Day 13. Stupid math.

inputs/day13.txt

@@ -0,0 +1,2 @@
+1007153
+29,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,433,x,x,x,x,x,x,x,x,x,x,x,x,13,17,x,x,x,x,19,x,x,x,23,x,x,x,x,x,x,x,977,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,41

inputs/day13_test.txt

@@ -0,0 +1,2 @@
+939
+7,13,x,x,59,x,31,19

src/bin/bus.rs

@@ -0,0 +1,101 @@
+use advent2020::errors::TopLevelError;
+use std::env;
+use std::fs;
+use std::str::FromStr;
+
+fn find_pattern(busses: &[(usize, usize)]) -> Result<usize, TopLevelError> {
+    if busses.is_empty() {
+        return Err(TopLevelError::NoInputFound);
+    }
+
+    let mut t = 1;
+    let mut increment = 1;
+
+    for (offset, factor) in busses {
+        while (t + offset) % factor != 0 {
+            t += increment;
+        }
+        increment = lcm(increment, *factor);
+    }
+
+    Ok(t)
+}
+
+fn lcm(x: usize, y: usize) -> usize {
+    let mut k = x;
+
+    while k % y != 0 {
+        k += x;
+    }
+
+    k
+}
+
+#[test]
+fn lcm_test() {
+    assert_eq!(9, lcm(3, 9));
+    assert_eq!(12, lcm(3, 4));
+    assert_eq!(60, lcm(15, 20));
+    assert_eq!(7, lcm(1, 7));
+    assert_eq!(7, lcm(7, 1));
+}
+
+#[test]
+fn pattern_examples() {
+    assert_eq!(
+        1068781,
+        find_pattern(&[(0, 7), (1, 13), (4, 59), (6, 31), (7, 19)]).unwrap()
+    );
+    assert_eq!(3417, find_pattern(&[(0, 17), (2, 13), (3, 19)]).unwrap());
+    assert_eq!(
+        754018,
+        find_pattern(&[(0, 67), (1, 7), (2, 59), (3, 61)]).unwrap()
+    );
+    assert_eq!(
+        779210,
+        find_pattern(&[(0, 67), (2, 7), (3, 59), (4, 61)]).unwrap()
+    );
+    assert_eq!(
+        1261476,
+        find_pattern(&[(0, 67), (1, 7), (3, 59), (4, 61)]).unwrap()
+    );
+    assert_eq!(
+        1202161486,
+        find_pattern(&[(0, 1789), (1, 37), (2, 47), (3, 1889)]).unwrap()
+    );
+}
+
+fn main() -> Result<(), TopLevelError> {
+    let filename = env::args().nth(1).expect("No file argument given.");
+    let contents = fs::read_to_string(filename)?;
+    let mut lines = contents.lines();
+    let start_time = usize::from_str(lines.next().ok_or(TopLevelError::NoInputFound)?)?;
+    let bus_notes = lines.next().ok_or(TopLevelError::NoInputFound)?;
+    let mut busses = Vec::new();
+
+    for (idx, bus) in bus_notes.split(',').enumerate() {
+        match usize::from_str(bus) {
+            Err(_) if bus == "x" => {}
+            Err(e) => return Err(TopLevelError::NumConversionError(e)),
+            Ok(x) => busses.push((idx, x)),
+        }
+    }
+
+    let (next_bus, when) = busses
+        .iter()
+        .map(|(_, x)| (x, x - (start_time % x)))
+        .min_by(|(_, a), (_, b)| a.cmp(b))
+        .ok_or(TopLevelError::NoSolutionFound)?;
+    println!(
+        "The next #{} bus is in {} minutes. [{}]",
+        next_bus,
+        when,
+        next_bus * when
+    );
+    println!(
+        "The pattern you're looking for starts at {}",
+        find_pattern(&busses)?
+    );
+
+    Ok(())
+}