Solution for day 13

day13-hard/Cargo.toml

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+[package]
+name = "day13-hard"
+version = "0.1.0"
+authors = ["inga-lovinde <52715130+inga-lovinde@users.noreply.github.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

day13-hard/src/main.rs

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+use std::cmp::Reverse;
+use std::io::{self, BufRead};
+
+#[derive(Clone, Copy, Debug)]
+struct Condition {
+    pub base: u128,
+    pub remainder: u128,
+}
+
+impl Condition {
+    fn new(base: u128, remainder: u128) -> Condition {
+        Condition {
+            base,
+            remainder,
+        }
+    }
+}
+
+impl Default for Condition {
+    fn default() -> Self {
+        Self {
+            base: 1,
+            remainder: 0,
+        }
+    }
+}
+
+// chinese remainder theorem, sieve method
+// assuming all bases are coprime
+fn solve(current: Condition, remaining_conditions: &[Condition]) -> Option<Condition> {
+    if remaining_conditions.len() == 0 {
+        return Some(current);
+    }
+
+    let first_condition = remaining_conditions[0];
+    for i in 0..first_condition.base {
+        if (i * current.base + current.remainder) % first_condition.base == first_condition.remainder {
+            return solve(Condition::new(current.base * first_condition.base, i * current.base + current.remainder), &remaining_conditions[1..]);
+        }
+    }
+
+    return None;
+}
+
+fn main() {
+    let stdin = io::stdin();
+    let mut stdin_lines = stdin.lock().lines();
+    let _timestamp: u128 = stdin_lines.next().unwrap().unwrap().parse().unwrap();
+    let mut conditions: Vec<_> = stdin_lines.next().unwrap().unwrap().split(",")
+        .enumerate()
+        .filter(|(_index, entry)| *entry != "x")
+        .map(|(index, entry)| (entry.parse::<u128>().unwrap(), index))
+        .map(|(base, index)| Condition::new(base, (-(index as i128)).rem_euclid(base as i128) as u128))
+        .collect();
+
+    conditions.sort_by_key(|condition| Reverse(condition.base));
+
+    for condition in conditions.iter() {
+        println!("{:?}", condition);
+    }
+
+    let solution = solve(Condition::default(), &conditions).unwrap();
+    println!("{:?}", solution);
+}

day13/Cargo.toml

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+[package]
+name = "day13"
+version = "0.1.0"
+authors = ["inga-lovinde <52715130+inga-lovinde@users.noreply.github.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

day13/src/main.rs

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+use std::io::{self, BufRead};
+
+fn get_wait_time(timestamp: u64, bus_number: u64) -> u64 {
+    let remainder = timestamp % bus_number;
+    if remainder == 0 {
+        return 0;
+    }
+
+    bus_number - remainder
+}
+
+fn main() {
+    let stdin = io::stdin();
+    let mut stdin_lines = stdin.lock().lines();
+    let timestamp: u64 = stdin_lines.next().unwrap().unwrap().parse().unwrap();
+    let earliest_bus_number = stdin_lines.next().unwrap().unwrap().split(",")
+        .filter(|&entry| entry != "x")
+        .map(|entry| entry.parse().unwrap())
+        .min_by_key(|&bus_number| get_wait_time(timestamp, bus_number))
+        .unwrap();
+
+    println!("{}", earliest_bus_number * get_wait_time(timestamp, earliest_bus_number));
+}