From ff7f326e1d162e3983b70e217db98b77dac8bae0 Mon Sep 17 00:00:00 2001
From: Grimmauld <Grimmauld@grimmauld.de>
Date: Mon, 2 Dec 2024 22:36:05 +0100
Subject: [PATCH] alternative day 2 solution: zipListsWith for deltas, just two
  checks

---
 day2/default.nix | 17 ++++-------------
 utils.nix        |  1 +
 2 files changed, 5 insertions(+), 13 deletions(-)

diff --git a/day2/default.nix b/day2/default.nix
index e74c200..cee8a33 100644
--- a/day2/default.nix
+++ b/day2/default.nix
@@ -4,20 +4,11 @@ with builtins;
 with utils;
 let
   records = map (l: map toInt (splitWhitespace l)) (readLines ./input);
-  getMaxDelta = l:
-    (foldl' (prev: value: {
-      inherit value;
-      maxDelta = max prev.maxDelta (delta prev.value value);
-    }) {
-      value = head l;
-      maxDelta = 0;
-    } l).maxDelta;
+  getDeltas = r: zipListsWith sub (init r) (drop 1 r);
   isSafe = r:
-    let sorted = sort lessThan r;
-    in (r == sorted || r == (reverseList sorted)) && allUnique r
-    && (3 >= getMaxDelta r);
-  isSafeDampened = r:
-    any isSafe ([ r ] ++ (map (deleteAt r) (range 0 (length r))));
+    let deltas = getDeltas r;
+    in (all (d: -3 <= d && d < 0) deltas) || (all (d: 3 >= d && d > 0) deltas);
+  isSafeDampened = r: any isSafe (map (deleteAt r) (listRange r));
 in {
   part1 = count isSafe records;
   part2 = count isSafeDampened records;
diff --git a/utils.nix b/utils.nix
index bcc0236..5b041ad 100644
--- a/utils.nix
+++ b/utils.nix
@@ -9,4 +9,5 @@ with builtins; rec {
   deleteAt = l: i: (take i l) ++ (drop (i + 1) l);
   eq = x: y: x == y;
   greaterThan = x: y: x > y;
+  listRange = l: range 0 (length l);
 }