"""
Sphinx extension to add ReadTheDocs-style "Edit on GitHub" links to the
sidebar.

Loosely based on https://github.com/astropy/astropy/pull/347
"""

import os
import warnings

__licence__ = 'BSD (3 clause)'


def get_github_url(app, view, path):
    github_fmt = 'https://github.com/{}/{}/{}/{}{}'
    return (
        github_fmt.format(app.config.edit_on_github_project, view,
                          app.config.edit_on_github_branch,
                          app.config.edit_on_github_src_path, path))


def html_page_context(app, pagename, templatename, context, doctree):
    if templatename != 'page.html':
        return

    if not app.config.edit_on_github_project:
        warnings.warn("edit_on_github_project not specified")
        return
    if not doctree:
        warnings.warn("doctree is None")
        return
    path = os.path.relpath(doctree.get('source'), app.builder.srcdir)
    show_url = get_github_url(app, 'blob', path)
    edit_url = get_github_url(app, 'edit', path)

    context['show_on_github_url'] = show_url
    context['edit_on_github_url'] = edit_url


def setup(app):
    app.add_config_value('edit_on_github_project', '', True)
    app.add_config_value('edit_on_github_branch', 'master', True)
    app.add_config_value('edit_on_github_src_path', '', True)  # 'eg' "docs/"
    app.connect('html-page-context', html_page_context)