4 files changed, 146 insertions, 1 deletions
diff --git a/Cargo.lock b/Cargo.lock
index 168a14b..2e0bc8d 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -14,3 +14,7 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
 ]
+
+[[package]]
+name = "day2"
+version = "0.1.0"
diff --git a/Cargo.toml b/Cargo.toml
index b512d30..d8418cc 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,3 +1,3 @@
 [workspace]
 resolver = "3"
-members = ["day1"]
+members = ["day1", "day2"]
diff --git a/day2/Cargo.toml b/day2/Cargo.toml
new file mode 100644
index 0000000..6c22320
--- /dev/null
+++ b/day2/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "day2"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
diff --git a/day2/src/main.rs b/day2/src/main.rs
new file mode 100644
index 0000000..5bb661f
--- /dev/null
+++ b/day2/src/main.rs
@@ -0,0 +1,135 @@
+use std::{env, fs, num::ParseIntError, ops::Range, str::FromStr};
+
+#[derive(Debug, Clone, Copy)]
+struct IDRange(u64, u64);
+
+#[derive(Debug)]
+struct ParseIDRangeError;
+
+impl From<ParseIntError> for ParseIDRangeError {
+    fn from(_: ParseIntError) -> Self {
+        Self
+    }
+}
+
+impl FromStr for IDRange {
+    type Err = ParseIDRangeError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let (start, end) = s.split_once('-').ok_or(ParseIDRangeError)?;
+        Ok(Self(start.parse()?, end.parse()?))
+    }
+}
+
+struct IDRangeIntoIter {
+    range: IDRange,
+    cur: u64,
+}
+
+impl Iterator for IDRangeIntoIter {
+    type Item = ID;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.cur <= self.range.1 {
+            let id = ID(self.cur);
+            self.cur += 1;
+            Some(id)
+        } else {
+            None
+        }
+    }
+}
+
+impl IntoIterator for IDRange {
+    type Item = ID;
+
+    type IntoIter = IDRangeIntoIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        Self::IntoIter {
+            cur: self.0,
+            range: self,
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq)]
+struct ID(u64);
+
+impl ID {
+    fn is_invalid(&self) -> bool {
+        let s = self.0.to_string();
+        s.len() % 2 == 0 && {
+            let (a, b) = s.split_at(s.len() / 2);
+            a == b
+        }
+    }
+}
+
+#[derive(Debug)]
+enum Error {
+    BadArgument,
+    BadRange(String),
+}
+
+impl From<std::io::Error> for Error {
+    fn from(_: std::io::Error) -> Self {
+        Self::BadArgument
+    }
+}
+
+fn main() -> Result<(), Error> {
+    let x: Range<u32> = 0..99;
+    let mut args = env::args();
+    let input_path = args.nth(1).ok_or_else(|| Error::BadArgument)?;
+    let input = fs::read_to_string(input_path)?;
+
+    let ranges: Vec<IDRange> = input
+        .split_whitespace()
+        .flat_map(|s| s.split(','))
+        .filter(|s| s.len() != 0)
+        .map(|s| s.parse().map_err(|_| Error::BadRange(s.into())))
+        .collect::<Result<_, _>>()?;
+
+    let result: u64 = ranges
+        .iter()
+        .flat_map(|range| range.into_iter().filter(ID::is_invalid).map(|id| id.0))
+        .sum();
+
+    println!("Result: {}", result);
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parse_id_range() -> Result<(), ParseIDRangeError> {
+        let range: IDRange = "99-100".parse()?;
+        assert_eq!(range.0, 99);
+        assert_eq!(range.1, 100);
+        Ok(())
+    }
+
+    #[test]
+    fn id_range_into_iter() {
+        let range = IDRange(2, 4);
+        let ids: Vec<ID> = range.into_iter().collect();
+        assert_eq!(ids, vec![ID(2), ID(3), ID(4)]);
+    }
+
+    #[test]
+    fn id_is_valid() {
+        assert!(!ID(0).is_invalid());
+        assert!(!ID(1).is_invalid());
+        assert!(!ID(2).is_invalid());
+        assert!(!ID(10).is_invalid());
+        assert!(!ID(12).is_invalid());
+        assert!(!ID(21).is_invalid());
+
+        assert!(ID(11).is_invalid());
+        assert!(ID(5555).is_invalid());
+    }
+}