from PySide2.QtCore import QObject, Slot, QSize, QUrl, Qt
from PySide2.QtGui import QImageReader, QImageWriter

import os
import pathlib
import hashlib
import time


class ThumbnailCache(QObject):
    """
    ThumbnailCache provides an abstraction for the thumbnail cache on disk, available in QML.

    For a given image file, it ensures the corresponding thumbnail exists (by creating it if necessary)
    and gives access to it.

    The default cache location is a subdirectory of the user's home directory:
    ~/Meshroom/thumbnails.
    This location can be overriden with the MESHROOM_THUMBNAIL_DIR environment variable.

    This class also takes care of cleaning the thumbnail directory,
    i.e. scanning this directory and removing thumbnails that have not been used for too long.
    The default time limit is one week.

    The main use case for thumbnails in Meshroom is in the ImageGallery.
    """

    # Thumbnail cache directory
    thumbnailDir = os.path.join(pathlib.Path.home(), 'Meshroom', 'thumbnails')

    # Thumbnail dimensions limit (the actual dimensions of a thumbnail will depend on the aspect ratio)
    thumbnailSize = QSize(100, 100)

    # Time limit for thumbnail storage on disk, expressed in days
    storageTimeLimit = 7

    @Slot(QUrl, result=QUrl)
    def thumbnail(self, imgSource):
        """
        Retrieve the filepath of the thumbnail corresponding to a given image.
        If the thumbnail does not exist on disk, it is created.

        Args:
            imgSource (QUrl): the filepath to the input image

        Returns:
            QUrl: the filepath to the corresponding thumbnail
        """
        # Safety check
        if not imgSource.isValid():
            return None

        imgPath = imgSource.toLocalFile()

        # Use SHA1 hashing to associate a unique thumbnail to the image
        digest = hashlib.sha1(imgPath.encode('utf-8')).hexdigest()
        path = os.path.join(ThumbnailCache.thumbnailDir, f'{digest}.jpg')
        source = QUrl.fromLocalFile(path)

        # Check if thumbnail already exists
        if os.path.exists(path):
            # Update last modification time
            pathlib.Path(path).touch(exist_ok=True)
            return source

        # Thumbnail does not exist, therefore we create it:
        # 1. read the image
        # 2. scale it to thumbnail dimensions
        # 3. write it in the cache
        print(f'[ThumbnailCache] Creating thumbnail {path} for image {imgPath}')

        # Initialize image reader object
        reader = QImageReader()
        reader.setFileName(imgPath)
        reader.setAutoTransform(True)

        # Read image and check for potential errors
        img = reader.read()
        if img.isNull():
            print(f'[ThumbnailCache] Error when reading image: {reader.errorString()}')
            return None

        # Make sure the thumbnail directory exists before writing into it
        os.makedirs(ThumbnailCache.thumbnailDir, exist_ok=True)

        # Scale image while preserving aspect ratio
        thumbnail = img.scaled(ThumbnailCache.thumbnailSize, aspectMode=Qt.KeepAspectRatio)

        # Write thumbnail to disk and check for potential errors
        writer = QImageWriter(path)
        success = writer.write(thumbnail)
        if not success:
            print(f'[ThumbnailCache] Error when writing thumbnail: {writer.errorString()}')
            return None

        return source

    @staticmethod
    def clean():
        """
        Scan the thumbnail directory and
        remove all thumbnails that have not been used for more than storageTimeLimit days.
        """
        # Get current time
        now = time.time()

        # Scan thumbnail directory and gather all thumbnails to remove
        toRemove = []
        for entry in os.scandir(ThumbnailCache.thumbnailDir):
            if not entry.is_file():
                continue

            # Compute storage duration since last usage of thumbnail
            lastUsage = os.path.getmtime(entry.path)
            storageTime = now - lastUsage
            print(f'[ThumbnailCache] Thumbnail {entry.name} has been stored for {storageTime}s')

            # Mark as removable if storage time exceeds limit
            if storageTime > ThumbnailCache.storageTimeLimit * 3600 * 24:
                print(f'[ThumbnailCache] {entry.name} exceeded storage time limit')
                toRemove.append(entry.path)

        # Remove all thumbnails marked as removable
        for path in toRemove:
            print(f'[ThumbnailCache] Remove {path}')
            os.remove(path)