Implement day 18 2021

2021/src/common.rs

@@ -4,6 +4,9 @@ use std::io::Read;
 use std::marker::PhantomData;
 use std::str::FromStr;
 
+use nom::Finish;
+use nom::IResult;
+
 pub struct LineIter<'a> {
     reader: BufReader<&'a mut dyn Read>,
     buffer: String,
@@ -78,3 +81,15 @@ pub fn ordered<O: PartialOrd>(a: O, b: O) -> (O, O) {
         (b, a)
     }
 }
+
+pub fn read_input<P, O>(input: &mut dyn Read, parser: P) -> O
+where
+    P: for<'a> FnOnce(&'a [u8]) -> IResult<&'a [u8], O>,
+{
+    let mut buffer = Vec::new();
+    input.read_to_end(&mut buffer).unwrap();
+
+    let (_, output) = parser(&buffer).finish().unwrap();
+
+    output
+}

2021/src/day18.rs

@@ -1,9 +1,245 @@
+use std::fmt::Debug;
 use std::io::Read;
+use std::mem::replace;
 
-pub fn part1(_input: &mut dyn Read) -> String {
-    todo!()
+use nom::branch::alt;
+use nom::character::complete::newline;
+use nom::combinator::map;
+use nom::multi::many0;
+use nom::sequence::delimited;
+use nom::sequence::separated_pair;
+use nom::sequence::terminated;
+use nom::IResult;
+
+use crate::common::read_input;
+
+#[derive(Clone, Eq, PartialEq)]
+enum TurtleNumber {
+    Literal(u8),
+    Pair(Box<(TurtleNumber, TurtleNumber)>),
 }
 
-pub fn part2(_input: &mut dyn Read) -> String {
-    todo!()
+impl TurtleNumber {
+    pub fn add(self, other: Self) -> Self {
+        let mut new = TurtleNumber::Pair(Box::new((self, other)));
+        new.reduce();
+
+        new
+    }
+
+    pub fn magnitude(&self) -> u32 {
+        match self {
+            TurtleNumber::Literal(num) => *num as u32,
+            TurtleNumber::Pair(pair) => 3 * pair.0.magnitude() + 2 * pair.1.magnitude(),
+        }
+    }
+
+    fn reduce(&mut self) {
+        loop {
+            while self.try_explode(0).is_some() {}
+
+            if self.split() {
+                continue;
+            } else {
+                break;
+            }
+        }
+    }
+
+    fn leftmost_add(&mut self, value: u8) {
+        match self {
+            TurtleNumber::Literal(num) => *num += value,
+            TurtleNumber::Pair(pair) => pair.0.leftmost_add(value),
+        }
+    }
+
+    fn rightmost_add(&mut self, value: u8) {
+        match self {
+            TurtleNumber::Literal(num) => *num += value,
+            TurtleNumber::Pair(pair) => pair.1.rightmost_add(value),
+        }
+    }
+
+    fn try_explode(&mut self, depth: usize) -> Option<(Option<u8>, Option<u8>)> {
+        let pair = match self {
+            TurtleNumber::Literal(_) => return None,
+            TurtleNumber::Pair(pair) => pair,
+        };
+
+        if depth == 4 {
+            let original = replace(self, TurtleNumber::Literal(0));
+            let pair = match original {
+                TurtleNumber::Pair(pair) => *pair,
+                _ => unreachable!("Already checked for pair above"),
+            };
+
+            if let (TurtleNumber::Literal(first), TurtleNumber::Literal(second)) = pair {
+                Some((Some(first), Some(second)))
+            } else {
+                panic!("Too deeply nested turtle number: {:?}", pair);
+            }
+        } else {
+            match pair.0.try_explode(depth + 1) {
+                Some((left, Some(right))) => {
+                    pair.1.leftmost_add(right);
+                    Some((left, None))
+                }
+                Some((left, None)) => Some((left, None)),
+                None => match pair.1.try_explode(depth + 1) {
+                    Some((Some(left), right)) => {
+                        pair.0.rightmost_add(left);
+                        Some((None, right))
+                    }
+                    other => other,
+                },
+            }
+        }
+    }
+
+    pub fn split(&mut self) -> bool {
+        match self {
+            TurtleNumber::Literal(num) if *num >= 10 => {
+                let half = *num / 2;
+                let other = *num - half;
+                *self = TurtleNumber::from((half, other));
+                true
+            }
+            TurtleNumber::Pair(pair) => pair.0.split() || pair.1.split(),
+            _ => false,
+        }
+    }
+}
+
+impl Debug for TurtleNumber {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Literal(num) => write!(f, "{}", num),
+            Self::Pair(pair) => write!(f, "[{:?},{:?}]", pair.0, pair.1),
+        }
+    }
+}
+
+// Helper traits to easily convert tuples to turtle numbers
+impl From<u8> for TurtleNumber {
+    fn from(num: u8) -> Self {
+        TurtleNumber::Literal(num)
+    }
+}
+
+impl<T, U> From<(T, U)> for TurtleNumber
+where
+    T: Into<TurtleNumber>,
+    U: Into<TurtleNumber>,
+{
+    fn from((left, right): (T, U)) -> Self {
+        TurtleNumber::Pair(Box::new((left.into(), right.into())))
+    }
+}
+
+fn parse_turtle(input: &[u8]) -> IResult<&[u8], TurtleNumber> {
+    use nom::character::complete::char;
+    use nom::character::complete::u8;
+
+    alt((
+        map(u8, TurtleNumber::Literal),
+        map(
+            delimited(
+                char('['),
+                separated_pair(parse_turtle, char(','), parse_turtle),
+                char(']'),
+            ),
+            |pair| TurtleNumber::Pair(Box::new(pair)),
+        ),
+    ))(input)
+}
+
+fn parse_input(input: &[u8]) -> IResult<&[u8], Vec<TurtleNumber>> {
+    many0(terminated(parse_turtle, newline))(input)
+}
+
+pub fn part1(input: &mut dyn Read) -> String {
+    let turtles = read_input(input, parse_input);
+    turtles
+        .into_iter()
+        .reduce(|a, b| a.add(b))
+        .map(|num| num.magnitude())
+        .unwrap()
+        .to_string()
+}
+
+pub fn part2(input: &mut dyn Read) -> String {
+    let turtles = read_input(input, parse_input);
+
+    turtles
+        .iter()
+        .flat_map(|a| turtles.iter().map(|b| a.clone().add(b.clone()).magnitude()))
+        .max()
+        .unwrap()
+        .to_string()
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::test_implementation;
+
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/18.txt");
+
+    #[test]
+    fn test_magnitude() {
+        let num: TurtleNumber = (
+            (((8, 7), (7, 7)), ((8, 6), (7, 7))),
+            (((0, 7), (6, 6)), (8, 7)),
+        )
+            .into();
+
+        assert_eq!(num.magnitude(), 3488);
+    }
+
+    #[test]
+    fn test_add() {
+        let input = TurtleNumber::from(((((4, 3), 4), 4), (7, ((8, 4), 9))));
+        let result = input.add((1, 1).into());
+
+        let expected = TurtleNumber::from(((((0, 7), 4), ((7, 8), (6, 0))), (8, 1)));
+        assert_eq!(result, expected);
+    }
+
+    #[test]
+    fn test_explode() {
+        let mut input1 = TurtleNumber::from((((((9, 8), 1), 2), 3), 4));
+        let output1 = TurtleNumber::from(((((0, 9), 2), 3), 4));
+
+        input1.reduce();
+        assert_eq!(input1, output1);
+
+        let mut input2 = TurtleNumber::from((7, (6, (5, (4, (3, 2))))));
+        let output2 = TurtleNumber::from((7, (6, (5, (7, 0)))));
+
+        input2.reduce();
+        assert_eq!(input2, output2);
+
+        let mut input3: TurtleNumber = TurtleNumber::from(((6, (5, (4, (3, 2)))), 1));
+        let output3 = TurtleNumber::from(((6, (5, (7, 0))), 3));
+
+        input3.reduce();
+        assert_eq!(input3, output3);
+
+        let mut input4 = TurtleNumber::from(((3, (2, (1, (7, 3)))), (6, (5, (4, (3, 2))))));
+        let output4 = TurtleNumber::from(((3, (2, (8, 0))), (9, (5, (7, 0)))));
+
+        input4.reduce();
+        assert_eq!(input4, output4);
+    }
+
+    #[test]
+    fn sample_part1() {
+        test_implementation(part1, SAMPLE, 4140);
+    }
+
+    #[test]
+    fn sample_part2() {
+        test_implementation(part2, SAMPLE, 3993);
+    }
 }

2021/src/samples/18.txt

@@ -0,0 +1,10 @@
+[[[0,[5,8]],[[1,7],[9,6]]],[[4,[1,2]],[[1,4],2]]]
+[[[5,[2,8]],4],[5,[[9,9],0]]]
+[6,[[[6,2],[5,6]],[[7,6],[4,7]]]]
+[[[6,[0,7]],[0,9]],[4,[9,[9,0]]]]
+[[[7,[6,4]],[3,[1,3]]],[[[5,5],1],9]]
+[[6,[[7,3],[3,2]]],[[[3,8],[5,7]],4]]
+[[[[5,4],[7,7]],8],[[8,3],8]]
+[[9,3],[[9,9],[6,[4,9]]]]
+[[2,[[7,7],7]],[[5,8],[[9,3],[0,2]]]]
+[[[[5,2],5],[8,[3,7]]],[[5,[7,5]],[4,4]]]