1 files changed, 135 insertions, 0 deletions
diff --git a/2020/rust/day05/src/main.rs b/2020/rust/day05/src/main.rs
new file mode 100644
index 0000000..4d6072a
--- /dev/null
+++ b/2020/rust/day05/src/main.rs
@@ -0,0 +1,135 @@
+use anyhow::{anyhow, Result};
+use std::env::args;
+use std::fs::File;
+use std::io::{BufRead, BufReader};
+use std::ops::RangeInclusive;
+use std::path::Path;
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+enum BinarySpaceError {
+    #[error("Space is not fully reduced")]
+    NotReduced,
+}
+
+struct BinarySpace {
+    start: usize,
+    end: usize,
+}
+
+impl BinarySpace {
+    fn new(range: RangeInclusive<usize>) -> BinarySpace
+    // FIXME It would be nice with a cleaner interface. The intention was to use something like the
+    //       RangeBounds trait, but it seems to require support of Unbounded ranges, which this
+    //       BinarySpace will not be able to work with.
+    {
+        BinarySpace {
+            start: *range.start(),
+            end: *range.end(),
+        }
+    }
+
+    fn size(&self) -> usize {
+        self.end - self.start
+    }
+
+    fn split(&mut self, op: char) {
+        let size = self.size() + 1;
+        let half = size / 2;
+        match op {
+            'F' | 'L' => self.end -= half,
+            'B' | 'R' => self.start += half,
+            c => panic!("Invalid op to BinarySpace.split(): {}", c),
+        }
+    }
+
+    fn value(&self) -> std::result::Result<usize, BinarySpaceError> {
+        if self.start == self.end {
+            Ok(self.start)
+        } else {
+            Err(BinarySpaceError::NotReduced)
+        }
+    }
+}
+
+fn read_input<T: AsRef<Path>>(filename: T) -> std::io::Result<Vec<String>> {
+    let f = File::open(filename)?;
+    let reader = BufReader::new(f);
+
+    reader.lines().collect()
+}
+
+fn parse_boarding_passes(input: &[String]) -> Result<Vec<usize>> {
+    input.iter().map(|ops| {
+        let mut row = BinarySpace::new(0..=127);
+        let mut col = BinarySpace::new(0..=7);
+        for op in ops.clone().drain(..) {
+            match op {
+                'F' | 'B' => row.split(op),
+                'L' | 'R' => col.split(op),
+                _ => return Err(anyhow!("Invalid op encountered")),
+            }
+        }
+
+        Ok(row.value()? * 8 + col.value()?)
+    }).collect()
+}
+
+fn part1(input: &[usize]) -> Option<usize> {
+    input.iter().max().copied()
+}
+
+fn part2(input: &[usize]) -> Option<usize> {
+    let mut ret = None;
+
+    for seat in 1..(128*8) {
+        if (input.contains(&seat),
+            input.contains(&(seat-1)),
+            input.contains(&(seat+1))
+        ) == (false, true, true) {
+            match ret {
+                None => ret = Some(seat),
+                Some(_) => return None, /* No more than one solution allowed */
+            }
+        }
+    }
+
+    ret
+}
+
+fn main() {
+    let (do_part_1, do_part_2) = aoc::do_parts();
+
+    let filename = match args().nth(1) {
+        Some(f) => f,
+        None => {
+            eprintln!("Missing input filename");
+            std::process::exit(1);
+        },
+    };
+    match read_input(filename) {
+        Ok(input) => {
+            match parse_boarding_passes(&input) {
+                Ok(seat_ids) => {
+                    if do_part_1 {
+                        match part1(&seat_ids) {
+                            Some(solution) =>
+                                println!("Part1, highest seat ID found to be: {}", solution),
+                            None => eprintln!("Part1, no soluton found"),
+                        }
+                    }
+                    if do_part_2 {
+                        match part2(&seat_ids) {
+                            Some(solution) =>
+                                println!("Part2, seat {} is lacking a boarding pass", solution),
+                            None => eprintln!("Part2, no soluton found"),
+                        }
+                    }
+                },
+                Err(err) => eprintln!("Could not parse input: {}", err),
+            }
+
+        },
+        Err(err) => eprintln!("Could not read input: {}", err),
+    }
+}