Naive solution

day-13/src/main.rs

@@ -9,7 +9,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     println!("{}", (arrival - time) * earliest);
 
     // Part 2
-    let timestamp = find_gauss(&ids);
+    let timestamp = find_incrementing(&ids);
     println!("{}", timestamp);
 
     Ok(())
@@ -28,7 +28,7 @@ fn parse_notes(input: &str) -> Result<(u32, Vec<u32>), &str> {
         .next()
         .ok_or("Input too short")?
         .split(',')
-        .map(|id| id.parse().unwrap_or(1))
+        .map(|id| id.parse().unwrap_or(0))
         .collect();
 
     Ok((estimate, ids))
@@ -36,7 +36,7 @@ fn parse_notes(input: &str) -> Result<(u32, Vec<u32>), &str> {
 
 fn find_closest_bus(time: u32, ids: &Vec<u32>) -> (u32, u32) {
     ids.iter()
-        .filter(|id| **id != 1)
+        .filter(|id| **id != 0)
         .map(|id| {
             let mut arrival = *id;
 
@@ -50,39 +50,48 @@ fn find_closest_bus(time: u32, ids: &Vec<u32>) -> (u32, u32) {
         .expect("Must have at least one bus ID")
 }
 
-fn find_gauss(ids: &Vec<u32>) -> u128 {
+fn find_incrementing(ids: &Vec<u32>) -> u128 {
     let ids: Vec<u128> = ids.iter().map(|id| *id as u128).collect();
-    let product: u128 = ids.iter().product();
+    let (largest, increment) = ids
+        .iter()
+        .enumerate()
+        .max_by_key(|id| id.1)
+        .expect("Must have at least one bus ID");
 
-    let gauss = |mut n: u128, mut d: u128, m: u128| -> u128 {
-        while d != 1 {
-            if m == 1 {
-                return 0;
-            };
+    let mut iterations = (14166999999493 - largest as u128) / increment;
 
-            let multiplier = if m > d {
-                m / d + (m % d != 0) as u128
+    loop {
+        iterations += 1;
+        let max = increment * iterations;
+
+        if ids.iter().enumerate().rev().all(|(i, number)| {
+            if *number == 0 {
+                true
             } else {
-                1
-            };
-
-            n = (n * multiplier) % m;
-            d = (d * multiplier) % m;
+                let target = max - largest as u128 + i as u128;
+                target % number == 0
+            }
+        }) {
+            break max - largest as u128;
+        } else {
+            if iterations % 100000000 == 0 {
+                let fun = ids
+                    .iter()
+                    .enumerate()
+                    .filter(|(i, number)| {
+                        if **number == 0 {
+                            true
+                        } else {
+                            let target = max - largest as u128 + *i as u128;
+                            target % **number == 0
                         }
-
-        n / d
-    };
-
-    (0..ids.len())
-        .map(|a| {
-            let b = product / ids[a];
-            let bi = gauss(1, b, ids[a]);
-            let a = (ids[a] * a as u128 - a as u128) % ids[a] as u128;
-
-            a * b * bi
                     })
-        .sum::<u128>()
-        % product
+                    .count();
+
+                println!("{} - {}/{}", max - largest as u128, fun, ids.len());
+            }
+        }
+    }
 }
 
 #[cfg(test)]
@@ -115,7 +124,7 @@ mod tests {
         );
 
         let (time, ids) = parse_notes(input)?;
-        let timestamp = find_gauss(&ids);
+        let timestamp = find_incrementing(&ids);
 
         assert_eq!(timestamp, 3417);
 
@@ -131,7 +140,7 @@ mod tests {
         );
 
         let (time, ids) = parse_notes(input)?;
-        let timestamp = find_gauss(&ids);
+        let timestamp = find_incrementing(&ids);
 
         assert_eq!(timestamp, 1068781);
 
@@ -147,7 +156,7 @@ mod tests {
         );
 
         let (time, ids) = parse_notes(input)?;
-        let timestamp = find_gauss(&ids);
+        let timestamp = find_incrementing(&ids);
 
         assert_eq!(timestamp, 754018);
 
@@ -163,7 +172,7 @@ mod tests {
         );
 
         let (time, ids) = parse_notes(input)?;
-        let timestamp = find_gauss(&ids);
+        let timestamp = find_incrementing(&ids);
 
         assert_eq!(timestamp, 779210);