2025: day02: part 1

aoc2025/src/day02.rs

@@ -0,0 +1,116 @@
+use anyhow::{anyhow, Context, Result};
+use std::{fmt::Write, ops::RangeInclusive, str::FromStr};
+
+const INPUT: &str = include_str!("../input/day02.txt");
+
+pub fn run() -> Result<String> {
+    let mut res = String::with_capacity(128);
+    writeln!(res, "part 1: {}", part1(INPUT)?)?;
+    Ok(res)
+}
+
+struct IdRange(RangeInclusive<u64>);
+
+impl FromStr for IdRange {
+    type Err = anyhow::Error;
+
+    fn from_str(s: &str) -> Result<Self> {
+        let (left, right) = s
+            .split_once('-')
+            .ok_or_else(|| anyhow!("couldn't find dash in range: `{}'", s))?;
+        let (left, right) = (
+            left.parse()
+                .with_context(|| format!("couldn't parse left member of range: `{}'", left))?,
+            right
+                .trim()
+                .parse()
+                .with_context(|| format!("couldn't parse right member of range: `{}'", right))?,
+        );
+
+        Ok(IdRange(left..=right))
+    }
+}
+
+impl Iterator for IdRange {
+    type Item = <RangeInclusive<u64> as Iterator>::Item;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.next()
+    }
+}
+
+fn get_num_digits(num: &u64) -> usize {
+    let mut digits = 0;
+    let mut num = *num;
+    while num != 0 {
+        num /= 10;
+        digits += 1;
+    }
+    digits
+}
+
+fn is_repeated_twice(num: &u64) -> bool {
+    let num_digits = get_num_digits(num);
+    if !num_digits.is_multiple_of(2) {
+        return false;
+    }
+    let half = num_digits / 2;
+    let mask = 10_u64.pow(half as u32);
+
+    *num / mask == *num % mask
+}
+
+fn part1(input: &str) -> Result<u64> {
+    let ranges = input
+        .split(',')
+        .map(IdRange::from_str)
+        .collect::<Result<Vec<_>>>()?;
+
+    Ok(ranges
+        .into_iter()
+        .map(|range| range.filter(is_repeated_twice).sum::<u64>())
+        .sum())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const PROVIDED: &str = include_str!("../input/day02_provided.txt");
+
+    #[test]
+    fn repeated_twice() {
+        assert!(is_repeated_twice(&11));
+        assert!(is_repeated_twice(&22));
+        assert!(is_repeated_twice(&99));
+        assert!(is_repeated_twice(&1010));
+        assert!(is_repeated_twice(&1188511885));
+        assert!(is_repeated_twice(&222222));
+        assert!(is_repeated_twice(&446446));
+        assert!(is_repeated_twice(&38593859));
+
+        assert!(!is_repeated_twice(&111));
+        assert!(!is_repeated_twice(&121));
+        assert!(!is_repeated_twice(&1));
+    }
+
+    #[test]
+    fn part1_provided() {
+        assert_eq!(part1(PROVIDED).unwrap(), 1227775554);
+    }
+
+    #[test]
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 26255179562);
+    }
+
+    //#[test]
+    //fn part2_provided() {
+    //    assert_eq!(part2(PROVIDED).unwrap(), 6);
+    //}
+
+    //#[test]
+    //fn part2_real() {
+    //    assert_eq!(part2(INPUT).unwrap(), 6616);
+    //}
+}