day 17 part 2

Author: maco

2021/elixir/lib/advent_of_code/day_17.ex

@@ -1,18 +1,14 @@
 defmodule AdventOfCode.Day17 do
   def part1(args) do
-
-    %{x1: x1, x2: x2, y1: y1, y2: y2} = Regex.named_captures(~r/.*x=(?<x1>[0-9\-]*)\.\.(?<x2>[0-9\-]*), y=(?<y1>[0-9\-\.]*)\.\.(?<y2>[0-9\-\.]*)/, args)
-    |> Enum.map(fn {k,v} -> {String.to_atom(k), String.to_integer(v)} end)
-    |> Map.new()
-    start = {0,0}
+    %{x1: x1, x2: x2, y1: y1, y2: y2} = parse(args)
     target = {x1..x2, y1..y2}
 
     {x, y} = range(x1, x2, y1, y2)
 
     {highest, _velocity} =
     Enum.reduce(x, {-1_000_000, {0,0}}, fn vx, {bestest_height, _v}->
         Enum.reduce(y, {bestest_height, {vx, 0}}, fn vy, {best_height, best_vel}->
-          {final_pos, new_height} = try_velocity(start, {vx, vy}, target)
+          {final_pos, new_height} = try_velocity({0,0}, {vx, vy}, target)
           max_height = max(best_height, new_height)
           cond do
             inside_target?(final_pos, target) and new_height == max_height ->
@@ -29,8 +25,8 @@ defmodule AdventOfCode.Day17 do
 
   @spec range(number, number, number, number) :: {Range.t(), Range.t()}
   def range(x1, x2, y1, y2) do
-    y = max(abs(x1), abs(x2))
-    x = max(abs(y1), abs(y2))
+    y = max(abs(x1), abs(x2)) * 3
+    x = max(abs(y1), abs(y2)) * 3
     {-x..x, -y..y}
   end
 
@@ -85,5 +81,30 @@ defmodule AdventOfCode.Day17 do
   end
 
   def part2(args) do
+    %{x1: x1, x2: x2, y1: y1, y2: y2} = parse(args)
+    target = {x1..x2, y1..y2}
+
+    {x, y} = range(x1, x2, y1, y2)
+
+    Enum.reduce(x, [], fn vx, vs ->
+      new_vs = Enum.reduce(y, [], fn vy, velocities->
+        {final_pos, _new_height} = try_velocity({0,0}, {vx, vy}, target)
+        case inside_target?(final_pos, target) do
+          true ->
+            [{vx, vy} | velocities] 
+          false ->
+            velocities
+        end
+      end)
+      [new_vs | vs]
+    end)
+    |> List.flatten
+    |> Enum.count
+  end
+
+  def parse(input) do
+    Regex.named_captures(~r/.*x=(?<x1>[0-9\-]*)\.\.(?<x2>[0-9\-]*), y=(?<y1>[0-9\-\.]*)\.\.(?<y2>[0-9\-\.]*)/, input)
+    |> Enum.map(fn {k,v} -> {String.to_atom(k), String.to_integer(v)} end)
+    |> Map.new()
   end
 end

2021/elixir/test/advent_of_code/day_17_test.exs

@@ -11,11 +11,10 @@ defmodule AdventOfCode.Day17Test do
     assert result == 45
   end
 
-  @tag :skip
   test "part2" do
-    input = nil
+    input = "target area: x=20..30, y=-10..-5"
     result = part2(input)
 
-    assert result
+    assert result == 112
   end
 end