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|
//! This module provides a way to construct a `File`.
//! It is intended to be completely decoupled from the
//! parser, so as to allow to evolve the tree representation
//! and the parser algorithm independently.
//!
//! The `TreeSink` trait is the bridge between the parser and the
//! tree builder: the parser produces a stream of events like
//! `start node`, `finish node`, and `FileBuilder` converts
//! this stream to a real tree.
use std::mem;
use crate::{
ParseError, TreeSink,
SyntaxKind::{self, *},
};
/// `Parser` produces a flat list of `Event`s.
/// They are converted to a tree-structure in
/// a separate pass, via `TreeBuilder`.
#[derive(Debug)]
pub(crate) enum Event {
/// This event signifies the start of the node.
/// It should be either abandoned (in which case the
/// `kind` is `TOMBSTONE`, and the event is ignored),
/// or completed via a `Finish` event.
///
/// All tokens between a `Start` and a `Finish` would
/// become the children of the respective node.
///
/// For left-recursive syntactic constructs, the parser produces
/// a child node before it sees a parent. `forward_parent`
/// saves the position of current event's parent.
///
/// Consider this path
///
/// foo::bar
///
/// The events for it would look like this:
///
///
/// START(PATH) IDENT('foo') FINISH START(PATH) COLONCOLON IDENT('bar') FINISH
/// | /\
/// | |
/// +------forward-parent------+
///
/// And the tree would look like this
///
/// +--PATH---------+
/// | | |
/// | | |
/// | '::' 'bar'
/// |
/// PATH
/// |
/// 'foo'
///
/// See also `CompletedMarker::precede`.
Start {
kind: SyntaxKind,
forward_parent: Option<u32>,
},
/// Complete the previous `Start` event
Finish,
/// Produce a single leaf-element.
/// `n_raw_tokens` is used to glue complex contextual tokens.
/// For example, lexer tokenizes `>>` as `>`, `>`, and
/// `n_raw_tokens = 2` is used to produced a single `>>`.
Token {
kind: SyntaxKind,
n_raw_tokens: u8,
},
Error {
msg: ParseError,
},
}
impl Event {
pub(crate) fn tombstone() -> Self {
Event::Start { kind: TOMBSTONE, forward_parent: None }
}
}
/// Generate the syntax tree with the control of events.
pub(super) fn process(sink: &mut dyn TreeSink, mut events: Vec<Event>) {
let mut forward_parents = Vec::new();
for i in 0..events.len() {
match mem::replace(&mut events[i], Event::tombstone()) {
Event::Start { kind: TOMBSTONE, .. } => (),
Event::Start { kind, forward_parent } => {
// For events[A, B, C], B is A's forward_parent, C is B's forward_parent,
// in the normal control flow, the parent-child relation: `A -> B -> C`,
// while with the magic forward_parent, it writes: `C <- B <- A`.
// append `A` into parents.
forward_parents.push(kind);
let mut idx = i;
let mut fp = forward_parent;
while let Some(fwd) = fp {
idx += fwd as usize;
// append `A`'s forward_parent `B`
fp = match mem::replace(&mut events[idx], Event::tombstone()) {
Event::Start { kind, forward_parent } => {
if kind != TOMBSTONE {
forward_parents.push(kind);
}
forward_parent
}
_ => unreachable!(),
};
// append `B`'s forward_parent `C` in the next stage.
}
for kind in forward_parents.drain(..).rev() {
sink.start_branch(kind);
}
}
Event::Finish => sink.finish_branch(),
Event::Token { kind, n_raw_tokens } => {
sink.leaf(kind, n_raw_tokens);
}
Event::Error { msg } => sink.error(msg),
}
}
}
|
