3 files changed, 113 insertions, 1 deletions
diff --git a/2022/rust/Cargo.toml b/2022/rust/Cargo.toml
index b2641a6..b7ab11d 100644
--- a/2022/rust/Cargo.toml
+++ b/2022/rust/Cargo.toml
@@ -3,7 +3,7 @@ members = [
      "day01",
      "day02",
      "day03",
-#     "day04",
+     "day04",
 #     "day05",
 #     "day06",
 #     "day07",
diff --git a/2022/rust/day04/Cargo.toml b/2022/rust/day04/Cargo.toml
new file mode 100644
index 0000000..b418e99
--- /dev/null
+++ b/2022/rust/day04/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "day04"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+aoc = { path = "../../../common/rust/aoc" }
+anyhow = "1.0"
+regex = "1.7.0"
diff --git a/2022/rust/day04/src/main.rs b/2022/rust/day04/src/main.rs
new file mode 100644
index 0000000..d32f1a3
--- /dev/null
+++ b/2022/rust/day04/src/main.rs
@@ -0,0 +1,103 @@
+use {
+    anyhow::{
+        anyhow,
+        Context,
+        Result,
+    },
+    regex::Regex,
+    std::{
+        env::args,
+        fmt,
+        fs::File,
+        io::{
+            BufRead,
+            BufReader,
+        },
+        path::Path,
+    },
+};
+
+struct SectionAssignment {
+    start: usize,
+    end: usize,
+}
+
+impl SectionAssignment {
+    fn new(start: usize, end: usize) -> Self {
+        Self {
+            start,
+            end,
+        }
+    }
+
+    fn contains(&self, other: &SectionAssignment) -> bool {
+        self.start <= other.start && self.end >= other.end
+    }
+
+    fn overlaps(&self, other: &SectionAssignment) -> bool {
+        (self.start >= other.start && self.start <= other.end) ||
+        (self.end >= other.start && self.end <= other.end) ||
+        (other.start >= self.start && other.start <= self.end) ||
+        (other.end >= self.start && other.end <= self.end)
+    }
+}
+
+impl fmt::Debug for SectionAssignment {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (0..=9).map(|i| if i >= self.start && i <= self.end {
+            write!(f, "{i}")
+        } else {
+            write!(f, ".")
+        }).last();
+        write!(f, " {}-{}", self.start, self.end)?;
+        Ok(())
+    }
+}
+
+type AssignmentPair = [SectionAssignment; 2];
+
+fn read_input<T: AsRef<Path>>(filename: T) -> Result<Vec<AssignmentPair>> {
+    let reader = BufReader::new(File::open(filename)?);
+    let re = Regex::new(r"(?P<s0>[0-9]+)-(?P<e0>[0-9]+),(?P<s1>[0-9]+)-(?P<e1>[0-9]+)")
+        .context("Regex compilation failed.")?;
+
+    reader.lines().map(
+        |v| {
+            let s = v?;
+            let caps = re.captures(&s).ok_or_else(|| anyhow!("Regex matching failed."))?;
+            match (caps.name("s0"), caps.name("e0"), caps.name("s1"), caps.name("e1")) {
+                ( Some(s0), Some(e0), Some(s1), Some(e1) ) => Ok([
+                    SectionAssignment::new(s0.as_str().parse()?, e0.as_str().parse()?),
+                    SectionAssignment::new(s1.as_str().parse()?, e1.as_str().parse()?),
+                ]),
+                _ => Err(anyhow!("Parse error.")),
+            }
+        }
+    ).collect()
+}
+
+fn part1(input: &[AssignmentPair]) -> Result<usize> {
+    Ok(input.iter().filter(|pair|
+        pair[0].contains(&pair[1]) ||
+        pair[1].contains(&pair[0])).count())
+}
+
+fn part2(input: &[AssignmentPair]) -> Result<usize> {
+    Ok(input.iter().filter(|pair| pair[0].overlaps(&pair[1])).count())
+}
+
+fn main() -> Result<()> {
+    let ( do_part_1, do_part_2 ) = aoc::do_parts();
+
+    let filename = args().nth(1).ok_or_else(|| anyhow!("Missing input filename"))?;
+    let input = read_input(filename).context("Could not read input")?;
+    if do_part_1 {
+        let solution = part1(&input).context("No solution for part 1")?;
+        println!("Part1, solution found to be: {}", solution);
+    }
+    if do_part_2 {
+        let solution = part2(&input).context("No solution for part 2")?;
+        println!("Part2, solution found to be: {}", solution);
+    }
+    Ok(())
+}