From def813adf8ca5bf6f42edf9489ed07f32795355b Mon Sep 17 00:00:00 2001
From: DJ O'Leary <dijitol@proton.me>
Date: Tue, 19 Aug 2025 10:13:19 +0200
Subject: feat: add WIP solution to day 5, part 1

---
 2024/OCaml/lib/day_05/part_01.ml | 79 ++++++++++++++++++++++++++++++----------
 1 file changed, 59 insertions(+), 20 deletions(-)

(limited to '2024')

diff --git a/2024/OCaml/lib/day_05/part_01.ml b/2024/OCaml/lib/day_05/part_01.ml
index 92bb105..f32bef1 100644
--- a/2024/OCaml/lib/day_05/part_01.ml
+++ b/2024/OCaml/lib/day_05/part_01.ml
@@ -1,23 +1,62 @@
 (* Day 5: Part 1 *)
 
-(*
-    01. Parse pages that need updating into list, e.g. "75,47,61,53,29" -> [ 75; 47; 61; 53; 29; ]
-    02. Parse page ordering rules into DTOs, e.g. "47|53" -> {"before": 47, "after": 53}
-    03. Find all rules that have pages from the parsed page list on both sides, e.g. "47|53" is valid but "97|47" and "47|13" are not
-      - filter down to "before" values that are in page list
-      - filter down to "after" values that are in page list
-    04. Map across the page list and ensure that all rules for each page are upheld
-    05. Map across the rules for the current page and ensure that all rules are upheld
-      - filter rules down to rules with "before" equal to current page
-      - walk the page list and ensure that all "after" values do no appear before the "before" value
-      - return true if all rules are valid for the current page
-    06. Reduce result down to single boolean
-    07. IF FALSE, move on to next page in page list
-    08. IF TRUE, continue until all rules are validated
-    09. IF ALL RULES ARE NOT VALID FOR PAGE LIST, move on to next page list
-    10. IF ALL RULES ARE VALID FOR PAGE LIST, save middle number in page list
-    11. ONCE ALL PAGE LISTS HAVE BEEN PROCESSED, add all saved middle numbers
-    12. Return sum of middle numbers
-*)
+module StringMap = Map.Make (String)
 
-let solve _ = ()
+let inverse_page_rule page_rule =
+  String.split_on_char '|' page_rule |> fun lst ->
+  match lst with [] -> "" | _ :: _ -> String.concat "|" (List.rev lst)
+
+(** [parse_aux acc page_rules] is a map of all invalid transitions as defined by
+    [page_rules] *)
+let rec get_invalid_transitions_aux acc page_rules : unit StringMap.t =
+  match page_rules with
+  | [] -> acc
+  | h :: t ->
+      get_invalid_transitions_aux
+        (acc |> StringMap.add (inverse_page_rule h) ())
+        t
+
+let get_invalid_transitions page_rules =
+  get_invalid_transitions_aux StringMap.empty page_rules
+
+(** [solve page_rules updates] is the solution for this puzzle, given the
+    [page_rules] and a list of [updates].
+
+    BELOW DOESN'T WORK: no guarantee that second page comes immediately after
+    the first
+    - pass in updates to solve
+    - make sliding window of 2 elements
+    - invert element order and concat
+    - check if key is in map from [parse page_rules],
+    - if yes, then it is an illegal transition
+    - if no, then add the list to a list for further processing *)
+let solve page_rules _ =
+  let invalid_transitions = get_invalid_transitions page_rules in
+  ()
+
+let _ =
+  solve
+    [
+      "47|53";
+      "97|13";
+      "97|61";
+      "97|47";
+      "75|29";
+      "61|13";
+      "75|53";
+      "29|13";
+      "97|29";
+      "53|29";
+      "61|53";
+      "97|53";
+      "61|29";
+      "47|13";
+      "75|47";
+      "97|75";
+      "47|61";
+      "75|61";
+      "47|29";
+      "75|13";
+      "53|13";
+    ]
+    []
-- 
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