From dbc723e2d13593277b317b74cc81d400aae0b573 Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Tue, 10 Dec 2024 09:06:07 +0000
Subject: [PATCH] 2024 day 10

---
 bin/day2410.ml | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++
 bin/dune       |  6 +++--
 2 files changed, 65 insertions(+), 2 deletions(-)
 create mode 100644 bin/day2410.ml

diff --git a/bin/day2410.ml b/bin/day2410.ml
new file mode 100644
index 0000000..6ffdfe5
--- /dev/null
+++ b/bin/day2410.ml
@@ -0,0 +1,61 @@
+type grid = { grid : string; width : int; height : int }
+
+let grid_of_file fname =
+  let strs = Aoc.strings_of_file fname in
+  let grid = List.fold_left String.cat "" strs in
+  let width = String.length (List.hd strs) in
+  let height = String.length grid / width in
+  { grid; width; height }
+
+let grid_length grid = String.length grid.grid
+let grid_get_by_idx grid idx = grid.grid.[idx]
+let grid_get_by_pos grid (x, y) = grid.grid.[x + (y * grid.width)]
+let grid_pos_of_idx grid idx = (idx mod grid.width, idx / grid.width)
+
+let grid_pos_is_valid grid (x, y) =
+  x >= 0 && y >= 0 && x < grid.width && y < grid.height
+
+let next_char ch = Char.chr (1 + Char.code ch)
+let ten = next_char '9'
+let one = '1'
+
+let find_trail grid pos0 =
+  let add_pos lst pos digit =
+    if grid_pos_is_valid grid pos && grid_get_by_pos grid pos = digit then
+      pos :: lst
+    else lst
+  in
+  let add_poses lst (x, y) digit =
+    let lst = add_pos lst (x - 1, y) digit in
+    let lst = add_pos lst (x + 1, y) digit in
+    let lst = add_pos lst (x, y - 1) digit in
+    let lst = add_pos lst (x, y + 1) digit in
+    lst
+  in
+  let rec find_next acc digit = function
+    | [] -> acc
+    | h :: t -> find_next (add_poses acc h digit) digit t
+  in
+  let rec impl acc digit =
+    if digit = ten then acc else impl (find_next [] digit acc) (next_char digit)
+  in
+  impl [ pos0 ] one
+
+let find_trails grid =
+  let rec impl acc idx =
+    if idx >= grid_length grid then acc
+    else if grid_get_by_idx grid idx <> '0' then impl acc (idx + 1)
+    else (* grid_get_by_idx grid idx = 0 *)
+      impl (find_trail grid (grid_pos_of_idx grid idx) :: acc) (idx + 1)
+  in
+  impl [] 0 |> List.rev
+
+let part1 grid =
+  find_trails grid
+  |> List.map (List.sort_uniq compare)
+  |> List.map List.length |> List.fold_left ( + ) 0
+
+let part2 grid =
+  find_trails grid |> List.map List.length |> List.fold_left ( + ) 0
+
+let _ = Aoc.main grid_of_file [ (string_of_int, part1); (string_of_int, part2) ]
diff --git a/bin/dune b/bin/dune
index 6b6c429..a607d83 100644
--- a/bin/dune
+++ b/bin/dune
@@ -8,7 +8,8 @@
   day2406
   day2407
   day2408
-  day2409)
+  day2409
+  day2410)
  (names
   day2401
   day2402
@@ -18,5 +19,6 @@
   day2406
   day2407
   day2408
-  day2409)
+  day2409
+  day2410)
  (libraries str aoc))