import requests
import os
import shutil

def download_file(url: str, save_dir: str, filename: str = None) -> str | None:
    """
    Downloads a file from a URL and saves it to the specified directory.

    Args:
        url: The URL of the file to download.
        save_dir: The directory where the file should be saved.
        filename: Optional. The name to save the file as. 
                  If None, tries to infer from URL or Content-Disposition.

    Returns:
        The full path to the downloaded file if successful, None otherwise.
    """
    if not url:
        print("Error: Download URL cannot be empty.")
        return None

    os.makedirs(save_dir, exist_ok=True)

    try:
        with requests.get(url, stream=True, timeout=30) as r:
            r.raise_for_status() # Raise an exception for bad status codes

            if not filename:
                # Try to get filename from Content-Disposition header
                content_disposition = r.headers.get('content-disposition')
                if content_disposition:
                    import re
                    fname_match = re.findall('filename="?([^"]+)"?', content_disposition)
                    if fname_match:
                        filename = fname_match[0]
                if not filename: # Fallback to last part of URL
                    filename = url.split('/')[-1]
                    if not filename: # If URL ends with '/', generate a name
                        filename = "downloaded_file" 
            
            # Sanitize filename (basic example, might need more robust sanitization)
            filename = "".join(c for c in filename if c.isalnum() or c in ['.', '_', '-']).strip()
            if not filename: # If sanitization results in empty filename
                filename = "downloaded_file_unnamed"

            local_filepath = os.path.join(save_dir, filename)

            with open(local_filepath, 'wb') as f:
                shutil.copyfileobj(r.raw, f)
            
            print(f"Successfully downloaded '{filename}' to '{local_filepath}'")
            return local_filepath
    except requests.exceptions.RequestException as e:
        print(f"Error downloading file from {url}: {e}")
        return None
    except IOError as e:
        print(f"Error saving file to {local_filepath}: {e}")
        return None
    except Exception as e:
        print(f"An unexpected error occurred during download: {e}")
        return None

if __name__ == '__main__':
    # Example Usage:
    test_url = "https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf" # A sample PDF
    download_dir = "test_downloads"
    
    # Test 1: Basic download
    print("\n--- Test 1: Basic Download ---")
    downloaded_path = download_file(test_url, download_dir)
    if downloaded_path and os.path.exists(downloaded_path):
        print(f"Test 1 Success: File at {downloaded_path}")
    else:
        print("Test 1 Failed.")

    # Test 2: Download with specified filename
    print("\n--- Test 2: Download with specified filename ---")
    custom_filename = "my_custom_dummy.pdf"
    downloaded_path_custom = download_file(test_url, download_dir, filename=custom_filename)
    if downloaded_path_custom and os.path.exists(downloaded_path_custom) and os.path.basename(downloaded_path_custom) == custom_filename:
        print(f"Test 2 Success: File at {downloaded_path_custom}")
    else:
        print("Test 2 Failed.")

    # Test 3: Invalid URL
    print("\n--- Test 3: Invalid URL ---")
    invalid_url = "http://invalid.url/nonexistentfile.txt"
    downloaded_path_invalid = download_file(invalid_url, download_dir)
    if downloaded_path_invalid is None:
        print("Test 3 Success: Handled invalid URL correctly.")
    else:
        print("Test 3 Failed.")
    
    # Cleanup (optional)
    # if os.path.exists(download_dir):
    #     import shutil
    #     shutil.rmtree(download_dir)
    #     print(f"\nCleaned up '{download_dir}' directory.")