import os
import urequests
from sd_util import SDCardManager

def download_file(url, dest_filename, use_sd=True):
    """Downloads a file from a URL over the network in memory-efficient chunks.
    
    If use_sd is True, it automatically mounts the microSD card and saves to /sd/dest_filename.
    Otherwise, it saves to the local flash storage.
    
    Returns:
        str: The full path to the downloaded file on success, None on failure.
    """
    dest_path = dest_filename
    sd_manager = None
    
    if use_sd:
        sd_manager = SDCardManager(mount_point='/sd')
        if not sd_manager.mount():
            print("Download Error: Could not mount SD card.")
            return None
        dest_path = f"/sd/{dest_filename}"
        
    print(f"Starting download from: {url} -> {dest_path}")
    
    try:
        res = urequests.get(url, stream=True)
    except Exception as e:
        print(f"HTTP Connection failed: {e}")
        return None
        
    if res.status_code != 200:
        print(f"HTTP Error: Received status code {res.status_code}")
        res.close()
        return None
        
    try:
        # Read from socket stream in chunks to avoid OutOfMemory errors
        chunk_size = 4096
        total_downloaded = 0
        
        with open(dest_path, 'wb') as f:
            while True:
                # Read chunk from the raw socket connection stream
                chunk = res.raw.read(chunk_size)
                if not chunk:
                    break
                f.write(chunk)
                total_downloaded += len(chunk)
                
                # Dynamic logging
                if total_downloaded % (chunk_size * 25) == 0:
                     print(f"Downloaded {total_downloaded // 1024} KB...")
                     
        print(f"Download complete! Saved {total_downloaded} bytes to '{dest_path}'.")
        return dest_path
        
    except Exception as e:
        print(f"Error writing to file: {e}")
        # Clean up partial file on failure
        try:
            os.remove(dest_path)
        except:
            pass
        return None
        
    finally:
        res.close()