day 9 part 1

lib/2023/day9.ex

@@ -0,0 +1,57 @@
+defmodule Mix.Tasks.Day9 do
+  use Mix.Task
+
+  def run(_) do
+    {_, content} = File.read("./input/2023_9.txt")
+
+    solve(content)|> inspect() |> IO.puts()
+  end
+
+  def solve(content) do
+    content
+      |> String.split("\n")
+      |> Stream.reject(&(&1 === ""))
+      |> Stream.map(fn line ->
+        line
+        |> String.split(" ")
+        |> Stream.map(&String.to_integer/1)
+        |> Enum.to_list()
+      end) 
+      |> Stream.map(fn list -> 
+      IO.puts(inspect(list))
+      extrapolate(list)
+      end)
+      |> Enum.reduce([], 
+        fn l, acc -> 
+        [l |> Enum.sum()
+        | acc]
+      end)  |> Enum.sum()
+      
+  end
+
+  def extrapolate(list) do
+    extrapolate(list, [])
+  end
+
+  def to_difference_list(nums) do
+    IO.puts("differencing #{inspect(nums)}")
+    list = nums 
+      |> Stream.chunk_every(2, 1, :discard)
+      |> Enum.reduce([], fn [a,b], acc -> [ b - a | acc] end)
+    IO.puts("differenc was #{inspect(list)}")
+    list
+  end
+
+  def extrapolate([0], acc) do
+    [0 | acc]
+  end
+
+  def extrapolate(list, acc) do
+    IO.puts("extrapolating #{inspect(list)}")
+    extrapolate(
+      list |> to_difference_list() |> Enum.reverse(),
+      [list |> List.last() | acc]
+    )
+  end
+
+  end