Add a TreeCursor type that travels over injection layers

This uses the layer parentage information from the parent commit to
traverse the layers. It's a similar API to `tree_sitter:TreeCursor`
but internally it does not use a `tree_sitter::TreeCursor` currently
because that interface is behaving very unexpectedly. Using the
`next_sibling`/`prev_sibling`/`parent` API on `tree_sitter::Node`
reflects the previous code's behavior so this should result in no
surprising changes.

1 files changed, 160 insertions, 0 deletions

diff --git a/helix-core/src/syntax/tree_cursor.rs b/helix-core/src/syntax/tree_cursor.rs
new file mode 100644
index 00000000..d9d140c9
--- /dev/null
+++ b/helix-core/src/syntax/tree_cursor.rs
@@ -0,0 +1,160 @@
+use std::{cmp::Reverse, ops::Range};
+
+use super::{LanguageLayer, LayerId};
+
+use slotmap::HopSlotMap;
+use tree_sitter::Node;
+
+/// The byte range of an injection layer.
+///
+/// Injection ranges may overlap, but all overlapping parts are subsets of their parent ranges.
+/// This allows us to sort the ranges ahead of time in order to efficiently find a range that
+/// contains a point with maximum depth.
+#[derive(Debug)]
+struct InjectionRange {
+    start: usize,
+    end: usize,
+    layer_id: LayerId,
+    depth: u32,
+}
+
+pub struct TreeCursor<'a> {
+    layers: &'a HopSlotMap<LayerId, LanguageLayer>,
+    root: LayerId,
+    current: LayerId,
+    injection_ranges: Vec<InjectionRange>,
+    // TODO: Ideally this would be a `tree_sitter::TreeCursor<'a>` but
+    // that returns very surprising results in testing.
+    cursor: Node<'a>,
+}
+
+impl<'a> TreeCursor<'a> {
+    pub(super) fn new(layers: &'a HopSlotMap<LayerId, LanguageLayer>, root: LayerId) -> Self {
+        let mut injection_ranges = Vec::new();
+
+        for (layer_id, layer) in layers.iter() {
+            // Skip the root layer
+            if layer.parent.is_none() {
+                continue;
+            }
+            for byte_range in layer.ranges.iter() {
+                let range = InjectionRange {
+                    start: byte_range.start_byte,
+                    end: byte_range.end_byte,
+                    layer_id,
+                    depth: layer.depth,
+                };
+                injection_ranges.push(range);
+            }
+        }
+
+        injection_ranges.sort_unstable_by_key(|range| (range.end, Reverse(range.depth)));
+
+        let cursor = layers[root].tree().root_node();
+
+        Self {
+            layers,
+            root,
+            current: root,
+            injection_ranges,
+            cursor,
+        }
+    }
+
+    pub fn node(&self) -> Node<'a> {
+        self.cursor
+    }
+
+    pub fn goto_parent(&mut self) -> bool {
+        if let Some(parent) = self.node().parent() {
+            self.cursor = parent;
+            return true;
+        }
+
+        // If we are already on the root layer, we cannot ascend.
+        if self.current == self.root {
+            return false;
+        }
+
+        // Ascend to the parent layer.
+        let range = self.node().byte_range();
+        let parent_id = self.layers[self.current]
+            .parent
+            .expect("non-root layers have a parent");
+        self.current = parent_id;
+        let root = self.layers[self.current].tree().root_node();
+        self.cursor = root
+            .descendant_for_byte_range(range.start, range.end)
+            .unwrap_or(root);
+
+        true
+    }
+
+    /// Finds the injection layer that has exactly the same range as the given `range`.
+    fn layer_id_of_byte_range(&self, search_range: Range<usize>) -> Option<LayerId> {
+        let start_idx = self
+            .injection_ranges
+            .partition_point(|range| range.end < search_range.end);
+
+        self.injection_ranges[start_idx..]
+            .iter()
+            .take_while(|range| range.end == search_range.end)
+            .find_map(|range| (range.start == search_range.start).then_some(range.layer_id))
+    }
+
+    pub fn goto_first_child(&mut self) -> bool {
+        // Check if the current node's range is an exact injection layer range.
+        if let Some(layer_id) = self
+            .layer_id_of_byte_range(self.node().byte_range())
+            .filter(|&layer_id| layer_id != self.current)
+        {
+            // Switch to the child layer.
+            self.current = layer_id;
+            self.cursor = self.layers[self.current].tree().root_node();
+            true
+        } else if let Some(child) = self.cursor.child(0) {
+            // Otherwise descend in the current tree.
+            self.cursor = child;
+            true
+        } else {
+            false
+        }
+    }
+
+    pub fn goto_next_sibling(&mut self) -> bool {
+        if let Some(sibling) = self.cursor.next_sibling() {
+            self.cursor = sibling;
+            true
+        } else {
+            false
+        }
+    }
+
+    pub fn goto_prev_sibling(&mut self) -> bool {
+        if let Some(sibling) = self.cursor.prev_sibling() {
+            self.cursor = sibling;
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Finds the injection layer that contains the given start-end range.
+    fn layer_id_containing_byte_range(&self, start: usize, end: usize) -> LayerId {
+        let start_idx = self
+            .injection_ranges
+            .partition_point(|range| range.end < end);
+
+        self.injection_ranges[start_idx..]
+            .iter()
+            .take_while(|range| range.start < end)
+            .find_map(|range| (range.start <= start).then_some(range.layer_id))
+            .unwrap_or(self.root)
+    }
+
+    pub fn reset_to_byte_range(&mut self, start: usize, end: usize) {
+        self.current = self.layer_id_containing_byte_range(start, end);
+        let root = self.layers[self.current].tree().root_node();
+        self.cursor = root.descendant_for_byte_range(start, end).unwrap_or(root);
+    }
+}