# file_utils.py

## Purpose
File I/O operations for Nano Banana Streamlit. Centralized handling of image saving/loading, metadata management, filename generation, and directory operations.

## Responsibilities
- Generate safe, unique filenames with timestamps
- Save/load images to/from disk
- Save/load metadata as JSON
- Create standardized metadata dictionaries
- Compute image hashes for change detection
- Manage output directory structure
- List recent generations

## Dependencies

### Imports
- `json` - JSON serialization
- `hashlib` - Image hashing (SHA-256)
- `re` - Filename sanitization (regex)
- `datetime` - Timestamps
- `pathlib.Path` - Path operations
- `PIL.Image` - Image handling
- `config.settings.Settings` - Directory paths
- `utils.logging_utils.get_logger` - Logging

### Used By
- All services - Save generation results
- All pages - Load/display images
- Backend clients - Save API responses
- `models/generation_result.py` - Metadata creation

## Public Interface

### Filename Utilities

#### `sanitize_filename(name: str) -> str`
Remove unsafe characters from filename.

**Rules:**
- Removes: `< > : " / \ | ? *`
- Replaces with underscore
- Strips leading/trailing spaces and dots
- Limits to 100 characters
- Falls back to "generated" if empty

**Example:**
```python
safe = sanitize_filename("My Character: v2.0")
# Returns: "My_Character__v2_0"
```

#### `generate_timestamp_filename(base_name: str, extension: str = "png") -> str`
Generate filename with timestamp.

**Format:** `{base_name}_{YYYYMMDD_HHMMSS}.{extension}`

**Example:**
```python
filename = generate_timestamp_filename("character", "png")
# Returns: "character_20251023_143052.png"
```

#### `get_unique_filename(directory: Path, base_name: str, extension: str = "png") -> Path`
Generate unique filename that doesn't exist in directory.

If file exists, appends counter: `_1`, `_2`, etc.

**Example:**
```python
path = get_unique_filename(Settings.CHARACTER_SHEETS_DIR, "hero", "png")
# Returns: Path("outputs/character_sheets/hero_20251023_143052.png")
# If exists: Path("outputs/character_sheets/hero_20251023_143052_1.png")
```

### Image Operations

#### `save_image(image: Image, directory: Path, base_name: str, metadata: dict = None) -> Tuple[Path, Path]`
Save image and optional metadata.

**Parameters:**
- `image`: PIL Image to save
- `directory`: Target directory (created if doesn't exist)
- `base_name`: Base filename (will add timestamp)
- `metadata`: Optional metadata dict (saved as JSON)

**Returns:** `(image_path, metadata_path)` tuple

**Example:**
```python
metadata = {"prompt": "sunset", "backend": "Gemini"}
img_path, meta_path = save_image(
    image=generated_image,
    directory=Settings.CHARACTER_SHEETS_DIR,
    base_name="hero",
    metadata=metadata
)
# Saves:
#   outputs/character_sheets/hero_20251023_143052.png
#   outputs/character_sheets/hero_20251023_143052.json
```

#### `load_image(file_path: Path) -> Image`
Load image from disk.

**Raises:**
- `FileNotFoundError`: If file doesn't exist
- `IOError`: If file can't be read as image

**Example:**
```python
image = load_image(Path("outputs/character_sheets/hero_20251023_143052.png"))
```

### Metadata Operations

#### `save_metadata(file_path: Path, metadata: dict)`
Save metadata dictionary as JSON.

**Format:** Indented JSON with UTF-8 encoding

**Raises:** `IOError` if write fails

#### `load_metadata(file_path: Path) -> dict`
Load metadata from JSON file.

**Raises:**
- `FileNotFoundError`: If file doesn't exist
- `json.JSONDecodeError`: If invalid JSON

**Example:**
```python
meta = load_metadata(Path("outputs/character_sheets/hero_20251023_143052.json"))
prompt = meta["prompt"]
```

#### `create_generation_metadata(...) -> dict`
Create standardized metadata dictionary.

**Parameters:**
- `prompt`: Generation prompt (required)
- `backend`: Backend used (required)
- `aspect_ratio`: Aspect ratio (required)
- `temperature`: Temperature value (required)
- `input_images`: List of input image paths (optional)
- `generation_time`: Time taken in seconds (optional)
- `**kwargs`: Additional custom fields

**Returns:** Metadata dictionary with standard fields

**Standard Fields:**
- `timestamp`: ISO format timestamp
- `prompt`: Generation prompt
- `backend`: Backend name
- `aspect_ratio`: Aspect ratio string
- `temperature`: Temperature value
- `version`: Application version ("2.0.0-streamlit")
- `input_images`: List of input paths (if provided)
- `generation_time_seconds`: Time taken (if provided)

**Example:**
```python
metadata = create_generation_metadata(
    prompt="sunset over mountains",
    backend="Gemini API (Cloud)",
    aspect_ratio="16:9",
    temperature=0.4,
    generation_time=3.5,
    character_name="Hero",  # Custom field
    stage="front_portrait"   # Custom field
)
```

### Image Hashing

#### `compute_image_hash(image: Image) -> str`
Compute SHA-256 hash of image data.

Useful for detecting if input images have changed.

**Returns:** Hex string (64 characters)

**Example:**
```python
hash1 = compute_image_hash(image1)
hash2 = compute_image_hash(image2)
if hash1 == hash2:
    print("Images are identical")
```

### Directory Operations

#### `ensure_output_directories()`
Ensure all output directories exist.

Creates all directories defined in Settings if they don't exist.
Called on startup.

#### `get_output_directory_for_type(generation_type: str) -> Path`
Get appropriate output directory for generation type.

**Types:**
- `"character_sheet"` → `Settings.CHARACTER_SHEETS_DIR`
- `"wardrobe"` → `Settings.WARDROBE_CHANGES_DIR`
- `"composition"` → `Settings.COMPOSITIONS_DIR`
- `"standard"` → `Settings.STANDARD_DIR`

**Raises:** `ValueError` if unknown type

**Example:**
```python
output_dir = get_output_directory_for_type("character_sheet")
# Returns: Path("outputs/character_sheets")
```

#### `list_recent_generations(generation_type: str, count: int = 10) -> list`
List recent generation files in a directory.

**Returns:** List of `(image_path, metadata_path)` tuples, newest first

Metadata path is `None` if JSON file doesn't exist.

**Example:**
```python
recent = list_recent_generations("character_sheet", count=5)
for img_path, meta_path in recent:
    image = load_image(img_path)
    if meta_path:
        metadata = load_metadata(meta_path)
```

## Usage Examples

### Service Saving Output
```python
from utils.file_utils import save_image, create_generation_metadata, get_output_directory_for_type

class CharacterForgeService:
    def generate(self, prompt, backend, ...):
        # ... generation code ...

        # Create metadata
        metadata = create_generation_metadata(
            prompt=prompt,
            backend=backend,
            aspect_ratio="3:4",
            temperature=0.35,
            generation_time=elapsed_time,
            character_name=character_name,
            stage="front_portrait"
        )

        # Save image and metadata
        output_dir = get_output_directory_for_type("character_sheet")
        img_path, meta_path = save_image(
            image=generated_image,
            directory=output_dir,
            base_name=character_name,
            metadata=metadata
        )

        return img_path
```

### Page Displaying Recent Generations
```python
import streamlit as st
from utils.file_utils import list_recent_generations, load_image

st.subheader("Recent Character Sheets")

recent = list_recent_generations("character_sheet", count=4)

cols = st.columns(4)
for idx, (img_path, meta_path) in enumerate(recent):
    with cols[idx]:
        image = load_image(img_path)
        st.image(image, caption=img_path.stem, use_container_width=True)
```

### Loading Previous Generation
```python
from utils.file_utils import load_image, load_metadata

# User selects a previous generation
image_path = st.selectbox("Load previous", [...])

if image_path:
    # Load image
    image = load_image(Path(image_path))
    st.image(image)

    # Load metadata (if exists)
    meta_path = Path(image_path).with_suffix(".json")
    if meta_path.exists():
        metadata = load_metadata(meta_path)
        st.json(metadata)

        # Restore settings
        st.session_state.prompt = metadata["prompt"]
        st.session_state.backend = metadata["backend"]
```

## Error Handling

### File Operations
All functions raise appropriate exceptions:
- `FileNotFoundError`: File doesn't exist
- `IOError`: Read/write error
- `json.JSONDecodeError`: Invalid JSON
- `ValueError`: Invalid parameters

Errors are logged before raising.

### Automatic Recovery
- Directories created automatically if they don't exist
- Filename conflicts resolved with counter suffix
- Missing metadata handled gracefully (returns None)

## Known Limitations
- Filename length limit: 100 characters (base name)
- No image format conversion (saves as PNG only)
- No image compression options
- No batch operations
- No cloud storage integration
- Hash only detects exact pixel matches (not perceptual similarity)

## Future Improvements
- Support multiple image formats (JPEG, WEBP)
- Add image compression/quality options
- Add batch save/load operations
- Add cloud storage backends (S3, GCS)
- Add perceptual image hashing (pHash)
- Add image metadata embedding (EXIF)
- Add file cleanup/archiving utilities
- Add generation statistics tracking

## Testing
- Test sanitize_filename() with various unsafe characters
- Test generate_timestamp_filename() format
- Test get_unique_filename() collision handling
- Test save_image() creates files correctly
- Test load_image() with valid/invalid files
- Test save/load_metadata() round-trip
- Test create_generation_metadata() includes all fields
- Test compute_image_hash() consistency
- Test list_recent_generations() sorting

## Related Files
- `config/settings.py` - Directory path constants
- `utils/logging_utils.py` - Logging functions
- All services - Save generation results
- All pages - Load and display files
- `models/generation_result.py` - Uses metadata creation

## Security Considerations
- Filename sanitization prevents directory traversal
- No arbitrary file paths allowed (always in Settings directories)
- JSON encoding ensures no code injection
- File permissions inherited from parent directory

## Performance Considerations
- Image hashing loads full image into memory
- Large images may be slow to hash
- list_recent_generations() sorts by modification time (fast)
- JSON serialization is fast for typical metadata size

## Change History
- 2025-10-23: Initial creation for Streamlit migration
  - Centralized all file I/O operations
  - Added comprehensive filename handling
  - Added metadata standardization
  - Added directory management
  - Added recent generations listing
  - Integrated with Settings and logging