day 5 solution

src/day05.rs

@@ -0,0 +1,44 @@
+// ==== day 5
+
+use crate::utils;
+use anyhow::Result;
+
+fn seat_code_to_usize(code: &str) -> usize {
+    assert_eq!(code.len(), 10);
+    let bincode = str::replace(code, "B", "1");
+    let bincode = str::replace(&bincode, "F", "0");
+    let bincode = str::replace(&bincode, "R", "1");
+    let bincode = str::replace(&bincode, "L", "0");
+    usize::from_str_radix(&bincode, 2).unwrap()
+}
+
+fn usize_to_seat_id(x: usize) -> usize {
+    ((x >> 3) * 8) + (x & 0b111)
+}
+
+pub fn run() -> Result<(usize, usize)> {
+    let data = utils::read_file("data/input-05.txt")?;
+    let answers = utils::read_i64s("data/output-05.txt")?;
+
+    // compute all seats (as integers)
+    let mut seat_ints: Vec<usize> = data.lines().map(|code| seat_code_to_usize(code)).collect();
+    seat_ints.sort_unstable();
+
+    // find the seat with the highest seat code
+    let p1 = usize_to_seat_id(*seat_ints.last().unwrap());
+
+    // find the empty seat
+    let mut empty = 0;
+    for i in 0..seat_ints.len() - 1 {
+        if seat_ints[i] + 2 == seat_ints[i + 1] {
+            empty = seat_ints[i] + 1;
+            break;
+        }
+    }
+    let p2 = usize_to_seat_id(empty);
+
+    assert_eq!(p1, answers[0] as usize);
+    assert_eq!(p2, answers[1] as usize);
+
+    Ok((p1, p2))
+}