1(binary) :: list
+  def part1(input) do
+    run(input, 12, 2)
+  end
+
+  @spec part2(binary) :: integer
+  def part2(input) do
+    for(
+      n <- 0..100,
+      v <- 0..100,
+      do: {n, v, run(input, n, v)}
+    )
+    |> Enum.find(&(elem(&1, 2) == 19_690_720))
+    |> then(fn {n, v, _} -> n * 100 + v end)
+  end
+
+  defp run(input, noun, verb) do
+    input
+    |> parse()
+    |> List.replace_at(@noun_address, noun)
+    |> List.replace_at(@verb_address, verb)
+    |> execute()
+    |> hd()
+  end
+
+  defp binary_op(program, op, a, b, out) do
+    program |> List.replace_at(out, op.(Enum.at(program, a), Enum.at(program, b)))
+  end
+
+  defp execute(program, pc \\ 0) do
+    instruction = program |> Enum.drop(pc) |> Enum.take(4)
+
+    case instruction do
+      [@add_opcode, a, b, out] ->
+        program
+        |> binary_op(&+/2, a, b, out)
+        |> execute(pc + 4)
+
+      [@mul_opcode, a, b, out] ->
+        program
+        |> binary_op(&*/2, a, b, out)
+        |> execute(pc + 4)
+
+      [@ret_opcode | _] ->
+        program
+    end
+  end
+
+  defp parse(input) do
+    input
+    |> String.split(",")
+    |> Enum.map(&Util.Integer.parse!/1)
+  end
+end
+
+input = Util.Input.read_text("day02")
+input |> Day02.part1() |> IO.inspect()
+input |> Day02.part2() |> IO.inspect()