Day 3 Parts 1 and 2

.gitignore

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+template/

README.md

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 My (attempted) solutions to [Advent of Code 2020](https://adventofcode.com/2020) in Elixir.
 
-<img width="973" alt="image" src="https://user-images.githubusercontent.com/498229/100832848-05b99600-34ac-11eb-931f-4b3a95d428f7.png">
+<img width="973" alt="image" src="https://user-images.githubusercontent.com/498229/100967992-db331000-3573-11eb-8a48-aa73d01f1a53.png">
 
 ## Strategy
 

day3/README

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+Day 3 Notes
+
++--------+
+| Part 1 |
++--------|
+
+$ elixir day3part1.exs
+Travelled (969, 323) and encountereed 270 trees
+
+Thoughts:
+
+Input repeats, so use Stream.cycle/2.
+Need to keep track of the x-offset, pass it to Enum.at/2.
+
+
++--------+
+| Part 2 |
++--------|
+
+$ elixir day3part2.exs
+2122848000
+
+Thoughts:
+
+Modify original answer to take x and y velocity.
+Can just pass x velocity in as before.
+Use Enum.take_every/2 to skip unneeded rows for y velocity.
+
++------------------+
+| Overall Thoughts |
++------------------+
+
+Pretty simple, because Stream.cycle/2 takes care of the tricky bit.
+Thankfully I answered part 1 in a way that made part 2 just a small change to add Enum.take_every/2.

day3/day3part1.exs

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+defmodule Day3Part1 do
+  def run do
+    {x, y, trees} =
+      File.read!("input")
+      |> String.trim()
+      |> String.split("\n")
+      |> Enum.map(&String.to_charlist/1)
+      |> Enum.reduce({0, 0, 0}, fn row, {x, y, trees} ->
+        trees =
+          case Stream.cycle(row) |> Enum.at(x) do
+            ?# -> trees + 1
+            ?. -> trees
+          end
+
+        {x + 3, y + 1, trees}
+      end)
+
+    IO.puts("Travelled (#{x}, #{y}) and encountereed #{trees} trees")
+  end
+end
+
+Day3Part1.run()

day3/day3part2.exs

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+defmodule Day3Part2 do
+  def run do
+    answer =
+      slope(1, 1) *
+        slope(3, 1) *
+        slope(5, 1) *
+        slope(7, 1) *
+        slope(1, 2)
+
+    IO.puts(answer)
+  end
+
+  def slope(v_x, v_y) do
+    {_, trees} =
+      File.read!("input")
+      |> String.trim()
+      |> String.split("\n")
+      |> Enum.take_every(v_y)
+      |> Enum.map(&String.to_charlist/1)
+      |> Enum.reduce({0, 0}, fn row, {x, trees} ->
+        trees =
+          case Stream.cycle(row) |> Enum.at(x) do
+            ?# -> trees + 1
+            ?. -> trees
+          end
+
+        {x + v_x, trees}
+      end)
+
+    trees
+  end
+end
+
+Day3Part2.run()

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