HackThisSite Programming Challenge 1 Algorithm (Unscrambling)

Recently hackthissite.org was recommended to me — it’s really fun to play around with, even if I think some of the challenges are not that realistic any more.

I thought it would be just as much fun to post some of my solutions to the programming challenges here. If not absolutely neccessary for understanding the underlying algorithm, I won’t post any information about how to use the programs, because the purpose of these posts shall be to understand it, not to use it in order to solve the HTS challenges.

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Posted by Uli Köhler in Algorithms

VNC: Scale VNC window


You want to view a VNC on a screen with a different resolution than the screen you’re viewing (e.g. you want to view your Full HD desktop display on your Notebook).

Usually VNC viewers show scroll bars, but in some cases you need to view the entire screen at once.

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Posted by Uli Köhler in Linux

Check Scanner number input boundary in Java

In many cases if you want to create an interactive command line interface, you need to check if a number entered by the user is valid and – if it isn’t – you want the user to re-input it.

Here’s a simple static method to check if a number typed by a user is within a given boundary:

 * Lets the user input an integer value until it satisfies the given
 * conditions
 * @param msg The prompt to ask the user for the value.
 * @param lower The lower boundary, inclusive
 * @param upper The upper boundary, inclusive
private static int guardedInput(String msg, int lower, int upper) {
    Scanner scanner = new Scanner(System.in);
    while (true) {
        int val = -1;
        try {
        val = scanner.nextInt();
        } catch (InputMismatchException ex) {
        System.out.println("Illegal value: Please type a number!");
        if (val < lower) {
        System.out.println("Illegal value: Must be greater than " + (lower - 1));
        } else if (val > upper) {
        System.out.println("Illegal value: Must be smaller than " + (upper + 1));
        } else {
        return val;


Posted by Uli Köhler in Java

How to fix npm “Cannot find module ‘graceful-fs'” error


When running any npm command, you get a stacktrace similar to the following:

Error: Cannot find module 'graceful-fs'
at Function.Module._resolveFilename (module.js:338:15)
at Function.Module._load (module.js:280:25)
at Module.require (module.js:362:17)
at require (module.js:378:17)
at Object.<anonymous> (/usr/share/npm/lib/utils/ini.js:32:10)
   at Module._compile (module.js:449:26)
at Object.Module._extensions..js (module.js:467:10)
at Module.load (module.js:356:32)
at Function.Module._load (module.js:312:12)
at Module.require (module.js:362:17)

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Posted by Uli Köhler in NodeJS

How to program Nexys3 FPGA board on Linux using Digilent Adept software: A guide for beginners

Instead of having to buy expensive JTAG adapters like the Xilinx Platform Cable USB, Digilent FPGA boards like the Nexys3 support programming by using the Adept protocol being translation into JTAG internally. The Adept software is pretty easy to use if you know the basics of how to use the executables.

This is a beginner’s tutorial, so if you already have some experience with FPGAs, you might want to skip some parts. Still you need to have some knowledge about how to use the shell.


  • You got a .bit file compiled for your specific FPGA model. To generate this from VHDL or Verilog code, you should use the Xilinx ISE (WebPack is free, but you need to register!) – if you use third-party software the calls to the Xilinx Toolchain may also be encapsulated in a Makefile or similar.
  • You got your Nexys3 plugged into your computer using a standard micro-USB cable. Note that there are two micro USB ports on the Nexys3 board: One labeled UART and one labeled USB PROG. You need to insert the USB cable into the port labeled  USB PROG
  • The Nexys3 is turned on: You can turn it on and off using the switch adjacent to the USB PROG port. If the board is turned on, a red light will appear. Actually the switch doesn’t turn the board off but sets it to receive power from the 5.5×2.1mm barrel adapter adjacent to the switch.

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Posted by Uli Köhler in FPGA

C++11 : Iterate over smart pointers using foreach loop


In C++11 you want to iterate over a smart pointer (auto_ptr, shared_ptr, …). collection, say a std::vector, using the new for loop syntax.

Let’s try it out:

using namespace std;
shared_ptr<vector<int> > smartptr(/* A ptr to your vector */);
for(int s : smartptr) {
    /* do something useful */

When trying to compile this code, GCC emits the following error message (other lines are omitted for the sake of simplicity)

error: no matching function for call to 'begin(std::shared_ptr<std::vector<int> >&)'
error: no matching function for call to 'end(std::shared_ptr<std::vector<int> >&)'

or, when LANG=de is set:

Fehler: keine passende Funktion für Aufruf von »begin(std::shared_ptr<std::vector<int> >&)«
Fehler: keine passende Funktion für Aufruf von »end(std::shared_ptr<std::vector<int> >&)«

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Posted by Uli Köhler in C/C++

Automatically format size string in Node.js


In NodeJS, you got a size of a file in bytes, but you want to format it for better readability.
For example, if your size is 10000 bytes, you want to print 10 kilobytes, but if it is 1200000, you want to print 1.20 Megabytes.

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Posted by Uli Köhler in C/C++

How to get filesize in Node.js

To determine the size of a file in NodeJS (e.g. to get the size of myfile.txt) use fs.stat() or fs.statSync() like this:

const fs = require("fs"); //Load the filesystem module
const stats = fs.statSync("myfile.txt");
const fileSizeInBytes = stats.size;
//Convert the file size to megabytes (optional)
const fileSizeInMegabytes = fileSizeInBytes / 1000000.0;

Another option is to use the following function:

function getFilesizeInBytes(filename) {
    const stats = fs.statSync(filename);
    const fileSizeInBytes = stats.size;
    return fileSizeInBytes;


Posted by Uli Köhler in NodeJS