From 65dfc7c940d6fe1d1fa29648e21107a32f5f7e16 Mon Sep 17 00:00:00 2001
From: Reiner Herrmann <reiner@reiner-h.de>
Date: Sat, 7 Dec 2024 21:16:46 +0100
Subject: day7 solution 2

---
 src/bin/day7.rs | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

(limited to 'src')

diff --git a/src/bin/day7.rs b/src/bin/day7.rs
index 60b08b1..934e0d9 100644
--- a/src/bin/day7.rs
+++ b/src/bin/day7.rs
@@ -2,26 +2,38 @@ static DAY: u8 = 7;
 
 fn main() {
     let input = advent::read_lines(DAY);
-    println!("{DAY}a: {}", calibration_result(&input));
-    println!("{DAY}b: {}", 0);
+    println!("{DAY}a: {}", calibration_result(&input, false));
+    println!("{DAY}b: {}", calibration_result(&input, true));
 }
 
-fn calculateable(result: i64, operands: &[i64]) -> bool {
+fn calculateable(result: i64, operands: &[i64], with_concat: bool) -> bool {
     if operands.len() == 1 {
         return result == operands[0];
     }
     let rem = &operands[..=operands.len()-2];
     let op = operands[operands.len()-1];
-    calculateable(result - op, rem) || (result % op == 0 && calculateable(result / op, rem))
+    if with_concat {
+        let result_str = result.to_string();
+        let op_str = op.to_string();
+        if result_str.ends_with(&op_str) && result_str.len() > op_str.len() {
+            let result_str = result_str.chars().take(result_str.len() - op_str.len()).collect::<String>();
+            let result = result_str.parse::<i64>().unwrap();
+            if calculateable(result, rem, with_concat) {
+                return true;
+            }
+        }
+    }
+    ((result - op >= 0) && calculateable(result - op, rem, with_concat))
+    || (result % op == 0 && calculateable(result / op, rem, with_concat))
 }
 
-fn calibration_result(input: &[String]) -> i64 {
+fn calibration_result(input: &[String], with_concat: bool) -> i64 {
     let mut sum = 0;
     for line in input {
         let (result, operands) = line.split_once(": ").unwrap();
         let result = result.parse::<i64>().unwrap();
         let operands = operands.split(" ").map(|x| x.parse::<i64>().unwrap()).collect::<Vec<_>>();
-        if calculateable(result, &operands) {
+        if calculateable(result, &operands, with_concat) {
             sum += result;
         }
     }
@@ -45,6 +57,7 @@ mod tests {
             "21037: 9 7 18 13",
             "292: 11 6 16 20",
         ].iter().map(|&x| String::from(x)).collect::<Vec<_>>();
-        assert_eq!(calibration_result(&input), 3749);
+        assert_eq!(calibration_result(&input, false), 3749);
+        assert_eq!(calibration_result(&input, true), 11387);
     }
 }
-- 
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