// ==UserScript==
// @name          GitHub First Commit
// @description   Add a link to a GitHub repo's first commit
// @author        chocolateboy
// @copyright     chocolateboy
// @version       2.7.0
// @namespace     https://github.com/chocolateboy/userscripts
// @license       GPL: https://www.gnu.org/copyleft/gpl.html
// @include       https://github.com/
// @include       https://github.com/*
// @require       https://cdn.jsdelivr.net/npm/cash-dom@8.1.0/dist/cash.min.js
// @grant         GM_log
// @inject-into   auto
// @downloadURL none
// ==/UserScript==

const COMMIT_BAR = 'div.js-details-container[data-issue-and-pr-hovercards-enabled] > *:last-child ul'
const FIRST_COMMIT_LABEL = '<span aria-label="First commit"><strong>1st</strong> commit</span>'

// this function extracts the URL of the repo's first commit and navigates to it.
// it is based on code by several developers, a list of whom can be found here:
// https://github.com/FarhadG/init#contributors
//
// XXX it doesn't work on private repos. a way to do that can be found here,
// but it requires an authentication token:
// https://gist.github.com/simonewebdesign/a70f6c89ffd71e6ba4f7dcf7cc74ccf8
function openFirstCommit (user, repo) {
    return fetch(`https://api.github.com/repos/${user}/${repo}/commits`)
        // the `Link` header has additional URLs for paging.
        // parse the original JSON for the case where no other pages exist
        .then(res => Promise.all([res.headers.get('link'), res.json()]))

        .then(([link, commits]) => {
            if (link) {
                // the link header contains two URLs and has the following
                // format (wrapped for readability):
                //
                //     <https://api.github.com/repositories/1234/commits?page=2>;
                //     rel="next",
                //     <https://api.github.com/repositories/1234/commits?page=9>;
                //     rel="last"

                // extract the URL of the last page (commits are ordered in
                // reverse chronological order, like the git CLI, so the oldest
                // commit is on the last page)
                const lastPage = link.match(/^.+?<([^>]+)>;/)[1]

                // fetch the last page of results
                return fetch(lastPage).then(res => res.json())
            }

            // if there's no link, we know we're on the only page
            return commits
        })

        // get the last commit and navigate to its target URL
        .then(commits => {
            location.href = commits[commits.length - 1].html_url
        })
}

// add the "First commit" link as the last child of the commit bar
//
// XXX on most sites, hitting the back button takes you back to a snapshot of
// the previous DOM tree, e.g. navigating away from a page on Hacker News
// highlighted via Hacker News Highlighter [1], then back to the highlighted
// page, retains the DOM changes (the highlighting). that isn't the case on
// GitHub, which triggers network requests and fragment/page reloads when
// navigating (back) to any (SPA) page, even when not logged in. this means, for
// example, we don't need to check if the first-commit widget already exists (it
// never does), and don't need to restore its old label when navigating away
// from a page (since the widget will always be created from scratch rather than
// reused)
//
// this has not always been the case, and may not be the case in the future (and
// may not be the case in some scenarios I haven't tested, e.g. on mobile), so
// rather than taking it for granted, we assume the site behaves like every
// other site in this respect and code to that (defensive coding). this costs
// nothing (apart from this explanation) and saves us having to scramble for a
// fix if the implementation changes.
//
// [1] https://greasyfork.org/en/scripts/39311-hacker-news-highlighter
function run () {
    const $commitBar = $(COMMIT_BAR)

    // bail if it's not a repo page
    if (!$commitBar.length) {
        return
    }

    // bail if the widget already exists
    let $firstCommit = $commitBar.find('#first-commit')

    if ($firstCommit.length) {
        return
    }

    /*
     * This is the first LI in the commit bar (UL), which we clone to create the
     * "First commit" widget.
     *
     *   <li class="ml-3">
     *     <a data-pjax="" href="/foo/bar/commits/master" class="link-gray-dark no-underline">
     *       <svg height="16">...</svg>
     *
     *       <span class="d-none d-sm-inline">
     *           <strong>42</strong>
     *           <span aria-label="Commits on master">commits</span>
     *       </span>
     *     </a>
     *   </li>
     */

    // create it
    $firstCommit = $commitBar
        .find('li')
        .eq(0)
        .clone()
        .attr('id', 'first-commit')

    const $link = $firstCommit
        .find('a')
        .removeAttr('href')
        .css('cursor', 'pointer')

    const $label = $(FIRST_COMMIT_LABEL)

    $link.find(':scope > span').empty().append($label)

    const [user, repo] = $('meta[name="octolytics-dimension-repository_network_root_nwo"]')
        .attr('content')
        .split('/')

    // before navigating away from the page, restore the original label. this
    // ensures it has the correct value if we navigate back to the repo page
    // without making a new request (e.g. via the back button)
    const oldLabelHtml = $label.html()

    $(window).on('unload', () => {
        $label.html(oldLabelHtml)
    })

    $link.on('click', () => {
        $label.text('Loading...')
        openFirstCommit(user, repo)
        return false // stop processing the click
    })

    $commitBar.append($firstCommit)
}

$(document).on('pjax:end', run) // run on pjax page loads
$(run) // run on full page loads