From 3b59a36e04e0e91c24bcdc611f851bcfe7cdc114 Mon Sep 17 00:00:00 2001
From: Micha Reiser <micha@reiser.io>
Date: Thu, 1 Feb 2024 11:48:05 +0100
Subject: [PATCH] Narrow text edits

---
 ruff_lsp/server.py   | 39 +++++++++++++++++++++++++++++++++++----
 tests/test_format.py |  6 +++++-
 2 files changed, 40 insertions(+), 5 deletions(-)

diff --git a/ruff_lsp/server.py b/ruff_lsp/server.py
index 5177df2..35a6ab7 100755
--- a/ruff_lsp/server.py
+++ b/ruff_lsp/server.py
@@ -16,7 +16,6 @@
 from pathlib import Path
 from typing import Any, List, NamedTuple, Sequence, Union, cast
 
-from lsprotocol import validators
 from lsprotocol.types import (
     CODE_ACTION_RESOLVE,
     INITIALIZE,
@@ -1379,13 +1378,45 @@ def _fixed_source_to_edits(
     if new_source == original_source:
         return []
 
+    # Reduce the text edit by omitting the common suffix and postfix (lines)
+    # from the text edit. I chose this basic diffing because "proper" diffing has
+    # the downside that it can be very slow in some cases. Black uses a diffing approach
+    # that takes time into consideration, but it requires spawning a thread to stop
+    # the diffing after a given time, which feels very heavy weight.
+    # This basic "diffing" has a guaranteed `O(n)` runtime and is sufficient to
+    # prevent transmitting the entire source document when formatting a range
+    # or formatting a document where most of the code remains unchanged.
+    #
+    # https://github.com/microsoft/vscode-black-formatter/blob/main/bundled/tool/lsp_edit_utils.py
+    new_lines = new_source.splitlines(True)
+    original_lines = original_source.splitlines(True)
+
+    start_offset = 0
+    end_offset = 0
+
+    for new_line, original_line in zip(new_lines, original_lines):
+        if new_line == original_line:
+            start_offset += 1
+        else:
+            break
+
+    for new_line, original_line in zip(
+        reversed(new_lines[start_offset:]), reversed(original_lines[start_offset:])
+    ):
+        if new_line == original_line:
+            end_offset += 1
+        else:
+            break
+
+    trimmed_new_source = "".join(new_lines[start_offset : len(new_lines) - end_offset])
+
     return [
         TextEdit(
             range=Range(
-                start=Position(line=0, character=0),
-                end=Position(line=validators.UINTEGER_MAX_VALUE, character=0),
+                start=Position(line=start_offset, character=0),
+                end=Position(line=len(original_lines) - end_offset, character=0),
             ),
-            new_text=new_source,
+            new_text=trimmed_new_source,
         )
     ]
 
diff --git a/tests/test_format.py b/tests/test_format.py
index 09a94b9..21ec62e 100644
--- a/tests/test_format.py
+++ b/tests/test_format.py
@@ -3,6 +3,7 @@
 from contextlib import nullcontext
 
 import pytest
+from lsprotocol.types import Position, Range
 from packaging.version import Version
 from pygls.workspace import Workspace
 
@@ -46,7 +47,10 @@ async def test_format(tmp_path, ruff_version: Version):
         [edit] = _fixed_source_to_edits(
             original_source=document.source, fixed_source=result.stdout.decode("utf-8")
         )
-        assert edit.new_text == expected
+        assert edit.new_text == ""
+        assert edit.range == Range(
+            start=Position(line=0, character=0), end=Position(line=1, character=0)
+        )
 
 
 @pytest.mark.asyncio