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

import io
import os
import sys

from django.apps import apps
from django.conf import settings
from django.core.exceptions import SuspiciousFileOperation
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():
        if not app_config.path:
            continue
        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 SuspiciousFileOperation:
                # The joined path was located outside of this template_dir
                # (it might be inside another one, so this isn't fatal).
                pass

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