From 0d910e8724b4e708f14b27c6d890a4fbc5e81b29 Mon Sep 17 00:00:00 2001
From: Johannes <freelon@gmx.net>
Date: Fri, 13 Dec 2024 15:32:45 +0100
Subject: [PATCH] day 13 part 2

---
 src/day13.rs | 73 +++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 50 insertions(+), 23 deletions(-)

diff --git a/src/day13.rs b/src/day13.rs
index 30988c5..4b3db5f 100644
--- a/src/day13.rs
+++ b/src/day13.rs
@@ -1,4 +1,4 @@
-use std::{borrow::Borrow, fs::read_to_string};
+use std::fs::read_to_string;
 
 use itertools::Itertools;
 use regex::Regex;
@@ -10,7 +10,7 @@ pub fn day_main() {
     println!(" part2: {}", part2(input));
 }
 
-type RiddleResult = u64;
+type RiddleResult = i64;
 
 fn part1(input: &str) -> RiddleResult {
     // Button A: X+30, Y+84
@@ -27,46 +27,73 @@ Prize: X=(\d+), Y=(\d+)",
             let (ax, ay, bx, by, px, py) = block
                 .iter()
                 .skip(1)
-                .map(|it| it.unwrap().as_str().parse::<u64>().unwrap())
+                .map(|it| it.unwrap().as_str().parse::<i64>().unwrap())
                 .collect_tuple()
                 .unwrap();
-            let mut solution = None;
-            for a in 0..=100 {
-                for b in 0..=100 {
-                    if a * ax + b * bx == px && a * ay + b * by == py {
-                        let s = 3 * a + b;
-                        if let Some(old) = solution {
-                            if old > s {
-                                solution = Some(s);
-                            }
-                        } else {
-                            solution = Some(s);
-                        }
-                    }
-                }
-            }
+            solve((ax, ay, bx, by, px, py)).unwrap_or(0)
+        })
+        .sum()
+}
+
+fn part2(input: &str) -> RiddleResult {
+    let r = Regex::new(
+        r"Button A: X\+(\d+), Y\+(\d+)
+Button B: X\+(\d+), Y\+(\d+)
+Prize: X=(\d+), Y=(\d+)",
+    )
+    .unwrap();
+    r.captures_iter(input)
+        .map(|block| {
+            let (ax, ay, bx, by, px, py) = block
+                .iter()
+                .skip(1)
+                .map(|it| it.unwrap().as_str().parse::<i64>().unwrap())
+                .collect_tuple()
+                .unwrap();
+            let solution = solve((ax, ay, bx, by, px + 10000000000000, py + 10000000000000));
             solution.unwrap_or(0)
         })
         .sum()
 }
 
-fn part2(_input: &str) -> RiddleResult {
-    0
+fn solve((ax, ay, bx, by, px, py): (i64, i64, i64, i64, i64, i64)) -> Option<RiddleResult> {
+    let b = (ay * px - ax * py) / (ay * bx - ax * by);
+    let a = (px - b * bx) / ax;
+    if a >= 0 && b >= 0 && a * ax + b * bx == px && a * ay + b * by == py {
+        Some(3 * a + b)
+    } else {
+        None
+    }
 }
 
 #[cfg(test)]
 mod test {
     use super::{part1, part2};
 
-    const TEST_INPUT: &str = r"";
+    const TEST_INPUT: &str = r"Button A: X+94, Y+34
+Button B: X+22, Y+67
+Prize: X=8400, Y=5400
+
+Button A: X+26, Y+66
+Button B: X+67, Y+21
+Prize: X=12748, Y=12176
+
+Button A: X+17, Y+86
+Button B: X+84, Y+37
+Prize: X=7870, Y=6450
+
+Button A: X+69, Y+23
+Button B: X+27, Y+71
+Prize: X=18641, Y=10279
+";
 
     #[test]
     fn test1() {
-        assert_eq!(part1(TEST_INPUT), 0);
+        assert_eq!(part1(TEST_INPUT), 480);
     }
 
     #[test]
     fn test2() {
-        assert_eq!(part2(TEST_INPUT), 0);
+        assert_eq!(part2(TEST_INPUT), 875318608908);
     }
 }