⚡ Optimize LSP boundary character lookups from O(N) to O(1)

tools/lsp/Cargo.toml

@@ -20,3 +20,10 @@ tower-lsp = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 tokio = { version = "1", features = ["full"] }
+
+[dev-dependencies]
+criterion = "0.8.2"
+
+[[bench]]
+name = "bench"
+harness = false

tools/lsp/benches/bench.rs

@@ -0,0 +1,64 @@
+use criterion::{criterion_group, criterion_main, Criterion};
+
+fn find_boundary_slow(line: &str, word: &str) -> usize {
+    let mut count = 0;
+    let word_len = word.len();
+    let mut start_pos = 0;
+    while let Some(pos_in_line) = line[start_pos..].find(word) {
+        let actual_pos = start_pos + pos_in_line;
+        let char_before = if actual_pos > 0 {
+            line.chars().nth(actual_pos - 1)
+        } else {
+            None
+        };
+        let char_after = line.chars().nth(actual_pos + word_len);
+
+        let is_boundary_before = char_before.is_none_or(|c| !c.is_alphanumeric() && c != '_');
+        let is_boundary_after = char_after.is_none_or(|c| !c.is_alphanumeric() && c != '_');
+
+        if is_boundary_before && is_boundary_after {
+            count += 1;
+        }
+        start_pos = actual_pos + word_len;
+    }
+    count
+}
+
+fn find_boundary_fast(line: &str, word: &str) -> usize {
+    let mut count = 0;
+    let word_len = word.len();
+    let mut start_pos = 0;
+    while let Some(pos_in_line) = line[start_pos..].find(word) {
+        let actual_pos = start_pos + pos_in_line;
+        let char_before = if actual_pos > 0 {
+            line[..actual_pos].chars().next_back()
+        } else {
+            None
+        };
+        let char_after = line[actual_pos + word_len..].chars().next();
+
+        let is_boundary_before = char_before.is_none_or(|c| !c.is_alphanumeric() && c != '_');
+        let is_boundary_after = char_after.is_none_or(|c| !c.is_alphanumeric() && c != '_');
+
+        if is_boundary_before && is_boundary_after {
+            count += 1;
+        }
+        start_pos = actual_pos + word_len;
+    }
+    count
+}
+
+fn criterion_benchmark(c: &mut Criterion) {
+    let long_line = "let abc = 123; ".repeat(1000);
+    let word = "abc";
+
+    c.bench_function("slow", |b| {
+        b.iter(|| find_boundary_slow(std::hint::black_box(&long_line), std::hint::black_box(word)))
+    });
+    c.bench_function("fast", |b| {
+        b.iter(|| find_boundary_fast(std::hint::black_box(&long_line), std::hint::black_box(word)))
+    });
+}
+
+criterion_group!(benches, criterion_benchmark);
+criterion_main!(benches);