Add snippet crate

Co-Authored-By: Nathan Sobo <nathan@zed.dev>
Co-Authored-By: Antonio Scandurra <me@as-cii.com>

crates/snippet/src/snippet.rs

@@ -0,0 +1,139 @@
+use anyhow::{anyhow, Result};
+use smallvec::SmallVec;
+use std::{collections::BTreeMap, ops::Range};
+use tree_sitter::{Parser, TreeCursor};
+
+#[derive(Default)]
+pub struct Snippet {
+    pub text: String,
+    pub tabstops: Vec<SmallVec<[Range<usize>; 2]>>,
+}
+
+impl Snippet {
+    pub fn parse(source: &str) -> Result<Self> {
+        let mut parser = Parser::new();
+        parser
+            .set_language(tree_sitter_snippet::language())
+            .unwrap();
+
+        let tree = parser.parse(source, None).unwrap();
+        if tree.root_node().has_error() {
+            return Err(anyhow!("invalid snippet"));
+        }
+
+        let mut text = String::new();
+        let mut tabstops = BTreeMap::new();
+        let mut cursor = tree.root_node().walk();
+        parse_snippet_node(&mut cursor, &mut text, &mut tabstops, source)?;
+
+        Ok(Snippet {
+            text,
+            tabstops: tabstops.into_values().collect(),
+        })
+    }
+}
+
+fn parse_snippet_node(
+    cursor: &mut TreeCursor,
+    text: &mut String,
+    tabstops: &mut BTreeMap<usize, SmallVec<[Range<usize>; 2]>>,
+    source: &str,
+) -> Result<()> {
+    cursor.goto_first_child();
+    loop {
+        let node = cursor.node();
+        match node.kind() {
+            "text" => text.push_str(&source[node.byte_range()]),
+            "tabstop" => {
+                if let Some(int_node) = node.named_child(0) {
+                    let index = source[int_node.byte_range()].parse::<usize>()?;
+                    tabstops
+                        .entry(index)
+                        .or_insert(SmallVec::new())
+                        .push(text.len()..text.len());
+                }
+            }
+            "placeholder" => {
+                cursor.goto_first_child();
+                cursor.goto_next_sibling();
+                let int_node = cursor.node();
+                let index = source[int_node.byte_range()].parse::<usize>()?;
+
+                cursor.goto_next_sibling();
+                cursor.goto_next_sibling();
+                let range_start = text.len();
+
+                parse_snippet_node(cursor, text, tabstops, source)?;
+                tabstops
+                    .entry(index)
+                    .or_insert(SmallVec::new())
+                    .push(range_start..text.len());
+
+                cursor.goto_parent();
+            }
+            _ => {}
+        }
+
+        if !cursor.goto_next_sibling() {
+            break;
+        }
+    }
+    cursor.goto_parent();
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_parse_snippet_with_tabstops() {
+        let snippet = Snippet::parse("one$1two").unwrap();
+        assert_eq!(snippet.text, "onetwo");
+        assert_eq!(
+            snippet
+                .tabstops
+                .iter()
+                .map(SmallVec::as_slice)
+                .collect::<Vec<_>>(),
+            &[vec![3..3]]
+        );
+    }
+
+    #[test]
+    fn test_parse_snippet_with_placeholders() {
+        let snippet = Snippet::parse("one${1:two}three").unwrap();
+        assert_eq!(snippet.text, "onetwothree");
+        assert_eq!(
+            snippet
+                .tabstops
+                .iter()
+                .map(SmallVec::as_slice)
+                .collect::<Vec<_>>(),
+            &[vec![3..6]]
+        );
+    }
+
+    #[test]
+    fn test_parse_snippet_with_nested_placeholders() {
+        let snippet = Snippet::parse(
+            "for (${1:var ${2:i} = 0; ${2:i} < ${3:${4:array}.length}; ${2:i}++}) {$5}",
+        )
+        .unwrap();
+        assert_eq!(snippet.text, "for (var i = 0; i < array.length; i++) {}");
+        assert_eq!(
+            snippet
+                .tabstops
+                .iter()
+                .map(SmallVec::as_slice)
+                .collect::<Vec<_>>(),
+            &[
+                vec![5..37],
+                vec![9..10, 16..17, 34..35],
+                vec![20..32],
+                vec![20..25],
+                vec![40..40],
+            ]
+        );
+    }
+}