from pathlib import Path
from difflib import unified_diff
root = Path(__file__).parent
parts = []
count = 0
for base in sorted((root / 'base').rglob('*.java')):
    rel = base.relative_to(root / 'base').as_posix()
    work = root / 'work' / rel
    old = base.read_text(encoding='utf-8').splitlines(keepends=True)
    new = work.read_text(encoding='utf-8').splitlines(keepends=True)
    if old != new:
        parts.append(''.join(unified_diff(old, new, fromfile='a/' + rel, tofile='b/' + rel)))
        count += 1
out = root / 't28-backend.patch'
out.write_text(''.join(parts), encoding='utf-8', newline='\n')
print(f'Prepared {out}: {count} changed files; no backend mutation or test execution.')