1 files changed, 52 insertions, 6 deletions

diff --git a/src/syntax/span.rs b/src/syntax/span.rs
index 5dcd8fc1..35425801 100644
--- a/src/syntax/span.rs
+++ b/src/syntax/span.rs
@@ -62,20 +62,25 @@ impl<T: Debug> Debug for Spanned<T> {
 pub struct Span(NonZeroU64);
 
 impl Span {
+    // Data layout:
+    // | 2 bits span pos | 16 bits source id | 46 bits number |
+
     // Number of bits for and minimum and maximum numbers assignable to spans.
-    const BITS: usize = 48;
+    const BITS: usize = 46;
     const DETACHED: u64 = 1;
-    const MIN: u64 = 2;
-    const MAX: u64 = (1 << Self::BITS) - 1;
 
     /// The full range of numbers available to spans.
-    pub const FULL: Range<u64> = Self::MIN .. Self::MAX + 1;
+    pub const FULL: Range<u64> = 2 .. (1 << Self::BITS);
 
     /// Create a new span from a source id and a unique number.
     ///
     /// Panics if the `number` is not contained in `FULL`.
     pub const fn new(id: SourceId, number: u64) -> Self {
-        assert!(number >= Self::MIN && number <= Self::MAX);
+        assert!(
+            Self::FULL.start <= number && number < Self::FULL.end,
+            "span number outside valid range"
+        );
+
         let bits = ((id.into_raw() as u64) << Self::BITS) | number;
         Self(to_non_zero(bits))
     }
@@ -85,6 +90,12 @@ impl Span {
         Self(to_non_zero(Self::DETACHED))
     }
 
+    /// Return a new span with updated position.
+    pub const fn with_pos(self, pos: SpanPos) -> Self {
+        let bits = (self.0.get() & ((1 << 62) - 1)) | ((pos as u64) << 62);
+        Self(to_non_zero(bits))
+    }
+
     /// The id of the source file the span points into.
     pub const fn source(self) -> SourceId {
         SourceId::from_raw((self.0.get() >> Self::BITS) as u16)
@@ -94,16 +105,37 @@ impl Span {
     pub const fn number(self) -> u64 {
         self.0.get() & ((1 << Self::BITS) - 1)
     }
+
+    /// Where in the node the span points to.
+    pub const fn pos(self) -> SpanPos {
+        match self.0.get() >> 62 {
+            0 => SpanPos::Full,
+            1 => SpanPos::Start,
+            2 => SpanPos::End,
+            _ => panic!("span pos encoding is invalid"),
+        }
+    }
 }
 
 /// Convert to a non zero u64.
 const fn to_non_zero(v: u64) -> NonZeroU64 {
     match NonZeroU64::new(v) {
         Some(v) => v,
-        None => unreachable!(),
+        None => panic!("span encoding is zero"),
     }
 }
 
+/// Where in a node a span points.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub enum SpanPos {
+    /// Over the full width of the node.
+    Full = 0,
+    /// At the start of the node.
+    Start = 1,
+    /// At the end of the node.
+    End = 2,
+}
+
 /// Result of numbering a node within an interval.
 pub type NumberingResult = Result<(), Unnumberable>;
 
@@ -118,3 +150,17 @@ impl Display for Unnumberable {
 }
 
 impl std::error::Error for Unnumberable {}
+
+#[cfg(test)]
+mod tests {
+    use super::{SourceId, Span, SpanPos};
+
+    #[test]
+    fn test_span_encoding() {
+        let id = SourceId::from_raw(5);
+        let span = Span::new(id, 10).with_pos(SpanPos::End);
+        assert_eq!(span.source(), id);
+        assert_eq!(span.number(), 10);
+        assert_eq!(span.pos(), SpanPos::End);
+    }
+}