3 files changed, 110 insertions, 9 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 52d0c35..168a14b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3,5 +3,14 @@
 version = 4
 
 [[package]]
+name = "anyhow"
+version = "1.0.100"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a23eb6b1614318a8071c9b2521f36b424b2c83db5eb3a0fead4a6c0809af6e61"
+
+[[package]]
 name = "day1"
 version = "0.1.0"
+dependencies = [
+ "anyhow",
+]
diff --git a/day1/Cargo.toml b/day1/Cargo.toml
index 8ffbbfb..c40a086 100644
--- a/day1/Cargo.toml
+++ b/day1/Cargo.toml
@@ -4,3 +4,4 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
+anyhow = "1.0.100"
diff --git a/day1/src/main.rs b/day1/src/main.rs
index 2849be2..aaee00b 100644
--- a/day1/src/main.rs
+++ b/day1/src/main.rs
@@ -1,4 +1,9 @@
-use std::str::FromStr;
+use anyhow::anyhow;
+use std::{
+    env, fs,
+    ops::{Add, Sub},
+    str::FromStr,
+};
 
 #[derive(Debug, PartialEq, Eq)]
 enum Rotation {
@@ -6,28 +11,99 @@ enum Rotation {
     Right(u16),
 }
 
-struct ParseRotationError(String);
+#[derive(Debug)]
+struct ParseRotationError;
 
 impl FromStr for Rotation {
     type Err = ParseRotationError;
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        let err = || ParseRotationError(s.into());
-
         let mut chars = s.chars();
-        let lr = chars.next().ok_or_else(err)?;
-        let n: u16 = chars.collect::<String>().parse().map_err(|_| err())?;
+        let lr = chars.next().ok_or(ParseRotationError)?;
+        let n: u16 = chars
+            .collect::<String>()
+            .parse()
+            .map_err(|_| ParseRotationError)?;
 
         match lr {
             'L' => Ok(Self::Left(n)),
             'R' => Ok(Self::Right(n)),
-            _ => Err(err()),
+            _ => Err(ParseRotationError),
         }
     }
 }
 
-fn main() {
-    println!("Hello, world!");
+#[derive(Clone, Copy, Debug)]
+struct Dial(u8);
+
+impl Dial {
+    fn new() -> Self {
+        Self(50)
+    }
+}
+
+impl Add<u16> for Dial {
+    type Output = Self;
+
+    fn add(self, rhs: u16) -> Self::Output {
+        let n = self.0 as u16;
+        let m = (n + rhs) % 100;
+        Self(m as u8)
+    }
+}
+
+impl Sub<u16> for Dial {
+    type Output = Self;
+
+    fn sub(self, rhs: u16) -> Self::Output {
+        let n = self.0 as u16;
+
+        if rhs > n {
+            let m = 100 - (n + rhs) % 100;
+            Self(m as u8)
+        } else {
+            let m = n - rhs;
+            Self(m as u8)
+        }
+    }
+}
+
+impl PartialEq<u8> for Dial {
+    fn eq(&self, other: &u8) -> bool {
+        self.0 == *other
+    }
+}
+
+fn main() -> anyhow::Result<()> {
+    let mut args = env::args();
+    let input_path = args.nth(1).ok_or(anyhow!("usage: day1 INPUT"))?;
+    let input = fs::read_to_string(input_path)?;
+
+    let rotations: Vec<Rotation> = input
+        .split_whitespace()
+        .map(|s| s.parse().map_err(|_| anyhow!("bad rotation: {}", s)))
+        .collect::<anyhow::Result<_>>()?;
+
+    let mut dial = Dial::new();
+    let mut zeros = 0;
+
+    for rot in rotations {
+        println!("dial: {:?}", dial);
+        println!("rotation: {:?}", rot);
+
+        match rot {
+            Rotation::Left(n) => dial = dial - n,
+            Rotation::Right(n) => dial = dial + n,
+        }
+
+        if dial == 0 {
+            zeros = zeros + 1;
+        }
+    }
+
+    println!("zeros: {}", zeros);
+
+    Ok(())
 }
 
 #[cfg(test)]
@@ -48,4 +124,19 @@ mod tests {
         assert!("L".parse::<Rotation>().is_err());
         assert!("2".parse::<Rotation>().is_err());
     }
+
+    #[test]
+    fn dial() {
+        let dial = Dial::new();
+
+        assert_eq!(dial + 1, 51);
+        assert_eq!(dial + 50, 0);
+        assert_eq!(dial + 100, 50);
+        assert_eq!(dial + 150, 0);
+
+        assert_eq!(dial - 1, 49);
+        assert_eq!(dial - 68, 82);
+        assert_eq!(dial - 68 - 30, 52);
+        assert_eq!(dial - 100, 50);
+    }
 }