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Day 3 Parts 1 and 2

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Adam Millerchip 2020-12-03 14:37:29 +09:00
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template/

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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

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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.

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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()

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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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