1 files changed, 92 insertions, 0 deletions
diff --git a/2023/src/bin/day_4.rs b/2023/src/bin/day_4.rs
new file mode 100644
index 0000000..7f5af6b
--- /dev/null
+++ b/2023/src/bin/day_4.rs
@@ -0,0 +1,92 @@
+use nom::{
+    bytes::complete::tag,
+    character::complete::{line_ending, space1, u32 as nom_u32},
+    combinator::map,
+    multi::separated_list1,
+    sequence::tuple,
+    IResult,
+};
+use std::{collections::BTreeSet, fs};
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let input = fs::read_to_string("inputs/day_4.txt")?;
+    let parsed = Scratchcards::parser(&input).unwrap().1;
+    dbg!(&parsed.points());
+    dbg!(&parsed.scratchcard_explosion_sum());
+
+    Ok(())
+}
+
+#[derive(Debug)]
+struct Scratchcards(Vec<Scratchcard>);
+
+#[derive(Debug)]
+struct Scratchcard {
+    winning: BTreeSet<u32>,
+    owned: BTreeSet<u32>,
+}
+
+impl Scratchcards {
+    fn parser(input: &str) -> IResult<&str, Self> {
+        map(
+            separated_list1(line_ending, Scratchcard::parser),
+            Scratchcards,
+        )(input)
+    }
+
+    fn points(&self) -> usize {
+        self.0.iter().map(|card| card.points()).sum()
+    }
+
+    fn scratchcard_explosion_sum(&self) -> u64 {
+        let mut scratchcard_pile = vec![1_u64.into(); self.0.len()];
+
+        for i in 0..scratchcard_pile.len() {
+            let points = self.0[i].winning_numbers();
+            let new_scratchcards: u64 = scratchcard_pile[i];
+            for offset in 0..points {
+                if let Some(pile) = scratchcard_pile.get_mut(i + 1 + offset) {
+                    *pile += new_scratchcards;
+                }
+            }
+        }
+
+        scratchcard_pile.into_iter().sum()
+    }
+}
+
+impl Scratchcard {
+    fn parser(input: &str) -> IResult<&str, Self> {
+        map(
+            tuple((
+                tag("Card"),
+                space1,
+                nom_u32,
+                tag(":"),
+                space1,
+                separated_list1(space1, nom_u32),
+                space1,
+                tag("|"),
+                space1,
+                separated_list1(space1, nom_u32),
+            )),
+            |(_, _, _, _, _, winning, _, _, _, owned)| Scratchcard {
+                winning: winning.into_iter().collect(),
+                owned: owned.into_iter().collect(),
+            },
+        )(input)
+    }
+
+    fn winning_numbers(&self) -> usize {
+        self.winning.intersection(&self.owned).count()
+    }
+
+    fn points(&self) -> usize {
+        let winning_numbers = self.winning_numbers();
+        if winning_numbers > 0 {
+            2_usize.pow(winning_numbers as u32 - 1)
+        } else {
+            0
+        }
+    }
+}