2020 Day 1 Part 1

2020/day1/day1.ex

@@ -0,0 +1,21 @@
+defmodule Day1 do
+  def all_pairs(list) do
+    Enum.reduce(0..(length(list) - 1), [], fn idx, pairs ->
+      item = Enum.at(list, idx)
+      other_items = Enum.slice(list, (idx + 1)..(length(list) - 1))
+      new_pairs = Enum.map(other_items, fn elt -> {item, elt} end)
+      pairs ++ new_pairs
+    end)
+  end
+
+  def part1(input) do
+    lines = String.split(input)
+    ints = Enum.map(lines, fn line -> String.to_integer(line) end)
+    pairs = Day1.all_pairs(ints)
+    [{a, b}] = Enum.filter(pairs, fn {a, b} -> a + b == 2020 end)
+    a * b
+  end
+end
+
+input = File.read!("input.txt")
+IO.inspect(Day1.part1(input))