Work a bit on Day 11

Author: ochsnerd

src/day11.rs

@@ -1,12 +1,23 @@
-use std::{collections::HashMap, fs, hash::Hash, ops::AddAssign};
+use std::{collections::HashMap, fs, hash::Hash, num::ParseIntError, ops::AddAssign, str::FromStr};
 
 pub fn day11(input_path: String) {
     let content = fs::read_to_string(input_path).unwrap();
 
-    let stones: Vec<_> = content.split_whitespace().map(Stone::parse).collect();
+    let stones: Vec<_> = content
+        .split_whitespace()
+        .map(|l| l.parse().unwrap())
+        .collect();
 
     println!("{:?}", part1(stones.iter().cloned().collect()));
-    println!("{:?}", part2(stones));
+    println!("{:?}", part2(&stones));
+
+    let stones: Vec<_> = content
+        .split_whitespace()
+        .map(|l| l.parse().unwrap()) // this deduces Stone2 because of the signature of part12
+        .collect();
+
+    println!("{:?}", part12(&stones));
+    println!("{:?}", part22(&stones));
 }
 
 // Most significant digit is last,
@@ -24,12 +35,12 @@ fn part1(stones: Vec<Stone>) -> usize {
     .len()
 }
 
-fn part2(stones: Vec<Stone>) -> usize {
-    let mut stone_map: HashMap<Stone, usize> =
-        accumulate_into_hashmap(stones.into_iter().map(|s| (s, 1)));
+fn part2(stones: &[Stone]) -> usize {
+    // order does not matter, so we can compress the stones into a hashmap, and do every operation just once
+    let mut stone_map: HashMap<Stone, usize> = accumulate(stones.iter().map(|s| (s.clone(), 1)));
 
     for _ in 0..75 {
-        stone_map = accumulate_into_hashmap(
+        stone_map = accumulate(
             stone_map
                 .into_iter()
                 .flat_map(|(stone, count)| blink_at(stone).into_iter().map(move |s| (s, count))),
@@ -38,20 +49,6 @@ fn part2(stones: Vec<Stone>) -> usize {
     stone_map.iter().map(|(_, count)| count).sum()
 }
 
-fn accumulate_into_hashmap<T, N, I>(values: I) -> HashMap<T, N>
-where
-    T: Eq + Hash,
-    N: Default + AddAssign,
-    I: IntoIterator<Item = (T, N)>,
-{
-    values
-        .into_iter()
-        .fold(HashMap::new(), |mut acc, (key, value)| {
-            *acc.entry(key).or_default() += value;
-            acc
-        })
-}
-
 fn blink_at(stone: Stone) -> Vec<Stone> {
     match stone {
         zero if zero.0.is_empty() => vec![Stone::from([1])],
@@ -61,17 +58,6 @@ fn blink_at(stone: Stone) -> Vec<Stone> {
 }
 
 impl Stone {
-    // should actually be proper parse
-    fn parse(string: &str) -> Stone {
-        Stone::new(
-            string
-                .chars()
-                .map(|c| c.to_digit(10).unwrap() as u8)
-                .rev()
-                .collect(),
-        )
-    }
-
     fn new(digits: Vec<u8>) -> Stone {
         Stone(digits).trimmed()
     }
@@ -108,29 +94,208 @@ impl Stone {
     }
 }
 
+#[derive(Debug, PartialEq, Eq)]
+struct ParseStoneError;
+
+impl FromStr for Stone {
+    type Err = ParseStoneError;
+
+    // this is just for illustrating
+    // fn from_str(s: &str) -> Result<Self, Self::Err> {
+    //     let digits: Result<Vec<u8>, ParseStoneError> = s
+    //         .chars()
+    //         .map(|c| c.to_digit(10).map(|d| d as u8).ok_or(ParseStoneError))
+    //         .rev()
+    //         // this collect flips and creates a vector,
+    //         // i.e. it does
+    //         // Iter<Result<u8, Error>> -> Result<Vec<u8>, Error>
+    //         // i.e. it does "early return with Result"
+    //         .collect();
+    //     Ok(Stone::new(digits?))
+    // }
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        s.chars()
+            .map(|c| c.to_digit(10).map(|d| d as u8).ok_or(ParseStoneError))
+            .rev()
+            .collect::<Result<_, _>>()
+            .map(Stone::new)
+    }
+}
+
+fn accumulate<T, N, I>(values: I) -> HashMap<T, N>
+where
+    T: Eq + Hash,
+    N: Default + AddAssign,
+    I: IntoIterator<Item = (T, N)>,
+{
+    values
+        .into_iter()
+        .fold(HashMap::new(), |mut acc, (key, value)| {
+            *acc.entry(key).or_default() += value;
+            acc
+        })
+}
+
+#[derive(Debug, PartialEq, Eq, Clone, Hash)]
+struct Stone2 {
+    number: usize,
+    digits: u32,
+}
+
+fn digits(number: usize) -> u32 {
+    if number == 0 {
+        return 1;
+    }
+    let mut temp = number;
+    let mut digit_count = 0;
+    while temp > 0 {
+        temp /= 10;
+        digit_count += 1;
+    }
+    digit_count
+}
+
+impl Stone2 {
+    fn new(number: usize) -> Self {
+        Stone2 {
+            number,
+            digits: digits(number),
+        }
+    }
+
+    fn split(self) -> [Self; 2] {
+        // calling this with odd number of digits is ub (I'm lazy)
+        let divisor = 10usize.pow(self.digits / 2);
+        let left = Stone2 {
+            number: self.number / divisor,
+            digits: self.digits / 2,
+        };
+        // right could have leading zeros, recompute digits
+        let right = Stone2::new(self.number % divisor);
+        [left, right]
+    }
+
+    fn muled_by(self, n: usize) -> Self {
+        Stone2::new(self.number * n)
+    }
+}
+
+// here, returning impl Iterator<Item = Stone2>
+// does not work, because all match arms would need to have the same concrete type
+// (which they cannot have, except when we'd implement a bespoke OneOrTwo-struct)
+fn blink_at2(stone: Stone2) -> Vec<Stone2> {
+    match stone {
+        Stone2 {
+            number: 0,
+            digits: 1,
+        } => vec![Stone2 {
+            number: 1,
+            digits: 1,
+        }],
+        even if even.digits % 2 == 0 => Vec::from(even.split()),
+        other => vec![other.muled_by(2024)],
+    }
+}
+
+fn do_rounds(stones: &[Stone2], rounds: usize) -> usize {
+    let mut stone_map = accumulate(stones.iter().map(|s| (s.clone(), 1)));
+
+    for _ in 0..rounds {
+        stone_map = accumulate(
+            stone_map
+                .into_iter()
+                .flat_map(|(stone, count)| blink_at2(stone).into_iter().map(move |s| (s, count))),
+        );
+    }
+    stone_map.iter().map(|(_, count)| count).sum()
+}
+
+fn part12(stones: &[Stone2]) -> usize {
+    do_rounds(stones, 25)
+}
+
+fn part22(stones: &[Stone2]) -> usize {
+    do_rounds(stones, 75)
+}
+
+impl FromStr for Stone2 {
+    type Err = ParseIntError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        s.parse().map(|ds| Stone2::new(ds))
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
     #[test]
     fn test_muled_by_zero() {
-        let stone = Stone::parse("123");
+        let stone: Stone = "123".parse().unwrap();
         let result = stone.muled_by(0);
         assert_eq!(result.0, vec![0, 0, 0]); // 123 * 0 = 0
     }
 
     #[test]
     fn test_muled_by_one() {
-        let stone = Stone::parse("456");
+        let stone: Stone = "456".parse().unwrap();
         let result = stone.muled_by(1);
         assert_eq!(result.0, vec![6, 5, 4]); // 456 * 1 = 456
     }
 
     #[test]
     fn test_muled_by_large_number() {
-        let stone = Stone::parse("999");
+        let stone: Stone = "999".parse().unwrap();
         let result = stone.muled_by(999);
         // 999 * 999 = 998001
         assert_eq!(result.0, vec![1, 0, 0, 8, 9, 9]);
     }
+
+    #[test]
+    fn test_muled_by_zero2() {
+        let stone: Stone2 = "123".parse().unwrap();
+        let result = stone.muled_by(0);
+        assert_eq!(
+            result,
+            Stone2 {
+                number: 0,
+                digits: 1
+            }
+        ); // 123 * 0 = 0
+    }
+
+    #[test]
+    fn test_muled_by_one2() {
+        let stone: Stone2 = "456".parse().unwrap();
+        let result = stone.muled_by(1);
+        assert_eq!(
+            result,
+            Stone2 {
+                number: 456,
+                digits: 3
+            }
+        ); // 456 * 1 = 456
+    }
+
+    #[test]
+    fn test_muled_by_large_number2() {
+        let stone: Stone2 = "999".parse().unwrap();
+        let result = stone.muled_by(999);
+        assert_eq!(
+            result,
+            Stone2 {
+                number: 998001,
+                digits: 6
+            }
+        ); // 999 * 999 = 998001
+    }
+
+    #[test]
+    fn test_split() {
+        let stone: Stone2 = "1001".parse().unwrap();
+        let result = stone.split();
+        assert_eq!(result, [Stone2::new(10), Stone2::new(1)]);
+    }
 }