From 3372c8819d048c5610623f67fb1c9cea660ad51d Mon Sep 17 00:00:00 2001
From: Sloane Perrault <sloane.perrault@gmail.com>
Date: Wed, 21 Sep 2022 09:19:53 -0400
Subject: [PATCH] 2021 day 17

---
 2021/README.md      |  3 +-
 2021/lib/2021/17.ex | 74 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 76 insertions(+), 1 deletion(-)
 create mode 100644 2021/lib/2021/17.ex

diff --git a/2021/README.md b/2021/README.md
index 2dd4e77..e9e1649 100644
--- a/2021/README.md
+++ b/2021/README.md
@@ -16,7 +16,7 @@
 | :-: | :-: | :-: | :-: | :-: | :-:  | :-: |
 |     |     |     | [1] | [2] | [3]  | [4] |
 | [5] | [6] | [7] | [8] | [9] | [10] | [11]|
-| [12]| [13]| [14]| [15]  | [16]  | 17   | 18  |
+| [12]| [13]| [14]| [15]  | [16]  | [17]   | 18  |
 | 19  | 20  | 21  | 22  | 23  | 24   | 25  |
 
 <!-- links -->
@@ -37,3 +37,4 @@
 [14]: ./lib/2021/14.ex
 [15]: ./lib/2021/15.ex
 [16]: ./lib/2021/16.ex
+[17]: ./lib/2021/17.ex
diff --git a/2021/lib/2021/17.ex b/2021/lib/2021/17.ex
new file mode 100644
index 0000000..40df34a
--- /dev/null
+++ b/2021/lib/2021/17.ex
@@ -0,0 +1,74 @@
+import AOC
+
+aoc 2021, 17 do
+  def p1 do
+    {target, {xb, yb} = bounds} = input_target()
+
+    for xv <- xb..0, yv <- yb..-yb do
+      launch_probe(xv, yv, target, bounds)
+    end
+    |> Enum.filter(&match?({true, _}, &1))
+    |> Enum.map(&elem(&1, 1))
+    |> Enum.map(&max_height/1)
+    |> Enum.max()
+  end
+
+  def p2 do
+    {target, {xb, yb} = bounds} = input_target()
+
+    for xv <- xb..0, yv <- yb..-yb do
+      launch_probe(xv, yv, target, bounds)
+    end
+    |> Enum.count(&match?({true, _}, &1))
+  end
+
+  def max_height(path) do
+    path
+    |> Enum.map(&elem(&1, 1))
+    |> Enum.max()
+  end
+
+  def launch_probe(xv0, yv0, target, bounds) do
+    path =
+      probe_path(xv0, yv0)
+      |> Enum.take_while(fn curr -> not past_target?(curr, bounds) end)
+
+    {Enum.any?(path, &MapSet.member?(target, &1)), path}
+  end
+
+  def probe_path(xv0, yv0) do
+    Stream.unfold({{0, 0}, {xv0, yv0}}, iterate(&next/1))
+    |> Stream.map(&elem(&1, 0))
+  end
+
+  def iterate(fun), do: fn curr -> {curr, fun.(curr)} end
+
+  def next({_, vel} = pos_vel), do: {next_pos(pos_vel), next_vel(vel)}
+
+  def next_pos({{x, y}, {xv, yv}}), do: {x + xv, y + yv}
+
+  def next_vel({xv, yv}), do: {next_xv(xv), next_yv(yv)}
+
+  def next_xv(0), do: 0
+  def next_xv(xv) when xv > 0, do: xv - 1
+  def next_xv(xv) when xv < 0, do: xv + 1
+
+  def next_yv(yv), do: yv - 1
+
+  def past_target?({x, y}, {xb, yb}), do: past_target?(x, xb) or past_target?(y, yb)
+  def past_target?(n, bound) when bound < 0, do: n < bound
+  def past_target?(n, bound), do: n > bound
+
+  def input_target(input_string \\ input_string()) do
+    [xs, ys] =
+      ~r/target area: x=(-?\d+)..(-?\d+), y=(-?\d+)..(-?\d+)/
+      |> Regex.run(input_string, capture: :all_but_first)
+      |> Enum.map(&String.to_integer/1)
+      |> Enum.chunk_every(2)
+      |> Enum.map(fn args -> apply(&Range.new/2, args) end)
+
+    target = for x <- xs, y <- ys, into: MapSet.new(), do: {x, y}
+    bounds = Enum.max_by(target, fn {x, y} -> abs(x) + abs(y) end)
+    {target, bounds}
+  end
+end