Complete day 5.1

day-5/src/main.rs

@@ -0,0 +1,94 @@
+use std::fs;
+
+type SeatCode = [char; 10];
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let input = fs::read_to_string("input")?;
+
+    // Part 1
+    let max_id: Result<usize, String> = input
+        .lines()
+        .map(|code| {
+            let (row, col) = find_seat(parse_seatcode(code)?);
+            Ok(row * 8 + col)
+        })
+        .max()
+        .ok_or("No seat codes set")?;
+
+    println!("{}", max_id?);
+
+    Ok(())
+}
+
+fn parse_seatcode(input: &str) -> Result<SeatCode, String> {
+    let mut output = ['0'; 10];
+
+    for (i, c) in input.chars().enumerate() {
+        if i > 10 {
+            Err(format!("Seat code too long: '{}'", input))?;
+        }
+
+        if !['F', 'B', 'L', 'R'].iter().any(|valid| &c != valid) {
+            Err(format!("Invalid character in seat code: '{}'", input))?;
+        }
+
+        output[i] = c;
+    }
+
+    Ok(output)
+}
+
+fn find_seat(code: SeatCode) -> (usize, usize) {
+    let midpoint = |(start, end): (usize, usize)| (start + end) / 2;
+    let binary_follow = |range: (usize, usize), direction: &char| match direction {
+        'F' | 'L' => (range.0, midpoint(range)),
+        'B' | 'R' => (midpoint(range) + 1, range.1),
+        _ => unreachable!("We already checked all characters are valid"),
+    };
+
+    let row = code[..7].iter().fold((0, 127), binary_follow).0;
+    let col = code[7..].iter().fold((0, 7), binary_follow).0;
+
+    (row, col)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_simple() -> Result<(), Box<dyn std::error::Error>> {
+        let input = "FBFBBFFRLR";
+        let seat = find_seat(parse_seatcode(input)?);
+
+        assert_eq!(seat, (44, 5));
+        Ok(())
+    }
+
+    #[test]
+    fn test_simple2() -> Result<(), Box<dyn std::error::Error>> {
+        let input = "BFFFBBFRRR";
+        let seat = find_seat(parse_seatcode(input)?);
+
+        assert_eq!(seat, (70, 7));
+        Ok(())
+    }
+
+    #[test]
+    fn test_simple3() -> Result<(), Box<dyn std::error::Error>> {
+        let input = "FFFBBBFRRR";
+        let seat = find_seat(parse_seatcode(input)?);
+
+        assert_eq!(seat, (14, 7));
+        Ok(())
+    }
+
+    #[test]
+    fn test_simple4() -> Result<(), Box<dyn std::error::Error>> {
+        let input = "BBFFBBFRLL";
+        let seat = find_seat(parse_seatcode(input)?);
+
+        assert_eq!(seat, (102, 4));
+        Ok(())
+    }
+}