add 2021 day 1

elixir/.formatter.exs

@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

elixir/.gitignore

@@ -26,3 +26,6 @@ aoc-*.tar
 /tmp/
 
 .session
+
+# Input files
+*.txt

elixir/lib/Y2021/day1.exs

@@ -0,0 +1,51 @@
+test_input = [ 
+  199,
+  200,
+  208,
+  210,
+  200,
+  207,
+  240,
+  269,
+  260,
+  263
+]
+
+# part 1 solution
+solve = fn input ->
+  input
+  |> Stream.chunk_every(2, 1, :discard)
+  |> Stream.map(fn [a,b] -> b - a end)
+  |> Stream.filter(fn a -> a > 0 end)
+  |> Enum.count()
+end
+
+# transform problem for part 2
+window_sum = fn input ->
+  input
+    |> Stream.chunk_every(3, 1, :discard)
+    |> Stream.map(&Enum.sum/1)
+end
+
+# verify solution for test input
+if not(solve.(test_input) == 7) or not(solve.(test_input |> window_sum.()) == 5) do
+  raise "solution doesn't solve test input"
+end
+
+# parse input from text data to solve
+parse = fn input -> 
+  input 
+  |> Stream.map(&String.trim/1)
+  |> Stream.reject(&(&1 == ""))
+  |> Stream.map(&String.to_integer/1)
+end
+
+
+File.stream!("./day1.txt")
+|> parse.()
+|> then(fn i ->
+    p1 = i |> solve.()
+    p2 = i |> window_sum.() |> solve.()
+    "p1: #{p1}\tp2: #{p2}"
+  end)
+|> IO.puts()