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

import os
import pathlib
import hashlib
import time
import logging


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.
    This operation also ensures that the number of thumbnails on disk does not exceed a certain limit, 
    by removing thumbnails if necessary (from least recently used to most recently used).

    The default time limit is 90 days, 
    and can be overriden with the MESHROOM_THUMBNAIL_TIME_LIMIT environment variable.

    The default maximum number of thumbnails on disk is 100000, 
    and can be overriden with the MESHROOM_MAX_THUMBNAILS_ON_DISK.

    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 = 90

    # Maximum number of thumbnails in the cache directory
    maxThumbnailsOnDisk = 100000

    @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
        logging.debug(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():
            logging.error(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:
            logging.error(f'[ThumbnailCache] Error when writing thumbnail: {writer.errorString()}')
            return None

        return source

    @staticmethod
    def clean():
        """
        Scan the thumbnail directory and: 
        1. remove all thumbnails that have not been used for more than storageTimeLimit days
        2. ensure that the number of thumbnails on disk does not exceed maxThumbnailsOnDisk.
        """
        # Check if thumbnail directory exists
        if not os.path.exists(ThumbnailCache.thumbnailDir):
            logging.debug('[ThumbnailCache] Thumbnail directory does not exist yet.')
            return

        # Get current time
        now = time.time()

        # Scan thumbnail directory and gather all thumbnails to remove
        toRemove = []
        remaining = []
        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
            logging.debug(f'[ThumbnailCache] Thumbnail {entry.name} has been stored for {storageTime}s')

            if storageTime > ThumbnailCache.storageTimeLimit * 3600 * 24:
                # Mark as removable if storage time exceeds limit
                logging.debug(f'[ThumbnailCache] {entry.name} exceeded storage time limit')
                toRemove.append(entry.path)
            else:
                # Store path and last usage time for potentially sorting and removing later
                remaining.append((entry.path, lastUsage))

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

        # Check if number of thumbnails on disk exceeds limit
        if len(remaining) > ThumbnailCache.maxThumbnailsOnDisk:
            nbToRemove = len(remaining) - ThumbnailCache.maxThumbnailsOnDisk
            logging.debug(f'[ThumbnailCache] Too many thumbnails: {len(remaining)} remaining, {nbToRemove} will be removed')

            # Sort by last usage order and remove excess
            remaining.sort(key=lambda elt: elt[1])
            for i in range(nbToRemove):
                path = remaining[i][0]
                logging.debug(f'[ThumbnailCache] Remove {path}')
                os.remove(path)