"""
Wrapper class that takes a list of template loaders as an argument and attempts
to load templates from them in order, caching the result.
"""

import hashlib
from django.template.base import TemplateDoesNotExist
from django.template.loader import get_template_from_string, find_template_loader, make_origin
from django.utils.encoding import force_bytes

from .base import Loader as BaseLoader


class Loader(BaseLoader):
    is_usable = True

    def __init__(self, loaders):
        self.template_cache = {}
        self.find_template_cache = {}
        self._loaders = loaders
        self._cached_loaders = []

    @property
    def loaders(self):
        # Resolve loaders on demand to avoid circular imports
        if not self._cached_loaders:
            # Set self._cached_loaders atomically. Otherwise, another thread
            # could see an incomplete list. See #17303.
            cached_loaders = []
            for loader in self._loaders:
                cached_loaders.append(find_template_loader(loader))
            self._cached_loaders = cached_loaders
        return self._cached_loaders

    def cache_key(self, template_name, template_dirs):
        if template_dirs:
            # If template directories were specified, use a hash to differentiate
            return '-'.join([template_name, hashlib.sha1(force_bytes('|'.join(template_dirs))).hexdigest()])
        else:
            return template_name

    def find_template(self, name, dirs=None):
        """
        Helper method. Lookup the template :param name: in all the configured loaders
        """
        key = self.cache_key(name, dirs)
        try:
            result = self.find_template_cache[key]
        except KeyError:
            result = None
            for loader in self.loaders:
                try:
                    template, display_name = loader(name, dirs)
                except TemplateDoesNotExist:
                    pass
                else:
                    result = (template, make_origin(display_name, loader, name, dirs))
                    break
        self.find_template_cache[key] = result
        if result:
            return result
        else:
            self.template_cache[key] = TemplateDoesNotExist
            raise TemplateDoesNotExist(name)

    def load_template(self, template_name, template_dirs=None):
        key = self.cache_key(template_name, template_dirs)
        template_tuple = self.template_cache.get(key)
        # A cached previous failure:
        if template_tuple is TemplateDoesNotExist:
            raise TemplateDoesNotExist
        elif template_tuple is None:
            template, origin = self.find_template(template_name, template_dirs)
            if not hasattr(template, 'render'):
                try:
                    template = get_template_from_string(template, origin, template_name)
                except TemplateDoesNotExist:
                    # If compiling the template we found raises TemplateDoesNotExist,
                    # back off to returning the source and display name for the template
                    # we were asked to load. This allows for correct identification (later)
                    # of the actual template that does not exist.
                    self.template_cache[key] = (template, origin)
            self.template_cache[key] = (template, None)
        return self.template_cache[key]

    def reset(self):
        "Empty the template cache."
        self.template_cache.clear()
        self.find_template_cache.clear()