While following a tutorial to install Virtualbox in order to have docker working on macOS, I
hit an issue where the docker-machine create command fails with an error that looks like this:
VBoxManage: error: Failed to create the host-only adapter
VBoxManage: error: VBoxNetAdpCtl: Error while adding new interface: failed to open /dev/vboxnetctl: No such file or directory
VBoxManage: error: Details: code NS_ERROR_FAILURE (0x80004005), component HostNetworkInterfaceWrap, interface IHostNetworkInterface
VBoxManage: error: Context: "RTEXITCODE handleCreate(HandlerArg *)" at line 95 of file VBoxManageHostonly.cpp
If you search on the Web, everybody says you have to open the Security & Privacy settings window
and allow the Oracle kernel extensions to run. But I didn’t have it. I tried uninstalling
Virtualbox, re-installing through the official website, reboot, uninstall, re-install with
brew cask but I always had the issue. Some people reported having a failed Virtualbox
installation but mine seemed ok.
I tried the spctl solution but it didn’t change anything.
In the end, I tried this StackOverflow answer:
sudo "/Library/Application Support/VirtualBox/LaunchDaemons/VirtualBoxStartup.sh" restart
It failed, but it told me to check the Security & Privacy setting window. I did, and I had the
button everyone was talking about. I enabled the kernel extension, rebooted, and it worked.
Hope this can save some time to anyone having the same issue!
Code-Golf is the art of writing the shortest program in a given language that
implements some given algorithm. It started in the 90’s in the Perl community
and spread to other languages; there are now languages dedicated to
code-golfing and StackExchange has a Q&A website for it.
In 2015, for example, I wrote a blog post showing how to write a JavaScript modules manager that fits in 140 chars (the maximum length
of a tweet at that time).
4clojure is a well-known website to learn Clojure
through exercises of increasing difficulty, but it has a lesser-known code-golf
challenge which you can enable by clicking on “Leagues” in the top menu.
If you check the code-golf checkbox, you then get a score on each problem that
is the number of non-whitespace characters of your solution; the smaller the
better.
The first thing you’ll note when code-golfing is that the reader syntax for
anonymous functions is a lot shorter than using fn:
; 18 chars
(fn [a b c] (* (+ a b) c))
; 13 chars
#(* (+ %1 %2) %3)
; 12 chars: -1 char because '%' is equivalent to '%1'
#(* (+ % %2) %3)
Unfortunately you can’t have a reader-syntax function inside another
reader-syntax one, so you often have to transform the code not to use anonymous
functions.
for is a very powerful tool for that, because it allows you to do the
equivalent of map, and a lot more, with no function:
; invalid!
#(map #(* 2 %) %)
; 19 chars
#(map (fn [x] (* 2 x)) %)
; 17 chars
#(map (partial * 2) %)
; 15 chars
#(for [x %] (* 2 x))
; Note that for this specific example
; the best solution uses `map`:
#(map + % %)
On some problems it can even be shorter than using map + filter:
; 31 chars
(fn [x a]
(map inc (filter #(< % a) x)))
; 26 chars
#(for [e x :when (< e a)] (inc e))
Some core functions are equivalent in some contexts and so the shorter one can
substitute a longer one:
; 18 chars
#(filter identity %)
; 14 chars
#(filter comp %)
; 6 chars
(inc x)
(dec x)
; 5 chars
(+ x 1)
(- x 1)
; 12 chars
(reduce str x)
; 11 chars
(apply str x)
; 14 chars
(apply concat x)
; 13 chars
(mapcat comp x)
When you must use a long function name in multiple places, it might be shorter
to let that function with a one-letter symbol:
; 120 chars
#(clojure.set/difference
(clojure.set/union % %2)
(clojure.set/union
(clojure.set/difference % %2)
(clojure.set/difference %2 %)))
; 73 chars
#(let [d clojure.set/difference u clojure.set/union]
(d (u % %2) (u (d % %2) (d %2 %))))
; Note that for this specific example
; there is a 17-chars solution
#(set (filter %2 %))
Other tricks
Use indexed access on vectors:
; 15 chars
(first [:a :b :c])
; 11 chars
([:a :b :c] 0)
Use set literals as functions:
; 16 chars
(remove #(= :a %) x)
; 14 chars
(remove #{:a} x)
Inverse conditions to use shorter functions:
; 15 chars
(if (empty? p) a b)
; 12 chars
(if (seq p) b a)
Inlined code is sometimes shorter:
; 24 chars
(let [p (* 3 a)]
(if (< p 5)
a
p))
; 19 chars
(if (< (* 3 a) 5)
a
(* 3 a))
Use 1 instead of :else/:default in cond:
; 24 chars
(cond
(= m p) a
(< m p) b
:else c)
; 20 chars
(cond
(= m p) a
(< m p) b
1 c)
Use maps instead of ifs for conditions on equality (this one really makes
the code harder to read):
; 13 chars
(if (= "L" x) a b)
; 12 chars
(case x "L" a b)
; 10 chars
({"L" a} x b)
For the latest StackExchange “time”-themed contest, I made
a gif showing the evolution of StackOverflow from 2008 to today:
(click on the image to play it again)
The first step was to find a decent API for the Internet Archive. It
supports Memento, an HTTP-based protocol defined in the RFC 7089 in
- Using the
memento_client wrapper, we can get the closest snapshot
of a website at a given date with the following Python code:
from datetime import datetime, timedelta
from memento_client import MementoClient
mc = MementoClient(timegate_uri="https://web.archive.org/web/",
check_native_timegate=False)
def get_snapshot_url(url, dt):
info = mc.get_memento_info(url, dt)
closest = info.get("mementos", {}).get("closest")
if closest:
return closest["uri"][0]
# As an example, let’s look at StackOverflow two weeks ago
url = "https://stackoverflow.com/"
two_weeks_ago = datetime.now() - timedelta(weeks=2)
snapshot_url = get_snapshot_url(url, two_weeks_ago)
print("StackOverflow from ~2 weeks ago: %s" % snapshot_url)
Don’t forget to install the memento_client lib:
pip install memento_client
Note this gives us the closest snapshot, so it might not be exactly two
weeks ago.
We can use this code to loop using an increasing time delta in order to get
snapshots at different times. But we don’t only want to get the URLs. We wants
to make a screenshot of each one.
The easiest way to programmatically take a screenshot of a webpage is probably
to use Selenium. I used Chrome as a driver; you can either download
it from the ChromeDriver website or run the following command
if you’re on a Mac with Homebrew:
brew install bfontaine/utils/chromedriver
We also need to install Selenium for Python:
The code is pretty short:
from selenium import webdriver
def make_screenshot(url, filename):
driver = webdriver.Chrome("chromedriver")
driver.get(url)
driver.save_screenshot(filename)
driver.quit()
url = "https://web.archive.org/web/20181119211854/https://stackoverflow.com/"
make_screenshot(url, "stackoverflow_20181119211854.png")
If you run the code above, you should see a Chrome window open, go at the URL
by itself, then close once the page is fully charged. You now have a screenshot
of this page in stackoverflow_20181119211854.png! However, you’ll quickly
notice the screenshot includes the Wayback Machine’s header over the top of the
website:
This is handy when browsing through snapshots by hand, but not so much when we
access them from Python.
Fortunately, we can get a header-less URL by changing it a bit: we can
append id_ to the end of the date in order to get the page exactly as it was
when the bot crawled it. However, this means it links to CSS and JS files that
may not exist anymore. We can get a URL to an archived page that has been
slightly modified to replace links with their archived version using im_
instead.
- Page with header and rewritten links:
https://web.archive.org/web/20181119211854/...- Original page, as it was when crawled:
https://web.archive.org/web/20181119211854id_/...- Original page with rewritten links:
https://web.archive.org/web/20181119211854im_/...
Re-running the code using the modified URL gives us the correct screenshot:
url = "https://web.archive.org/web/20181119211854im_/https://stackoverflow.com/"
make_screenshot(url, "stackoverflow_20181119211854.png")
Joining the two bits of code we can make screenshots of a URL at different
intervals. You may want to check the images once it’s done to remove
inconsistencies. For example, the archived snapshots of Google’s homepage
aren’t all in the same language.
Once we have all images, we can generate a gif using Imagemagick:
convert -delay 50 -loop 1 *.png stackoverflow.gif
I used the following parameters:
-delay 50: change frame every 0.5s. The number is in 100th of a second.-loop 1: loop only once over all the frames. The default is to make an
infinite loop but it doesn’t really make sense here.
You may want to play with the -delay parameter depending on how many images
you have as well as how often the website changes.
I also made a version with Google (~10MB) at 5 frames per second,
with -delay 20. I used the same delay
as the StackOverflow gif: at least 5 weeks between each screenshot. You
can see which year the screenshot is from by looking at the bottom of each
image.
