"""
Wrapper for loading templates from "templates" directories in INSTALLED_APPS
packages.
"""

import os
import sys

from django.apps import apps
from django.conf import settings
from django.template.base import TemplateDoesNotExist
from django.template.loader import BaseLoader
from django.utils._os import safe_join
from django.utils import six


def calculate_app_template_dirs():
    if six.PY2:
        fs_encoding = sys.getfilesystemencoding() or sys.getdefaultencoding()
    app_template_dirs = []
    for app_config in apps.get_app_configs():
        template_dir = os.path.join(app_config.path, 'templates')
        if os.path.isdir(template_dir):
            if six.PY2:
                template_dir = template_dir.decode(fs_encoding)
            app_template_dirs.append(template_dir)
    return tuple(app_template_dirs)


# At compile time, cache the directories to search.
app_template_dirs = calculate_app_template_dirs()


class Loader(BaseLoader):
    is_usable = True

    def get_template_sources(self, template_name, template_dirs=None):
        """
        Returns the absolute paths to "template_name", when appended to each
        directory in "template_dirs". Any paths that don't lie inside one of the
        template dirs are excluded from the result set, for security reasons.
        """
        if not template_dirs:
            template_dirs = app_template_dirs
        for template_dir in template_dirs:
            try:
                yield safe_join(template_dir, template_name)
            except UnicodeDecodeError:
                # The template dir name was a bytestring that wasn't valid UTF-8.
                raise
            except ValueError:
                # The joined path was located outside of template_dir.
                pass

    def load_template_source(self, template_name, template_dirs=None):
        for filepath in self.get_template_sources(template_name, template_dirs):
            try:
                with open(filepath, 'rb') as fp:
                    return (fp.read().decode(settings.FILE_CHARSET), filepath)
            except IOError:
                pass
        raise TemplateDoesNotExist(template_name)