"""
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 BaseLoader, get_template_from_string, find_template_loader, make_origin
from django.utils.encoding import force_bytes

class Loader(BaseLoader):
    is_usable = True

    def __init__(self, loaders):
        self.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 find_template(self, name, dirs=None):
        for loader in self.loaders:
            try:
                template, display_name = loader(name, dirs)
                return (template, make_origin(display_name, loader, name, dirs))
            except TemplateDoesNotExist:
                pass
        raise TemplateDoesNotExist(name)

    def load_template(self, template_name, template_dirs=None):
        key = template_name
        if template_dirs:
            # If template directories were specified, use a hash to differentiate
            key = '-'.join([template_name, hashlib.sha1(force_bytes('|'.join(template_dirs))).hexdigest()])

        if key not in self.template_cache:
            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.
                    return template, origin
            self.template_cache[key] = template
        return self.template_cache[key], None

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