From 7efb8f46e92303e63f9472ab09d65ed522717689 Mon Sep 17 00:00:00 2001
From: Justin Wernick <j.wernick@eyeo.com>
Date: Mon, 4 Dec 2023 10:47:16 +0200
Subject: Day 4

---
 2023/src/bin/day_4.rs | 91 ++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 82 insertions(+), 9 deletions(-)

(limited to '2023/src')

diff --git a/2023/src/bin/day_4.rs b/2023/src/bin/day_4.rs
index b3a610b..7f5af6b 100644
--- a/2023/src/bin/day_4.rs
+++ b/2023/src/bin/day_4.rs
@@ -1,19 +1,92 @@
-use nom::IResult;
-use std::fs;
+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_2.txt")?;
-    let parsed = Example::parser(&input).unwrap().1;
-    dbg!(&parsed);
+    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 Example;
+struct Scratchcards(Vec<Scratchcard>);
 
-impl Example {
-    fn parser(_input: &str) -> IResult<&str, Self> {
-        todo!()
+#[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
+        }
     }
 }
-- 
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