1 files changed, 74 insertions, 0 deletions
diff --git a/src/day_04.rs b/src/day_04.rs
new file mode 100644
index 0000000..95dd95c
--- /dev/null
+++ b/src/day_04.rs
@@ -0,0 +1,74 @@
+use std::ops::Range;
+
+use anyhow::Result;
+
+use crate::{Problem, Solution};
+
+pub fn parse_range(section: &str) -> Result<Range<usize>> {
+    let (start, end) = section.split_once('-').unwrap();
+    let start = start.parse::<usize>()?;
+    let end = end.parse::<usize>()?;
+    Ok(Range { start, end })
+}
+
+fn parse_pair(input: &str) -> Result<(Range<usize>, Range<usize>)> {
+    let (section_a, section_b) = input.split_once(',').unwrap();
+
+    Ok((parse_range(section_a)?, parse_range(section_b)?))
+}
+
+fn range_is_subset((a, b): &(Range<usize>, Range<usize>)) -> bool {
+    (a.start <= b.start && a.end >= b.end) || (b.start <= a.start && b.end >= a.end)
+}
+
+fn range_is_union((a, b): &(Range<usize>, Range<usize>)) -> bool {
+    (a.start <= b.start && a.end >= b.start) || (b.start <= a.start && b.end >= a.start)
+}
+
+pub struct Day04;
+
+impl Problem for Day04 {
+    const DAY: u8 = 4;
+
+    const INPUT: &'static str = include_str!("../input/day_4.txt");
+}
+
+impl Solution for Day04 {
+    type Answer1 = usize;
+
+    type Answer2 = usize;
+
+    fn part_1(input: &str) -> Result<Self::Answer1, anyhow::Error> {
+        let ranges: Vec<_> = input.lines().map(parse_pair).collect::<Result<_>>()?;
+        Ok(ranges.into_iter().filter(range_is_subset).count())
+    }
+
+    fn part_2(input: &str) -> Result<Self::Answer2, anyhow::Error> {
+        let ranges: Vec<_> = input.lines().map(parse_pair).collect::<Result<_>>()?;
+        Ok(ranges.into_iter().filter(range_is_union).count())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const TEST_INPUT: &str = r#"2-4,6-8
+2-3,4-5
+5-7,7-9
+2-8,3-7
+6-6,4-6
+2-6,4-8"#;
+
+    #[test]
+    fn test_part_1_example() -> Result<()> {
+        assert_eq!(2, Day04::part_1(TEST_INPUT)?);
+        Ok(())
+    }
+
+    #[test]
+    fn test_part_2_example() -> Result<()> {
+        assert_eq!(4, Day04::part_2(TEST_INPUT)?);
+        Ok(())
+    }
+}