Day 9

lib/day9/day9.ex

@@ -0,0 +1,114 @@
+defmodule Day9 do
+  @example """
+  35
+  20
+  15
+  25
+  47
+  40
+  62
+  55
+  65
+  95
+  102
+  117
+  150
+  182
+  127
+  219
+  299
+  277
+  309
+  576
+  """
+  @example_premable_length 5
+
+  def part1(example \\ false) do
+    input = if example, do: @example, else: File.read!("lib/day9/input.txt")
+    length = if example, do: @example_premable_length, else: 25
+
+    input
+    |> parse_input()
+    |> find_first_non_sum(length)
+  end
+
+  def part2(example \\ false) do
+    input = if example, do: @example, else: File.read!("lib/day9/input.txt")
+    length = if example, do: @example_premable_length, else: 25
+
+    input = parse_input(input)
+
+    non_sum = find_first_non_sum(input, length)
+
+    {start, length} = find_range_that_sums_to(input, non_sum)
+    {_, after_start} = Enum.split(input, start)
+    {range, _} = Enum.split(after_start, length)
+    Enum.min(range) + Enum.max(range)
+  end
+
+  def parse_input(input) do
+    input
+    |> String.trim()
+    |> String.split("\n")
+    |> Enum.map(&String.to_integer/1)
+  end
+
+  def find_first_non_sum([_ | rest] = input, length) do
+    if is_number_sum_of_pairs_first(input, length) do
+      find_first_non_sum(rest, length)
+    else
+      {_, [non_sum | _]} = Enum.split(input, length)
+      non_sum
+    end
+  end
+
+  def is_number_sum_of_pairs_first(inputs, count) do
+    {before, [first | _]} = Enum.split(inputs, count)
+
+    before
+    |> all_pairs()
+    |> Enum.any?(fn {a, b} ->
+      # IO.puts("check if #{a} + #{b} == #{first}")
+      a + b == first
+    end)
+  end
+
+  def all_pairs(inputs) do
+    inputs
+    |> Enum.drop(-1)
+    |> Enum.with_index()
+    |> Enum.flat_map(fn {a, a_index} ->
+      {_, rest} = Enum.split(inputs, a_index + 1)
+
+      Enum.map(rest, fn b ->
+        {a, b}
+      end)
+    end)
+  end
+
+  def find_range_that_sums_to([_ | rest] = numbers, target, start \\ 0) do
+    case do_find_leading_range_that_sums_to(numbers, target) do
+      false ->
+        find_range_that_sums_to(rest, target, start + 1)
+
+      count when is_integer(count) ->
+        {start, count}
+    end
+  end
+
+  def do_find_leading_range_that_sums_to(numbers, remaining, count \\ 0)
+
+  def do_find_leading_range_that_sums_to(_, 0, count) do
+    count
+  end
+
+  def do_find_leading_range_that_sums_to([head | rest], remaining, count) do
+    # IO.puts("trying start: #{start} count: #{count}, target: #{target}")
+
+    if head <= remaining do
+      do_find_leading_range_that_sums_to(rest, remaining - head, count + 1)
+    else
+      false
+    end
+  end
+end