2025 day 3

Author: ictrobot

crates/year2025/src/day03.rs

@@ -0,0 +1,73 @@
+use utils::prelude::*;
+
+/// Finding the maximum number from ordered digits.
+#[derive(Clone, Debug)]
+pub struct Day03<'a> {
+    lines: Vec<&'a str>,
+}
+
+impl<'a> Day03<'a> {
+    pub fn new(input: &'a str, _: InputType) -> Result<Self, InputError> {
+        let lines: Vec<&str> = input.lines().collect();
+        for &l in &lines {
+            if let Some(b) = l.bytes().find(|x| !x.is_ascii_digit()) {
+                return Err(InputError::new(input, b as char, "expected digit"));
+            }
+            if l.len() < 12 {
+                return Err(InputError::new(input, l, "expected at least 12 digits"));
+            }
+        }
+        Ok(Self { lines })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> u64 {
+        self.output_joltage::<2>()
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> u64 {
+        self.output_joltage::<12>()
+    }
+
+    fn output_joltage<const N: usize>(&self) -> u64 {
+        let mut sum = 0;
+        for l in &self.lines {
+            let mut nums = [0u8; N];
+            let (first, last) = l.as_bytes().split_last_chunk::<N>().unwrap();
+
+            for &b in first {
+                for j in 0..N {
+                    let x = b - b'0';
+                    if x > nums[j] {
+                        nums[j] = x;
+                        nums[j + 1..].fill(0);
+                        break;
+                    }
+                }
+            }
+
+            for (i, &b) in last.iter().enumerate() {
+                for j in i..N {
+                    let x = b - b'0';
+                    if x > nums[j] {
+                        nums[j] = x;
+                        nums[j + 1..].fill(0);
+                        break;
+                    }
+                }
+            }
+
+            sum += nums.iter().fold(0, |acc, &x| acc * 10 + x as u64);
+        }
+        sum
+    }
+}
+
+examples!(Day03<'_> -> (u64, u64) [
+    {
+        input: "987654321111111\n811111111111119\n234234234234278\n818181911112111",
+        part1: 357,
+        part2: 3121910778619,
+    },
+]);

crates/year2025/src/lib.rs

@@ -4,4 +4,5 @@
 utils::year!(2025 => year2025, ${
     1 => day01::Day01,
     2 => day02::Day02,
+    3 => day03::Day03<'_>,
 });