From 32bd49675fec7e4da94f06455c5747fb56780b33 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 15 pt 1

---
 2021/README.md      |   3 +-
 2021/lib/2021/15.ex | 102 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 104 insertions(+), 1 deletion(-)
 create mode 100644 2021/lib/2021/15.ex

diff --git a/2021/README.md b/2021/README.md
index 630eb49..0733edf 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 -->
@@ -35,3 +35,4 @@
 [12]: ./lib/2021/12.ex
 [13]: ./lib/2021/13.ex
 [14]: ./lib/2021/14.ex
+[15]: ./lib/2021/15.ex
diff --git a/2021/lib/2021/15.ex b/2021/lib/2021/15.ex
new file mode 100644
index 0000000..c2ad0bb
--- /dev/null
+++ b/2021/lib/2021/15.ex
@@ -0,0 +1,102 @@
+import AOC
+
+aoc 2021, 15 do
+  def input(input_string \\ input_string()) do
+    lists =
+      input_string
+      |> String.split("\n", trim: true)
+      |> Enum.map(fn line ->
+        line
+        |> String.graphemes()
+        |> Enum.map(&String.to_integer/1)
+      end)
+
+    for {row, i} <- Enum.with_index(lists), {cell, j} <- Enum.with_index(row), into: %{} do
+      {{i, j}, cell}
+    end
+  end
+
+  def move({i, j}, {g, h}), do: {g.(i), h.(j)}
+
+  def neighbors(map, pos) do
+    [
+      {&(&1 + 1), & &1},
+      {& &1, &(&1 + 1)},
+      {&(&1 - 1), & &1},
+      {& &1, &(&1 - 1)}
+    ]
+    |> Enum.map(&move(pos, &1))
+    |> Enum.filter(&Map.has_key?(map, &1))
+  end
+
+  def manhattan_dist({x1, y1}, {x2, y2}), do: abs(x1 - x2) + abs(y1 - y2)
+
+  def a_star(map, start, goal) do
+    open =
+      PriorityQueue.new()
+      |> PriorityQueue.push(start, 0)
+
+    g_scores = %{start => 0}
+    f_scores = %{start => manhattan_dist(start, goal)}
+
+    a_star(map, goal, open, g_scores, f_scores)
+  end
+
+  # Gets into PriorityQueue implementation details and is slow...
+  # https://github.com/bitwalker/libgraph/blob/46c435f507f8b99f3f14c96d380eda28e5e9c7de/lib/priority_queue.ex#L129-L132
+  def p_queue_member?(%PriorityQueue{priorities: tree}, v) do
+    v in (:gb_trees.to_list(tree) |> Enum.flat_map(fn {_, q} -> :queue.to_list(q) end))
+  end
+
+  def a_star(map, goal, open, g_scores, f_scores) do
+    case PriorityQueue.pop(open) do
+      {{:value, curr}, open} ->
+        if curr == goal do
+          f_scores[curr]
+        else
+          {open, g_scores, f_scores} =
+            for neighbor <- neighbors(map, curr), reduce: {open, g_scores, f_scores} do
+              {open, g_scores, f_scores} ->
+                tentative_g_score = g_scores[curr] + Map.get(map, neighbor)
+
+                if tentative_g_score < g_scores[neighbor] do
+                  g_scores = Map.put(g_scores, neighbor, tentative_g_score)
+
+                  f_scores =
+                    Map.put(
+                      f_scores,
+                      neighbor,
+                      tentative_g_score + manhattan_dist(neighbor, goal)
+                    )
+
+                  open =
+                    if p_queue_member?(open, neighbor),
+                      do: open,
+                      else: PriorityQueue.push(open, neighbor, f_scores[neighbor])
+
+                  {open, g_scores, f_scores}
+                else
+                  {open, g_scores, f_scores}
+                end
+            end
+
+          a_star(map, goal, open, g_scores, f_scores)
+        end
+
+      {:empty, _} ->
+        :error
+    end
+  end
+
+  def p1 do
+    map = input()
+
+    start = {0, 0}
+    goal = map |> Map.keys() |> Enum.max()
+
+    a_star(map, start, goal)
+  end
+
+  def p2 do
+  end
+end