# Download Filename Format Fix

## Problem
Songs downloaded from artist detail page and remote search results sometimes download the wrong file because:
- Multiple songs with the same title but different artists exist
- yt-dlp might pick a different version when searching by title only
- File organization is poor without artist information in filename

## Solution
Changed download filename format from:
```
Song Title.mp3
```

To:
```
Artist Name - Song Title.mp3
```

## Changes Made

### Backend

#### 1. `backend/app/api/download.py`
- Updated `process_download()` to accept `artist` parameter
- Format filename as `"{artist} - {title}"` when artist is provided
- Pass artist from request to background task

**Before:**
```python
success, file_path, error = await music_downloader.download_music(url, title)
```

**After:**
```python
output_filename = f"{artist} - {title}" if artist else title
success, file_path, error = await music_downloader.download_music(url, output_filename)
```

#### 2. `backend/app/api/auto_download.py`
- Updated auto-download to format filename with artist
- Uses artist from `selected_result`

**Before:**
```python
success, file_path, error = await music_downloader.download_music(
    selected_result.url,
    selected_result.title
)
```

**After:**
```python
output_filename = f"{selected_result.artist} - {selected_result.title}" if selected_result.artist else selected_result.title
success, file_path, error = await music_downloader.download_music(
    selected_result.url,
    output_filename
)
```

#### 3. `backend/app/services/downloader.py`
- Added `sanitize_filename()` function to handle special characters
- Prevents filesystem errors from characters like `/`, `\`, `:`, `*`, etc.
- Limits filename length to 200 characters

**New function:**
```python
def sanitize_filename(filename: str) -> str:
    """Sanitize filename by removing/replacing problematic characters"""
    replacements = {
        '/': '-',
        '\\': '-',
        ':': '-',
        '*': '',
        '?': '',
        '"': "'",
        '<': '',
        '>': '',
        '|': '-',
    }
    # ... sanitization logic
```

### Frontend

#### 4. `frontend/src/components/search/SearchPage.tsx`
- Fixed mutation to actually pass `artist` and `thumbnail` to API
- Was accepting parameters but not sending them

**Before:**
```typescript
mutationFn: ({ url, title }: { url: string; title: string; thumbnail?: string; artist?: string }) => 
  downloadApi.downloadMusic({ url, title }),
```

**After:**
```typescript
mutationFn: ({ url, title, thumbnail, artist }: { url: string; title: string; thumbnail?: string; artist?: string }) => 
  downloadApi.downloadMusic({ url, title, thumbnail, artist }),
```

## Examples

### Example 1: Search Page Download
**User searches for:** "Shape of You"

**Before:**
- Filename: `Shape of You.mp3`
- Could download any "Shape of You" (Ed Sheeran, cover, remix, etc.)

**After:**
- Filename: `Ed Sheeran - Shape of You.mp3`
- Downloads the correct version with artist specified

### Example 2: Artist Detail Page
**User is on "Taylor Swift" artist page, downloads "Love Story"**

**Before:**
- Filename: `Love Story.mp3`
- Might download a different artist's "Love Story"

**After:**
- Filename: `Taylor Swift - Love Story.mp3`
- Guaranteed to download the correct artist's version

### Example 3: Special Characters
**Song:** `AC/DC - Back In Black: Remastered`

**After sanitization:**
- Filename: `AC-DC - Back In Black- Remastered.mp3`
- Safe for all filesystems

## Testing

### Test Basic Download
```bash
# Via API
curl -X POST "http://localhost:8000/api/download/music" \
  -H "Content-Type: application/json" \
  -d '{
    "url": "https://youtube.com/watch?v=xxx",
    "title": "Shape of You",
    "artist": "Ed Sheeran"
  }'

# Check the downloaded file
ls -la data/music/
# Should see: "Ed Sheeran - Shape of You.mp3"
```

### Test Auto Download
```bash
# From artist page, download "Love Story" from Taylor Swift
# Should create: "Taylor Swift - Love Story.mp3"
```

### Test Special Characters
```bash
# Download song with special characters in name
curl -X POST "http://localhost:8000/api/download/music" \
  -H "Content-Type: application/json" \
  -d '{
    "url": "https://youtube.com/watch?v=xxx",
    "title": "Don'\''t Stop Me Now",
    "artist": "Queen"
  }'

# Should create: "Queen - Don't Stop Me Now.mp3"
```

## Migration

No database migration needed. Existing files are not affected.

New downloads will follow the new format:
- ✅ Downloads from search page: `Artist - Title.mp3`
- ✅ Downloads from artist detail page: `Artist - Title.mp3`
- ✅ Auto-downloads: `Artist - Title.mp3`

## Benefits

1. **Prevents wrong downloads** - Artist name ensures correct version
2. **Better organization** - Easy to identify songs by filename
3. **Consistent naming** - All downloads follow same format
4. **Filesystem safe** - Special characters are sanitized
5. **Backward compatible** - Old files still work

## Files Changed

- `backend/app/api/download.py` - Pass artist to formatter
- `backend/app/api/auto_download.py` - Format auto-download filenames
- `backend/app/services/downloader.py` - Add sanitization function
- `frontend/src/components/search/SearchPage.tsx` - Fix mutation to pass artist

## Related Issues

This fix addresses:
- Wrong songs being downloaded when title is ambiguous
- Poor file organization without artist info
- Filesystem errors from special characters in filenames
- Inconsistent naming across different download sources