github_compare_button.js

// ==UserScript==
// @name         GitHub Compare Button
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  This script can add compare button to GitHub commit log.
// @author       Shogo Kawahara <kawahara@bucyou.net>
// @match        https://github.com/*
// @grant        none
// ==/UserScript==
/* jshint -W097 */
'use strict';

var fromCommitId, toCommitId, repository, lastPathName;
var reset = function() {
    fromCommitId = null;
    toCommitId = null;
    $('.from-btn').removeClass('selected');
    $('.to-btn').removeClass('selected');
};
var open = function() {
    if (fromCommitId && toCommitId) {
        window.open('http://github.com/' + repository + '/compare/' + fromCommitId + '...' + toCommitId);
        reset();
    }
};
var updateRepositoryUrl = function() {
    var match =  window.location.pathname.match(/^(\/[^\/]+\/[^\/]+)/)
    if (match) {
        repository = match[1];
    }
};
var addButton = function() {
    $('.commit-links-cell').each(function(i, obj) {
        var cell = $(obj);
        var commitId =$(cell.parent()).attr('data-url').match(/\/commit\/([0-9a-f]+)/)[1];
        var fromButton = $('<button>')
        .append($('<span>').attr('class', 'octicon octicon-git-compare'))
        .attr('class', 'btn btn-outline tooltipped tooltipped-s from-btn')
        .attr('aria-label', 'compare from');
        if (commitId == fromCommitId) {
            fromButton.addClass('selected');
        }
        var toButton = $('<button>')
        .append($('<span>').attr('class', 'octicon octicon-git-compare'))
        .attr('class', 'btn btn-outline tooltipped tooltipped-s to-btn')
        .attr('aria-label', 'compare to');
        if (commitId == toCommitId) {
            toButton.addClass('selected');
        }
        var group = $('<div>').attr('class', 'commit-links-group btn-group');
        group.append(fromButton);
        group.append(toButton);
        fromButton.click(function() {
            fromCommitId = commitId;
            $('.from-btn').removeClass('selected');
            fromButton.addClass('selected');
            open();
        });
        toButton.click(function() {
            toCommitId = commitId;
            $('.to-btn').removeClass('selected');
            toButton.addClass('selected');
            open();
        });
        cell.css('width', '300px')
        cell.prepend(group);
    });
    updateRepositoryUrl();
};

addButton();
lastPathName = window.location.pathname;

var dc = $('[data-pjax-container]')
var mo = new MutationObserver(function() {
    addButton();
    if (window.location.pathname != lastPathName) {
        lastPathName = window.location.pathname;
        reset();
    }
});
mo.observe(dc[0], {childList: true});