LSP: Formatting minified JS/JSON files is slow

[Andriamanitra]

When formatting a 46KB minified .js file using textDocument/formatting request it takes about 5–6 seconds for deno lsp to respond. For my particular test file the language server responds with Content-Length: 546612 worth of TextEdit[] (that is about 11 times more than just sending the full file contents). For comparison formatting the same file takes only around 100ms using deno fmt.

Fix?

I think this could be fixed with some heuristics in the function that calculates text edits:

deno/cli/lsp/text.rs

Lines 209 to 258 in 8d24be1

	pub fn get_edits(a: &str, b: &str, line_index: &LineIndex) -> Vec<TextEdit> {
	if a == b {
	return vec![];
	}
	let chunks = diff(a, b);
	let mut text_edits = Vec::<TextEdit>::new();
	let mut iter = chunks.iter().peekable();
	let mut a_pos = TextSize::from(0);
	loop {
	let chunk = iter.next();
	match chunk {
	None => break,
	Some(Chunk::Equal(e)) => {
	a_pos += TextSize::from(e.encode_utf16().count() as u32);
	}
	Some(Chunk::Delete(d)) => {
	let start = line_index.position_utf16(a_pos);
	a_pos += TextSize::from(d.encode_utf16().count() as u32);
	let end = line_index.position_utf16(a_pos);
	let range = lsp::Range { start, end };
	match iter.peek() {
	Some(Chunk::Insert(i)) => {
	iter.next();
	text_edits.push(TextEdit {
	range,
	new_text: i.to_string(),
	});
	}
	_ => text_edits.push(TextEdit {
	range,
	new_text: "".to_string(),
	}),
	}
	}
	Some(Chunk::Insert(i)) => {
	let pos = line_index.position_utf16(a_pos);
	let range = lsp::Range {
	start: pos,
	end: pos,
	};
	text_edits.push(TextEdit {
	range,
	new_text: i.to_string(),
	});
	}
	}
	}
	text_edits
	}

A simple one would be to skip calculating the diff and treating the whole file as one big text edit if the lengths of a and b differ significantly.

Version information:

deno 1.37.0 (release, x86_64-unknown-linux-gnu)
v8 11.8.172.3
typescript 5.2.2