Archive for May, 2015

Upgrade Arch Linux for Raspberry Pi 2

Posted in Linux on May 23rd, 2015 by Pavel – 17 Comments

Raspberry Pi 2 model B v1.1 (see the note below)This articles shows how to transition from Raspberry Pi to Raspberry Pi 2 without reinstalling Arch Linux.

Arch Linux ARM images are tied to specific hardware architecture. While this may be seen as an inconvenience, there is an advantage in that Arch Linux binaries for Raspberry Pi 2 are compiled for ARM7 CPU and thus can benefit from the new instruction set.

Because ARM7 architecture is backward compatible with ARM6, it’s possible to upgrade your Raspberry Pi hardware to version 2 without reinstalling Arch Linux. Here is how we can do that (it goes without saying, that if you have any important data, consider backing it up first).
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Access your Raspberry Pi from anywhere

Posted in Linux on May 18th, 2015 by Pavel – 27 Comments

Console to Raspberry PiRaspberry Pi is an ideal platform for setting up personal services, like Git daemon, remote backup server, proxy server… you name it. However, while the device is small and handy it is still not so portable as a server, because conventional server installation usually includes domain name registration, setting up static IP addresses and configuring port forwarding on a router.

In this article I will show you how to utilize DDNS and UPnP technologies, so you will be able to simply plug your Raspberry Pi into an Ethernet port on (almost) any router and then securely access your own services from anywhere. The instructions are also applicable to a wireless connection (just use wlan0 instead of eth0 for Wi-Fi).

In a sense, the suggested approach solves the same problem as reverse SSH tunneling, but differently — there’s no need for a “visible” IP address on client side / middle machine (however, the provider’s IP on server side must be accessible), so it’s possible to establish connection with dynamic IPs, router’s NAT on server side and NAT on client side. Besides, this method is not limited to SSH and can handle other types of connections.
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Twitter, Puddles and foldlr

Posted in Programming on May 9th, 2015 by Pavel – 1 Comment

Blue Water DropsAbout a year ago I happen to devise a solution (spoiler!) to a puzzle latter known as “Twitter’s waterflow problem”. Despite the number of alternative solutions, my original approach still stand out in its own way, and can be used to demonstrate a few peculiar things about recursion and list folding.

The puzzle probably existed before Twitter itself, yet it received great popularity after Michael Kozakov’s post about his interview at Twitter. As Reddit commenter noticed, “luxury of time and hindsight are wonderful things indeed”, so here goes the problem specification.


We are given a list of numbers, representing walls of different height. The goal is to determine how much rain water can be accumulated between the walls.

Here’s a visual representation of [2,5,1,2,3,4,7,7,6]:


In this case, the amount of accumulated water is 10 (note that there are no implicit side walls).


Because of the puzzle popularity, many solutions, in different programming languages, were posted:

Approach, conciseness, performance and even correctness of those solutions vary greatly. What kind of solution do we want? It must be:

  • efficient (O(n)time complexity),
  • concise (1-5 lines of code),
  • compatible with linear-access sequences (like list),
  • preferably “functional”.

Now you may take your time and try to solve this problem by yourself before reading any further. In this way, you will not only enjoy a rather cute puzzle, but be able to comprehend with greater clarity what I’m going to show next.
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