14 June 2021

The One Thing

Last year, during second lockdown, I watched a message of my guro Jakob where he proposed reading some books in the time when you are not able to practice in the dojo. He specifically recommended two books, The One Thing: The Surprisingly Simple Truth Behind Extraordinary Results and Atomic Habits: An Easy and Proven Way to Build Good Habits and Break Bad Ones. I had seen both books in my Goodreads feed and had not paid attention. But when your master asks, you better follow ;-)

FocusThe Book
The book is about extraordinary results determined by how narrow you can make your focus. Gary Keller insists that it is possible to reach any goal if you focus on it. And by focus he means that you really focus i.e. dedicate four hours of each day to your one goal. It has to be one single goal. And then, by the thousands of hours you have put into your goal, you will see results. Yes obviously, when following this approach, you will see results. In this sense, the One Thing is extraordinary and boring at the same time. I highly recommend reading it.

First I planned to summarise the book for you, and others have done a better job doing that. The book's homepage is a start and you can find many summaries of the book online, like this one on Four Minute Books. While summaries are short and save time, you will not get the benefit of the book without reading it. A book is not only about its content. Here the book is cementing its message by many times the author tells you the same thing in different words, a.k.a. repetition. The total time it takes to read the book is also time you have to reflect on the material.

Maybe my summary above is a bit lame, so let's discuss the book's impact on my life. And it had a profound impact as you will see. The first and utmost thing I remember from reading the book is the permission to drop stuff from my agenda. Like all developers I know, my daily plan was way too full: work, write a blog post like this one, release an episode of the Coderetreat Facilitation podcast, run a public Coding Dojo for the community, follow up on this cool idea I read about, finish reading SICP and get back playing with Scheme, create more exercises for workshops, keep up to date with Twitter and other feeds, work out, spend time with family, fix this next thing in the flat my wife is complaining about, fight climate change and much more. How was I supposed to allocate 4 (FOUR!) hours each day for the thing I want to do? It was impossible. Obviously Gary was crazy. ;-) Or I had to drop some tasks from my agenda. The idea of focus and dropping tasks was not new to me. I remembered J.B. Rainsberger mentioning a "Not Doing" column in his task list in 2014. I myself have put whole technologies on my blacklist - which I would ignore from this time on, but I have never dropped tasks and ideas so aggressively. I had several TODO lists of low priority where tasks went to die without being honest to myself.

The First Month
I spent the first month following the One Thing on a meta level because I did not work on anything but worked on the process itself. First I increased my focus: I deleted most icons - usually things to do or bookmarks to look at - from my desktop. I removed TODO markers in documents and on my hard drive. Then I reduced my commitments, e.g. I dropped the schedule to blog and record, stopped visiting meetups, reduced the number of learning hours and decided on the maximum number of hours I would work for each client per week. And I was OK with that. Letting go became a mental state and I had been following the Minimalists and Marie Kondo already before I started.

Suddenly I had more time - I even had some unscheduled time to play video games which I almost never did. In hindsight this was not extraordinary at all. If you drop most things you are working on, you will have more time available. It is a matter of focus. Lockdown and home office helped as I saved more than two hours every day. While some colleagues complained that they worked two hours more, I used these two hours - giving me half of my required four hours already. And I used the first month - 80 hours total - to practice the One Thing process. I started each day with a few minutes of mediation to improve my focus. I read more books about focus, e.g. Leo Babauta's book on focus and simplicity, forming habits, e.g. Atomic Habits as mentioned above and learning. I studied these books, taking notes and collecting them. I recommend all of them. I clean up my "office" and spent some time decluttering it, storing stuff in drawers or trashing it to further improve my focus during work.

One example of my achievements since I started using the One Thing methods are the books I have been able to read this year. The Pragmatic Programmers once wrote that you should read at least four technical books a year and the last year I was able to follow their rule was 2016. Since then I have been struggling to read at least one technical book a year. Reading became hard, I had problems focusing and I usually fell asleep after reading a few pages. I decided to add some reading time every day to my four hours of focused work, reading on the topic of focus of course. So including the One Thing, I have read ten books in five months.

Spending four hours on focused work each day reduced the time I had available for other things. After some struggle, I had more clarity - and stopped my self-deception. For example, if I see an interesting article linked on Twitter I either read it immediately, or I forget about it. There is no point in adding it to my read-later list, because there is no read later list because there is no extra time for that. I would not read it anyway, my read-later list kept growing and growing. While I was dreaming of doing so much in the past, I have become a naysayer now. For example "this is a good idea. I might try it. (One second later.) No, not really this is not my One thing and I have no time. End of idea." This is refreshing and freeing. Clarity is one of my needs.

Crunch Time
One drawback is that I am always in crunch time. As my time is more limited, I have to prioritize harder and postpone many tasks I know I need to do. This makes me feel uneasy at times. And for the things I still want to do - not as my One thing, but from time to time, e.g. maintaining my blog - I have to live with tiny increments. For example, I was working on this blog post 15 minutes at a time for more than a month. It was a bit frustrating. Still writing and "Public Relations" are not my priority.

Not having time to attend meetups caused some FOMO (fear of missing out) and made me miss the people of the community I had not seen in a while. In general being unable to do things which I liked mad me sad. I have been attending every one of our local, bi-monthly Coderetreats since its inception January 2019. When I was unable to attend for the first time in more than two years, I felt very, very sad. I felt a loss, almost like mourning. Maybe this was the first time in my life when I really let something go. It is easy to let go things you are not attached to.

Crunch Time Summary
Time for a very short summary (as my latest 15 minutes increment is almost spent): Read the book. Find your One Thing. Focus on it. It is possible to allocate four hours each day to focus on your highest priority, though it might be hard to stick to it. I am half a year into the process and did not miss a single day. Keep pushing!

29 April 2021

My Code Katas

Ascending the StairsI run various trainings for software developers, at least one every week. I have used many different exercises - called code katas - most of them curtesy of my fellow coaches, especially Emily Bache who maintains a huge collection of excellent exercises on her GitHub. And I created some exercises myself. I have collected all of them on a page, each with a short description how to use it. Enjoy!

Check out My Code Katas (permanent link).

20 March 2021

11 Years of Prime Factors Kata

In this post I want to celebrate the Prime Factors Code Kata. Prime Factors is a small coding exercise first used by Robert C. Martin in 2005. It is my favourite code kata and it has been almost nine years since I last wrote about it - time to change that. Weird enough, the first code kata I ever worked on - outside of university assignments that turned out to be katas later - was Roman Numerals in 2007. The first time I did the Prime Factors was during Christmas holidays 2009 in Java and Ruby:
import java.util.ArrayList;
import java.util.List;

public class PrimeFactors {
  public static List<Integer> generate(int n) {
    List<Integer> primes = new ArrayList<Integer>();

    for (int candidate = 2; candidate <= n; candidate++) {
      for (; n % candidate == 0; n /= candidate) {

    return primes;
Now the Java code is exactly the code Robert Martin showed, I was following his example. The Ruby version from back then looks pretty similar, just using while instead of for.
module PrimeFactors
  def generate(n)
    prime_factors = []

    candidate = 2
    while n > 1 do
      while n % candidate == 0 do
        prime_factors << candidate
        n /= candidate
      candidate += 1
      candidate = n if candidate > Math.sqrt(n) # performance fix

If you are wondering how I am still able to find the code, I organise my code katas to allow lookup and comparison. Since then I did the kata 141 times and it has many uses.

Learn a language
Prime Factors is one of the first pieces of code I write - Test Driven of course - when revisiting old languages, like Commodore BASIC or looking at a new language, like Forth, using Gforth 0.7:
: prime_factors ( n -- n1 n2 n3 n4 )
  DUP 1 > IF           \ test for ?DO
    DUP 2 ?DO
        DUP I MOD 0 =  \ test candidate I
        I SWAP I /     \ add candidate, reduce number

T{ 1 prime_factors -> }T
T{ 2 prime_factors -> 2 }T
T{ 3 prime_factors -> 3 }T
T{ 4 prime_factors -> 2 2 }T
T{ 6 prime_factors -> 2 3 }T
T{ 8 prime_factors -> 2 2 2 }T
T{ 9 prime_factors -> 3 3 }T
Gforth came with a modified testing framework based on John Hayes S1I's tester.fs, defining the functions T{, -> and }T for testing. Note that the given function prime_factors is not realistic as the number of returned arguments is not known by the caller.

When I had a look at Scala, of course I did Prime Factors:
import java.lang.Math.sqrt

object PrimeFactors {
  def generate(number: Int): List[Int] = {

    def fold(current: Pair[Int, List[Int]], candidate: Int): Pair[Int, List[Int]] = {
      if (current._1 % candidate == 0)
        fold((current._1 / candidate, candidate :: current._2), candidate)

    val (remainder, factors) =
      (2 to sqrt(number).intValue).foldLeft((number, List[Int]()))(fold)

    if (remainder > 1)
      (remainder :: factors).reverse
This is crazy. The code creates a sequence of all candidate primes and folds it starting from left by dividing by the candidate recursively, appending to the begin of the list, which is cheap. Because of that the list is reversed at the end. I have no idea why I created this, probably I was playing around with foldLeft. This is not a good example, please do not copy it. After all these years, the Forth solution seems easier to grasp. ;-)

So which languages are missing? PowerShell looks much like my PHP (shown below) and my Python Prime Factors looks similar too, just with Python specific range(2, number + 1) and //= inside. And of course JavaScript is missing:
PrimeFactors = function() {
  this.factors = [];

PrimeFactors.prototype.generate = function(num) {
  var candidate;
  for (candidate = 2; candidate <= num; candidate += 1) {
    num = this.tryCandidate(num, candidate);
  return this.factors;

PrimeFactors.prototype.tryCandidate = function(num, candidate) {
  while (num % candidate === 0) {
    num = this.reduceByFactor(num, candidate);
  return num;

PrimeFactors.prototype.reduceByFactor = function(num, factor) {
  return num / factor;
Isn't that lovely? Again this is not good code, please do not copy it. At least I showed some creativity using prototype functions.

Learn a testing framework
Using TDD to write some known code is a perfect way to learn more about a testing framework. So I explored XSLTunit using Prime Factors in XSLT or NUnit in C#:
using NUnit.Framework;

public class PrimeFactorsTest
  [TestCase(new int[0], 1)]
  [TestCase(new int[] { 2 }, 2)]
  [TestCase(new int[] { 3 }, 3)]
  [TestCase(new int[] { 2, 2 }, 4)]
  [TestCase(new int[] { 2, 2, 2 }, 8)]
  [TestCase(new int[] { 3, 3 }, 9)]
  public void TestFactors(int[] expected, int number)
    CollectionAssert.AreEqual(expected, PrimeFactors.Generate(number));

  public void TestLarge()
    CollectionAssert.AreEqual(new int[] { int.MaxValue },
Test your own testing framework
Sometimes, when I need to create my own unit testing framework, e.g. TPUnit for old Turbo Pascal or ASM Unit for Windows Assembly, I use Prime Factors as test cases:
  mov     esi, 0          ; esi = number of factors
  mov     edi, ebx        ; edi = address of factors
  mov     ecx, eax        ; ecx = current number
  mov     ebx, 1          ; ebx = candidate

  jmp .not_diviseable

  inc     ebx             ; next candidate

; if candidate * candidate <= number then try candidate
  mov     eax, ebx
  mul     ebx
  cmp     eax, ecx
  jbe     .try_candidate

; else number is a (large) prime and we are done
  mov     [edi + esi * register_size], ecx
  add     esi, 1
  jmp     .done

; if number % candidate == 0 then add candidate
  mov     eax, ecx
  xor     edx, edx
  div     ebx
  cmp     edx, 0          ; remainder is 0
  jne     .not_diviseable

  mov     [edi + esi * register_size], ebx
                          ; store candidate in factors
  add     esi, 1          ; we found a factor
  mov     ecx, eax        ; number is remainder of division
  jmp     .try_candidate  ; try current candidate again

; if number > 1 then try next candidate
  cmp     ecx, 1
  jne     .loop_over_candidates

; return number of factors
  mov     eax, esi
This piece of assembly calcultes the prime factors of the number passed in EAX into in the dword array address EBX.

TDD Demo
In 2012 I started practising Prime Factors as kata performance, minimising the number of keys I pressed. I ran it around 50 times. In the end I used the practice to calm down when I was anxious - it was like mediation. I still have to perform it somewhere, adding music and all... I have used it demoing TDD in uncounted presentations - actually around 40: during my university guest lectures, user group meetings and at my clients. Most demos were in Java and some in PHP,
class PrimeFactors {
  static function generate($n) {
    $factors = [];
    for ($candidate = 2; $candidate <= $n; $candidate += 1) {
      while ($n % $candidate == 0) {
        $factors[]= $candidate;
        $n /= $candidate;
    return $factors;
and a single demo of test driving R code,
primeFactors <- function(number) {
  factors <- vector(mode="integer")

  candidate <- 2
  while (candidate <= sqrt(number)) {
    while (number %% candidate == 0) {
      factors <- c(factors, candidate)
      number <- number / candidate
    candidate = candidate + 1

  if (number > 1) {
    factors <- c(factors, number)

It seems, most programming languages look the same. The last sentence is not true for NATURAL, Cobol's cousin, which is ugly. I will not show it here as it would destroy this lovely post.

By writing this post, I learned that I still need to create Prime Factors in the programming languages Go, Kotlin, OpenOffice Basic, Oracle PL/SQL and of course TypeScript - I could - and I will, it is just a matter of time. So Prime Factors - in fact any small enough code kata - is a great tool for exploring, studying and practising programming languages, testing frameworks, IDE tools and Test Driven Development in general. I will leave you with my latest addition to my collection of Prime Factors, using C99. Have fun!
#include <math.h>

typedef struct {
  unsigned char count;
  unsigned int values[31];
} PrimeFactors;

void PrimeFactors_init(PrimeFactors* factors)
  (*factors).count = 0;

void PrimeFactors_add(PrimeFactors* factors, const unsigned int factor)
  int count = (*factors).count;
  (*factors).values[count] = factor;
  (*factors).count = count + 1;

void generate(const unsigned int number, PrimeFactors* factors)

  unsigned int remaining = number;
  for (unsigned int candidate = 2; candidate <= sqrtl(remaining); candidate += 1) {
    while (remaining % candidate == 0) {
      PrimeFactors_add(factors, candidate);
      remaining /= candidate;

  if (remaining > 1) {
    PrimeFactors_add(factors, remaining);