Zero-prefix numbers

2020/day01/day1.ex

@@ -0,0 +1,54 @@
+defmodule Day1 do
+  def part1(input) do
+    lines = String.split(input)
+    ints = Enum.map(lines, fn line -> String.to_integer(line) end)
+    pairs = all_pairs(ints)
+    [{a, b}] = Enum.filter(pairs, fn {a, b} -> a + b == 2020 end)
+    a * b
+  end
+
+  defp 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
+
+  # Wow, I'm out of shape when it comes to making sets recursively!
+
+  def part2(input) do
+    lines = String.split(input)
+    ints = Enum.map(lines, &String.to_integer/1)
+    pairs = all_triples(ints)
+    [{a, b, c}] = Enum.filter(pairs, fn {a, b, c} -> a + b + c == 2020 end)
+    a * b * c
+  end
+
+  defp all_triples(list) do
+    Enum.reduce(0..(length(list) - 1), [], fn idx, triples ->
+      item = Enum.at(list, idx)
+      pairs = all_pairs(Enum.slice(list, (idx + 1)..(length(list) - 1)))
+      new_triples = Enum.map(pairs, fn {a, b} -> {item, a, b} end)
+      triples ++ new_triples
+    end)
+  end
+
+  defp combinations(list, size) do
+    indices = 0..(length(list) - 1)
+
+    Enum.zip(for _ <- 1..size, do: indices)
+    |> IO.inspect()
+    |> Enum.filter(fn idxs -> length(Enum.uniq(Tuple.to_list(idxs))) == size end)
+    |> Enum.map(fn idxs ->
+      for i <- 0..(size - 1) do
+        elem(idxs, i)
+      end
+    end)
+  end
+end
+
+input = File.read!("input.txt")
+IO.inspect(Day1.part1(input))
+IO.inspect(Day1.part2(input))