commit a3ae4429f5c52e9c166786290ab2b5893ed6119b
Author: Gabriel Kaszewski <gabrielkaszewski@gmail.com>
Date:   Wed Apr 8 01:26:17 2026 +0200

    feat(domain): add Note enum with semitone math and display

diff --git a/crates/domain/src/lib.rs b/crates/domain/src/lib.rs
new file mode 100644
index 0000000..dc9d2cf
--- /dev/null
+++ b/crates/domain/src/lib.rs
@@ -0,0 +1,3 @@
+pub mod note;
+
+pub use note::Note;
diff --git a/crates/domain/src/note.rs b/crates/domain/src/note.rs
new file mode 100644
index 0000000..79f0134
--- /dev/null
+++ b/crates/domain/src/note.rs
@@ -0,0 +1,107 @@
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Note {
+    A, ASharpBFlat, B, C, CSharpDFlat, D,
+    DSharpEFlat, E, F, FSharpGFlat, G, GSharpAFlat,
+}
+
+impl Note {
+    pub fn semitone(&self) -> u8 {
+        match self {
+            Note::C => 0, Note::CSharpDFlat => 1, Note::D => 2,
+            Note::DSharpEFlat => 3, Note::E => 4, Note::F => 5,
+            Note::FSharpGFlat => 6, Note::G => 7, Note::GSharpAFlat => 8,
+            Note::A => 9, Note::ASharpBFlat => 10, Note::B => 11,
+        }
+    }
+
+    pub fn from_semitone(s: u8, use_sharps: bool) -> Note {
+        match s % 12 {
+            0 => Note::C, 1 => Note::CSharpDFlat, 2 => Note::D,
+            3 => Note::DSharpEFlat, 4 => Note::E, 5 => Note::F,
+            6 => Note::FSharpGFlat, 7 => Note::G, 8 => Note::GSharpAFlat,
+            9 => Note::A, 10 => Note::ASharpBFlat, 11 => Note::B,
+            _ => unreachable!(),
+        }
+    }
+
+    pub fn to_sharp_str(&self) -> &'static str {
+        match self {
+            Note::C => "C", Note::CSharpDFlat => "C#", Note::D => "D",
+            Note::DSharpEFlat => "D#", Note::E => "E", Note::F => "F",
+            Note::FSharpGFlat => "F#", Note::G => "G", Note::GSharpAFlat => "G#",
+            Note::A => "A", Note::ASharpBFlat => "A#", Note::B => "B",
+        }
+    }
+
+    pub fn to_flat_str(&self) -> &'static str {
+        match self {
+            Note::C => "C", Note::CSharpDFlat => "Db", Note::D => "D",
+            Note::DSharpEFlat => "Eb", Note::E => "E", Note::F => "F",
+            Note::FSharpGFlat => "Gb", Note::G => "G", Note::GSharpAFlat => "Ab",
+            Note::A => "A", Note::ASharpBFlat => "Bb", Note::B => "B",
+        }
+    }
+
+    /// Parse just the note portion from the start of a string.
+    /// Returns (Note, chars_consumed) or None.
+    pub fn parse_prefix(s: &str) -> Option<(Note, usize)> {
+        let mut chars = s.chars();
+        let root = match chars.next()? {
+            'A' => Note::A, 'B' => Note::B, 'C' => Note::C, 'D' => Note::D,
+            'E' => Note::E, 'F' => Note::F, 'G' => Note::G, _ => return None,
+        };
+        match chars.next() {
+            Some('#') => Some((Self::sharp_of(root), 2)),
+            Some('b') if s.len() > 1 => {
+                let flatted = Self::flat_of(root)?;
+                Some((flatted, 2))
+            }
+            _ => Some((root, 1)),
+        }
+    }
+
+    pub fn from_str(s: &str) -> Option<Note> {
+        let (note, consumed) = Self::parse_prefix(s)?;
+        if consumed == s.len() { Some(note) } else { None }
+    }
+
+    fn sharp_of(root: Note) -> Note {
+        Note::from_semitone((root.semitone() + 1) % 12, true)
+    }
+
+    fn flat_of(root: Note) -> Option<Note> {
+        if root.semitone() == 0 { return None; } // Cb is unusual, skip
+        Some(Note::from_semitone((root.semitone() + 11) % 12, false))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn semitone_roundtrip() {
+        assert_eq!(Note::from_semitone(Note::A.semitone(), true), Note::A);
+        assert_eq!(Note::from_semitone(Note::FSharpGFlat.semitone(), true), Note::FSharpGFlat);
+    }
+
+    #[test]
+    fn sharp_display() {
+        assert_eq!(Note::ASharpBFlat.to_sharp_str(), "A#");
+        assert_eq!(Note::FSharpGFlat.to_sharp_str(), "F#");
+    }
+
+    #[test]
+    fn flat_display() {
+        assert_eq!(Note::ASharpBFlat.to_flat_str(), "Bb");
+        assert_eq!(Note::CSharpDFlat.to_flat_str(), "Db");
+    }
+
+    #[test]
+    fn parse_note() {
+        assert_eq!(Note::from_str("F#"), Some(Note::FSharpGFlat));
+        assert_eq!(Note::from_str("Gb"), Some(Note::FSharpGFlat));
+        assert_eq!(Note::from_str("A"), Some(Note::A));
+        assert_eq!(Note::from_str("X"), None);
+    }
+}