From a7416ed0560a325a67bd4616a65a23115c123047 Mon Sep 17 00:00:00 2001
From: Jan-Bulthuis <git@bulthuis.dev>
Date: Sat, 7 Dec 2024 15:48:34 +0100
Subject: [PATCH] day7 fast

---
 aoc_2024/src/day7.rs | 45 ++++++++++++++++++++------------------------
 1 file changed, 20 insertions(+), 25 deletions(-)

diff --git a/aoc_2024/src/day7.rs b/aoc_2024/src/day7.rs
index eb4ea98..678b16a 100644
--- a/aoc_2024/src/day7.rs
+++ b/aoc_2024/src/day7.rs
@@ -23,19 +23,19 @@ fn parse(input: &str) -> Input {
 fn part1(input: &Input) -> usize {
     input
         .iter()
-        .filter(|v| part1_test(v.1[0], v.0, &v.1[1..]))
+        .filter(|v| part1_test(v.1[0] as i64, v.0 as i64, &v.1[1..]))
         .fold(0, |acc, v| acc + v.0 as usize)
 }
 
-fn part1_test(curr: u64, target: u64, numbers: &[u64]) -> bool {
-    if curr > target {
-        false
-    } else if numbers.is_empty() {
-        curr == target
+fn part1_test(target: i64, remaining: i64, numbers: &[u64]) -> bool {
+    if numbers.is_empty() {
+        remaining == target
     } else {
-        let num = numbers[0];
-        let rest = &numbers[1..];
-        part1_test(curr + num, target, rest) || part1_test(curr * num, target, rest)
+        let num = numbers[numbers.len() - 1] as i64;
+        let rest = &numbers[0..numbers.len() - 1];
+        let mul = remaining % num == 0 && part1_test(target, remaining / num, rest);
+        let add = part1_test(target, remaining - num, rest);
+        mul || add
     }
 }
 
@@ -43,26 +43,21 @@ fn part1_test(curr: u64, target: u64, numbers: &[u64]) -> bool {
 fn part2(input: &Input) -> usize {
     input
         .iter()
-        .filter(|v| part2_test(v.1[0], v.0, &v.1[1..]))
+        .filter(|v| part2_test(v.1[0] as i64, v.0 as i64, &v.1[1..]))
         .fold(0, |acc, v| acc + v.0 as usize)
 }
 
-fn concat(a: u64, b: u64) -> u64 {
-    let digits = b.ilog10() + 1;
-    a * 10u64.pow(digits) + b
-}
-
-fn part2_test(curr: u64, target: u64, numbers: &[u64]) -> bool {
-    if curr > target {
-        false
-    } else if numbers.is_empty() {
-        curr == target
+fn part2_test(target: i64, remaining: i64, numbers: &[u64]) -> bool {
+    if numbers.is_empty() {
+        remaining == target
     } else {
-        let num = numbers[0];
-        let rest = &numbers[1..];
-        part2_test(curr + num, target, rest)
-            || part2_test(curr * num, target, rest)
-            || part2_test(concat(curr, num), target, rest)
+        let num = numbers[numbers.len() - 1] as i64;
+        let rest = &numbers[0..numbers.len() - 1];
+        let mul = remaining % num == 0 && part2_test(target, remaining / num, rest);
+        let pow = 10i64.pow(num.ilog10() + 1);
+        let con = remaining % pow == num && part2_test(target, remaining / pow, rest);
+        let add = part2_test(target, remaining - num, rest);
+        mul || con || add
     }
 }