From 46755cea349a63a5b837229008ba7a8ea351fd38 Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Thu, 12 Dec 2024 08:58:13 +0000
Subject: [PATCH] 2024 day 12 part 1.

---
 bin/day2412.ml | 35 +++++++++++++++++++++++++++++++++++
 bin/dune       |  6 ++++--
 2 files changed, 39 insertions(+), 2 deletions(-)
 create mode 100644 bin/day2412.ml

diff --git a/bin/day2412.ml b/bin/day2412.ml
new file mode 100644
index 0000000..3ab88a8
--- /dev/null
+++ b/bin/day2412.ml
@@ -0,0 +1,35 @@
+let find_regions grid =
+  let working = Array.make (Aoc.Grid.length grid) ~-1 in
+  let add_pos region pos perimeter lst =
+    if Aoc.Grid.pos_is_valid grid pos && Aoc.Grid.get_by_pos grid pos = region
+    then (Aoc.Grid.idx_of_pos grid pos :: lst, perimeter - 1)
+    else (lst, perimeter)
+  in
+  let rec scan_pos perimeter area id = function
+    | [] -> (perimeter, area)
+    | idx :: t when working.(idx) <> -1 -> scan_pos perimeter area id t
+    | idx :: t ->
+        working.(idx) <- id;
+        let x, y = Aoc.Grid.pos_of_idx grid idx in
+        let area = succ area in
+        let perimeter = perimeter + 4 in
+        let region = Aoc.Grid.get_by_idx grid idx in
+        let t, perimeter = add_pos region (x - 1, y) perimeter t in
+        let t, perimeter = add_pos region (x + 1, y) perimeter t in
+        let t, perimeter = add_pos region (x, y - 1) perimeter t in
+        let t, perimeter = add_pos region (x, y + 1) perimeter t in
+        scan_pos perimeter area id t
+  in
+  let rec impl acc idx id =
+    if idx >= Aoc.Grid.length grid then List.rev acc
+    else
+      let perimeter, area = scan_pos 0 0 id [ idx ] in
+      if area = 0 then impl acc (idx + 1) id
+      else impl ((perimeter, area) :: acc) (idx + 1) (id + 1)
+  in
+  impl [] 0 0
+
+let part1 grid =
+  find_regions grid |> List.fold_left (fun acc (p, a) -> acc + (p * a)) 0
+
+let _ = Aoc.main Aoc.Grid.of_file [ (string_of_int, part1) ]
diff --git a/bin/dune b/bin/dune
index ac41748..506ca60 100644
--- a/bin/dune
+++ b/bin/dune
@@ -10,7 +10,8 @@
   day2408
   day2409
   day2410
-  day2411)
+  day2411
+  day2412)
  (names
   day2401
   day2402
@@ -22,5 +23,6 @@
   day2408
   day2409
   day2410
-  day2411)
+  day2411
+  day2412)
  (libraries str aoc))