1 files changed, 185 insertions, 0 deletions

diff --git a/crates/tt/src/buffer.rs b/crates/tt/src/buffer.rs
new file mode 100644
index 000000000..02c771f70
--- /dev/null
+++ b/crates/tt/src/buffer.rs
@@ -0,0 +1,185 @@
+//! FIXME: write short doc here
+
+use crate::{Subtree, TokenTree};
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+struct EntryId(usize);
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+struct EntryPtr(EntryId, usize);
+
+/// Internal type which is used instead of `TokenTree` to represent a token tree
+/// within a `TokenBuffer`.
+#[derive(Debug)]
+enum Entry<'t> {
+    // Mimicking types from proc-macro.
+    Subtree(&'t TokenTree, EntryId),
+    Leaf(&'t TokenTree),
+    // End entries contain a pointer to the entry from the containing
+    // token tree, or None if this is the outermost level.
+    End(Option<EntryPtr>),
+}
+
+/// A token tree buffer
+/// The safe version of `syn` [`TokenBuffer`](https://github.com/dtolnay/syn/blob/6533607f91686545cb034d2838beea338d9d0742/src/buffer.rs#L41)
+#[derive(Debug)]
+pub struct TokenBuffer<'t> {
+    buffers: Vec<Box<[Entry<'t>]>>,
+}
+
+impl<'t> TokenBuffer<'t> {
+    pub fn new(tokens: &'t [TokenTree]) -> TokenBuffer<'t> {
+        let mut buffers = vec![];
+
+        let idx = TokenBuffer::new_inner(tokens, &mut buffers, None);
+        assert_eq!(idx, 0);
+
+        TokenBuffer { buffers }
+    }
+
+    fn new_inner(
+        tokens: &'t [TokenTree],
+        buffers: &mut Vec<Box<[Entry<'t>]>>,
+        next: Option<EntryPtr>,
+    ) -> usize {
+        // Must contain everything in tokens and then the Entry::End
+        let start_capacity = tokens.len() + 1;
+        let mut entries = Vec::with_capacity(start_capacity);
+        let mut children = vec![];
+
+        for (idx, tt) in tokens.iter().enumerate() {
+            match tt {
+                TokenTree::Leaf(_) => {
+                    entries.push(Entry::Leaf(tt));
+                }
+                TokenTree::Subtree(subtree) => {
+                    entries.push(Entry::End(None));
+                    children.push((idx, (subtree, tt)));
+                }
+            }
+        }
+
+        entries.push(Entry::End(next));
+        let res = buffers.len();
+        buffers.push(entries.into_boxed_slice());
+
+        for (child_idx, (subtree, tt)) in children {
+            let idx = TokenBuffer::new_inner(
+                &subtree.token_trees,
+                buffers,
+                Some(EntryPtr(EntryId(res), child_idx + 1)),
+            );
+            buffers[res].as_mut()[child_idx] = Entry::Subtree(tt, EntryId(idx));
+        }
+
+        res
+    }
+
+    /// Creates a cursor referencing the first token in the buffer and able to
+    /// traverse until the end of the buffer.
+    pub fn begin(&self) -> Cursor {
+        Cursor::create(self, EntryPtr(EntryId(0), 0))
+    }
+
+    fn entry(&self, ptr: &EntryPtr) -> Option<&Entry> {
+        let id = ptr.0;
+        self.buffers[id.0].get(ptr.1)
+    }
+}
+
+/// A safe version of `Cursor` from `syn` crate https://github.com/dtolnay/syn/blob/6533607f91686545cb034d2838beea338d9d0742/src/buffer.rs#L125
+#[derive(Copy, Clone, Debug)]
+pub struct Cursor<'a> {
+    buffer: &'a TokenBuffer<'a>,
+    ptr: EntryPtr,
+}
+
+impl<'a> PartialEq for Cursor<'a> {
+    fn eq(&self, other: &Cursor) -> bool {
+        self.ptr == other.ptr && std::ptr::eq(self.buffer, other.buffer)
+    }
+}
+
+impl<'a> Eq for Cursor<'a> {}
+
+impl<'a> Cursor<'a> {
+    /// Check whether it is eof
+    pub fn eof(self) -> bool {
+        matches!(self.buffer.entry(&self.ptr), None | Some(Entry::End(None)))
+    }
+
+    /// If the cursor is pointing at the end of a subtree, returns
+    /// the parent subtree
+    pub fn end(self) -> Option<&'a Subtree> {
+        match self.entry() {
+            Some(Entry::End(Some(ptr))) => {
+                let idx = ptr.1;
+                if let Some(Entry::Subtree(TokenTree::Subtree(subtree), _)) =
+                    self.buffer.entry(&EntryPtr(ptr.0, idx - 1))
+                {
+                    return Some(subtree);
+                }
+
+                None
+            }
+            _ => None,
+        }
+    }
+
+    fn entry(self) -> Option<&'a Entry<'a>> {
+        self.buffer.entry(&self.ptr)
+    }
+
+    /// If the cursor is pointing at a `Subtree`, returns
+    /// a cursor into that subtree
+    pub fn subtree(self) -> Option<Cursor<'a>> {
+        match self.entry() {
+            Some(Entry::Subtree(_, entry_id)) => {
+                Some(Cursor::create(self.buffer, EntryPtr(*entry_id, 0)))
+            }
+            _ => None,
+        }
+    }
+
+    /// If the cursor is pointing at a `TokenTree`, returns it
+    pub fn token_tree(self) -> Option<&'a TokenTree> {
+        match self.entry() {
+            Some(Entry::Leaf(tt)) => Some(tt),
+            Some(Entry::Subtree(tt, _)) => Some(tt),
+            Some(Entry::End(_)) => None,
+            None => None,
+        }
+    }
+
+    fn create(buffer: &'a TokenBuffer, ptr: EntryPtr) -> Cursor<'a> {
+        Cursor { buffer, ptr }
+    }
+
+    /// Bump the cursor
+    pub fn bump(self) -> Cursor<'a> {
+        if let Some(Entry::End(exit)) = self.buffer.entry(&self.ptr) {
+            if let Some(exit) = exit {
+                Cursor::create(self.buffer, *exit)
+            } else {
+                self
+            }
+        } else {
+            Cursor::create(self.buffer, EntryPtr(self.ptr.0, self.ptr.1 + 1))
+        }
+    }
+
+    /// Bump the cursor, if it is a subtree, returns
+    /// a cursor into that subtree
+    pub fn bump_subtree(self) -> Cursor<'a> {
+        match self.entry() {
+            Some(Entry::Subtree(_, _)) => self.subtree().unwrap(),
+            _ => self.bump(),
+        }
+    }
+
+    /// Check whether it is a top level
+    pub fn is_root(&self) -> bool {
+        let entry_id = self.ptr.0;
+        entry_id.0 == 0
+    }
+}