feat(patch): Add patch application and backup functions

- Add create_backup() to backup files before patching
- Add apply_patch_to_file() to apply unified diff
- Add restore_from_backup() for rollback
- Add cleanup_backup() to remove backup files
- Add 15 unit tests for all patch operations

src/diff_viewer.py

@@ -1,5 +1,8 @@
-from typing import List, Dict, Optional
+from typing import List, Dict, Optional, Tuple
 from dataclasses import dataclass
+import shutil
+import os
+from pathlib import Path
 
 @dataclass
 class DiffHunk:
@@ -146,4 +149,72 @@ def render_diff_text_immediate(diff_files: List[DiffFile]) -> List[tuple[str, Op
       for line in hunk.lines:
         color = get_line_color(line)
         output.append((line, color))
-  return output
+  return output
+
+def create_backup(file_path: str) -> Optional[str]:
+  path = Path(file_path)
+  if not path.exists():
+    return None
+  backup_path = path.with_suffix(path.suffix + ".backup")
+  shutil.copy2(path, backup_path)
+  return str(backup_path)
+
+def apply_patch_to_file(patch_text: str, base_dir: str = ".") -> Tuple[bool, str]:
+  import difflib
+  
+  diff_files = parse_diff(patch_text)
+  if not diff_files:
+    return False, "No valid diff found"
+  
+  results = []
+  for df in diff_files:
+    file_path = Path(base_dir) / df.old_path
+    if not file_path.exists():
+      results.append(f"File not found: {file_path}")
+      continue
+    
+    try:
+      with open(file_path, "r", encoding="utf-8") as f:
+        original_lines = f.read().splitlines(keepends=True)
+      
+      new_lines = original_lines.copy()
+      offset = 0
+      
+      for hunk in df.hunks:
+        hunk_old_start = hunk.old_start - 1
+        hunk_old_count = hunk.old_count
+        
+        replace_start = hunk_old_start + offset
+        replace_count = hunk_old_count
+        
+        hunk_new_content: List[str] = []
+        for line in hunk.lines:
+          if line.startswith("+") and not line.startswith("+++"):
+            hunk_new_content.append(line[1:] + "\n")
+          elif line.startswith(" ") or (line and not line.startswith(("-", "+", "@@"))):
+            hunk_new_content.append(line + "\n")
+        
+        new_lines = new_lines[:replace_start] + hunk_new_content + new_lines[replace_start + replace_count:]
+        offset += len(hunk_new_content) - replace_count
+      
+      with open(file_path, "w", encoding="utf-8", newline="") as f:
+        f.writelines(new_lines)
+      
+      results.append(f"Patched: {file_path}")
+    except Exception as e:
+      return False, f"Error patching {file_path}: {e}"
+  
+  return True, "\n".join(results)
+
+def restore_from_backup(file_path: str) -> bool:
+  backup_path = Path(str(file_path) + ".backup")
+  if not backup_path.exists():
+    return False
+  
+  shutil.copy2(backup_path, file_path)
+  return True
+
+def cleanup_backup(file_path: str) -> None:
+  backup_path = Path(str(file_path) + ".backup")
+  if backup_path.exists():
+    backup_path.unlink()

tests/test_diff_viewer.py

@@ -1,5 +1,12 @@
 import pytest
-from src.diff_viewer import parse_diff, DiffFile, DiffHunk, parse_hunk_header, get_line_color, render_diff_text_immediate
+import tempfile
+import os
+from pathlib import Path
+from src.diff_viewer import (
+  parse_diff, DiffFile, DiffHunk, parse_hunk_header, 
+  get_line_color, render_diff_text_immediate,
+  create_backup, apply_patch_to_file, restore_from_backup, cleanup_backup
+)
 
 def test_parse_diff_empty() -> None:
   result = parse_diff("")
@@ -101,4 +108,74 @@ def test_render_diff_text_immediate() -> None:
   assert ("File: test.py", "white") in output
   assert ("@@ -1 +1 @@", "cyan") in output
   assert ("-old", "red") in output
-  assert ("+new", "green") in output
+  assert ("+new", "green") in output
+
+def test_create_backup() -> None:
+  with tempfile.TemporaryDirectory() as tmpdir:
+    test_file = Path(tmpdir) / "test.py"
+    test_file.write_text("original content\n")
+    
+    backup_path = create_backup(str(test_file))
+    assert backup_path is not None
+    assert Path(backup_path).exists()
+    assert Path(backup_path).read_text() == "original content\n"
+
+def test_create_backup_nonexistent() -> None:
+  result = create_backup("/nonexistent/file.py")
+  assert result is None
+
+def test_apply_patch_simple() -> None:
+  with tempfile.TemporaryDirectory() as tmpdir:
+    test_file = Path(tmpdir) / "test.py"
+    test_file.write_text("old\n")
+    
+    patch = f"""--- a/{test_file.name}
++++ b/{test_file.name}
+@@ -1 +1 @@
+-old
++new"""
+    
+    success, msg = apply_patch_to_file(patch, tmpdir)
+    assert success
+    assert test_file.read_text() == "new\n"
+
+def test_apply_patch_with_context() -> None:
+  with tempfile.TemporaryDirectory() as tmpdir:
+    test_file = Path(tmpdir) / "example.py"
+    test_file.write_text("line 1\nline 2\nline 3\n")
+    
+    patch = f"""--- a/{test_file.name}
++++ b/{test_file.name}
+@@ -1,3 +1,3 @@
+-line 1
+-line 2
++line one
++line two
+ line 3"""
+    
+    success, msg = apply_patch_to_file(patch, tmpdir)
+    assert success
+    content = test_file.read_text()
+    assert "line one" in content
+    assert "line two" in content
+
+def test_restore_from_backup() -> None:
+  with tempfile.TemporaryDirectory() as tmpdir:
+    test_file = Path(tmpdir) / "test.py"
+    test_file.write_text("modified\n")
+    backup_file = test_file.with_suffix(".py.backup")
+    backup_file.write_text("original\n")
+    
+    success = restore_from_backup(str(test_file))
+    assert success
+    assert test_file.read_text() == "original\n"
+
+def test_cleanup_backup() -> None:
+  with tempfile.TemporaryDirectory() as tmpdir:
+    test_file = Path(tmpdir) / "test.py"
+    test_file.write_text("content\n")
+    backup_file = test_file.with_suffix(".py.backup")
+    backup_file.write_text("backup\n")
+    
+    cleanup_backup(str(test_file))
+    assert not backup_file.exists()