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use std::fmt::{self, Debug, Formatter};
use std::num::NonZeroU64;
use std::ops::Range;
use super::SourceId;
/// A unique identifier for a syntax node.
///
/// This is used throughout the compiler to track which source section an error
/// or element stems from. Can be [mapped back](super::Source::range) to a byte
/// range for user facing display.
///
/// During editing, the span values stay mostly stable, even for nodes behind an
/// insertion. This is not true for simple ranges as they would shift. Spans can
/// be used as inputs to memoized functions without hurting cache performance
/// when text is inserted somewhere in the document other than the end.
///
/// Span ids are ordered in the syntax tree to enable quickly finding the node
/// with some id:
/// - The id of a parent is always smaller than the ids of any of its children.
/// - The id of a node is always greater than any id in the subtrees of any left
/// sibling and smaller than any id in the subtrees of any right sibling.
///
/// This type takes up 8 bytes and is null-optimized (i.e. `Option<Span>` also
/// takes 8 bytes).
#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
pub struct Span(NonZeroU64);
impl Span {
// Data layout:
// | 16 bits source id | 48 bits number |
// Number of bits for and minimum and maximum numbers assignable to spans.
const BITS: usize = 48;
const DETACHED: u64 = 1;
/// The full range of numbers available for span numbering.
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!(
Self::FULL.start <= number && number < Self::FULL.end,
"span number outside valid range"
);
let bits = ((id.into_u16() as u64) << Self::BITS) | number;
Self(to_non_zero(bits))
}
/// A span that does not point into any source file.
pub const fn detached() -> Self {
Self(to_non_zero(Self::DETACHED))
}
/// The id of the source file the span points into.
pub const fn source(self) -> SourceId {
SourceId::from_u16((self.0.get() >> Self::BITS) as u16)
}
/// The unique number of the span within its source file.
pub const fn number(self) -> u64 {
self.0.get() & ((1 << Self::BITS) - 1)
}
}
/// Convert to a non zero u64.
const fn to_non_zero(v: u64) -> NonZeroU64 {
match NonZeroU64::new(v) {
Some(v) => v,
None => panic!("span encoding is zero"),
}
}
/// A value with a span locating it in the source code.
#[derive(Copy, Clone, Eq, PartialEq, Hash)]
pub struct Spanned<T> {
/// The spanned value.
pub v: T,
/// The value's location in source code.
pub span: Span,
}
impl<T> Spanned<T> {
/// Create a new instance from a value and its span.
pub fn new(v: T, span: Span) -> Self {
Self { v, span }
}
/// Convert from `&Spanned<T>` to `Spanned<&T>`
pub fn as_ref(&self) -> Spanned<&T> {
Spanned { v: &self.v, span: self.span }
}
/// Map the value using a function.
pub fn map<F, U>(self, f: F) -> Spanned<U>
where
F: FnOnce(T) -> U,
{
Spanned { v: f(self.v), span: self.span }
}
}
impl<T: Debug> Debug for Spanned<T> {
fn fmt(&self, f: &mut Formatter) -> fmt::Result {
self.v.fmt(f)
}
}
#[cfg(test)]
mod tests {
use super::{SourceId, Span};
#[test]
fn test_span_encoding() {
let id = SourceId::from_u16(5);
let span = Span::new(id, 10);
assert_eq!(span.source(), id);
assert_eq!(span.number(), 10);
}
}
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