1 files changed, 156 insertions, 0 deletions

diff --git a/src/parser_impl/event.rs b/src/parser_impl/event.rs
new file mode 100644
index 000000000..eb5d0a4be
--- /dev/null
+++ b/src/parser_impl/event.rs
@@ -0,0 +1,156 @@
+//! 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 `Sink` 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 {
+    lexer::Token,
+    parser_impl::Sink,
+    SyntaxKind::{self, TOMBSTONE},
+};
+
+/// `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`
+    /// exists to allow to tweak parent-child relationships.
+    ///
+    /// 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: String,
+    },
+}
+
+pub(super) fn process(builder: &mut impl Sink, tokens: &[Token], events: Vec<Event>) {
+    let mut idx = 0;
+
+    let mut holes = Vec::new();
+    let mut forward_parents = Vec::new();
+
+    for (i, event) in events.iter().enumerate() {
+        if holes.last() == Some(&i) {
+            holes.pop();
+            continue;
+        }
+
+        match event {
+            &Event::Start {
+                kind: TOMBSTONE, ..
+            } => (),
+
+            &Event::Start { .. } => {
+                forward_parents.clear();
+                let mut idx = i;
+                loop {
+                    let (kind, fwd) = match events[idx] {
+                        Event::Start {
+                            kind,
+                            forward_parent,
+                        } => (kind, forward_parent),
+                        _ => unreachable!(),
+                    };
+                    forward_parents.push((idx, kind));
+                    if let Some(fwd) = fwd {
+                        idx += fwd as usize;
+                    } else {
+                        break;
+                    }
+                }
+                for &(idx, kind) in forward_parents.iter().into_iter().rev() {
+                    builder.start_internal(kind);
+                    holes.push(idx);
+                }
+                holes.pop();
+            }
+            &Event::Finish => {
+                while idx < tokens.len() {
+                    let token = tokens[idx];
+                    if token.kind.is_trivia() {
+                        idx += 1;
+                        builder.leaf(token.kind, token.len);
+                    } else {
+                        break;
+                    }
+                }
+                builder.finish_internal()
+            }
+            &Event::Token {
+                kind,
+                mut n_raw_tokens,
+            } => {
+                // FIXME: currently, we attach whitespace to some random node
+                // this should be done in a sensible manner instead
+                loop {
+                    let token = tokens[idx];
+                    if !token.kind.is_trivia() {
+                        break;
+                    }
+                    builder.leaf(token.kind, token.len);
+                    idx += 1
+                }
+                let mut len = 0.into();
+                for _ in 0..n_raw_tokens {
+                    len += tokens[idx].len;
+                    idx += 1;
+                }
+                builder.leaf(kind, len);
+            }
+            &Event::Error { ref msg } => builder.error(msg.clone()),
+        }
+    }
+}