# Diary, April 2020

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## Wednesday, April 1, 2020

### Bidirectional pseudo-random generator

A pseudo-random generator is a random generator which generates a very long sequence of numbers, which repeat exactly after a certain number of steps. This number is also called the cycle length. Often, when the state of the random number generator consists of n bits, it is possible to achieve a cycle length of a the n-th power of two minus one, meaning that the sequence contains all the possible values except for one. Because the sequences are usually too long to store in memory, an algorithm is used to calculate the next number. There are also cases where you would like to go back in the sequence and calculate the previous number in the sequence. Now, for one of the most simple random generators, Fibonacci LFSRs, which is based on shift-registers and the exclusive or operator, this is not very difficult. With these you have n bits and to calculate the next bit you take the exclusive or operation from the most right bit and some other bit and assign this to the most left bit after you have shifted all other bits one to the right. In a sense, it is like adding another bit to the left of the n and than shift the 'frame' of n bits one to the left. An important property of the exclusive or operator, which is often denoted with ⊕, is that when ab=c then a=bc. For this reason, when taking n bits, it is possible to calculate the bit that was on the right, by taking the exclusive or operation of the most left bit and the one other bit that was involved. Thus by shifting the frame back to the right, you get the previous value and can keep on doing this as long as you want. The following two C expression can be used to calculate the next and the previous value in a sequence defined by the constants A and B.
```v = ((((1 << (B-1))-1) & v) << 1) | (1 & ((v >> (A-1)) ^ (v >> (B-1))));

v = (v >> 1) | ((1 & (v ^ (v >> A))) << (B-1));
```
For example, when A is 5 and B is 9, a sequence of 511 values of 9 bits are produced. But with larger values for A and B, much larger sequences can be produced.

## Saturday, April 4, 2020

### Trees of random number generation

I have been thinking about dynamic procedural world generation for a long time. The idea of procedural generation is already very old and it has been applied to many computer games. I am not into computer games, although there are some games with really beautifull worlds. The problem that I have with most computer games, is the game aspect that requires you to fight or at least to achieve some goal. I often just want to look around, investigate a world and just experience the views. Many of the Windows scenic background make me want to look around. If you want to wander around, you need a large world, and than it is quite logical to make use of dynamic procedural generation, where you generate the part of the world that is visible. The parts that are closer, need to have more detail than those that are further away. The idea is to change the level of detail. At the highest level, you could divide the world into squares, where to each square you assign a random number based on two bidirectional pseudo-random generator. That random number is then used to generate the contents of the square. It could for example being used to generate a height map using perlin noise. The idea is to use the random number as the seed (or starting value) of another pseudo-random generator (perferable with a different cycle length). To generate a realistic landscape, one might want to use a number of overlapping perlin noise maps as explained in the video Procedural Landmass Generation (E01: Introduction) by Sebastian Lague. Each noise map has a higher frequency, but a lower amplitute. For parts that are further away from the viewer, the noise maps with a higher frequency and a lower amplitute are not visible. So, there is actually no reason to calculate them. The idea is to subdivide the square in smaller squares and only generate these noise maps for the nearby smaller squares. Somehow, this means that for every smaller square, we need another pseudo-random generator. In case of a noise map, it is possible to another set of two bidirectional pseudo-random generators. But is we are generating objects in the landscape, where the number of objects differ per square, this is no longer easy, especially if the objects themselves (for example, a tree) consists of objects that could have an even deeper level of detail (such as the leaves). What we actually need, is a tree of pseudo-random generators, where on each level we use a random number to seed an arbitrary number of other random generators. One way of dealing with this, when using, for example, a Fibonacci LFSR, is to perform an excluse-or operation with a successive sequence before using it as a seed value for the next level random generator. One could use another pseudo-random generator for this purpose.

## Monday, April 6, 2020

### 23.3° Celsius

We have been seeing a lot of blue sky in the past week. Today, the temperature at Twenthe Airport went up to 23.3° Celsius, which breaks the previous record of 22.7° for the temperature on this date in 1961. It is also the first day that temperatures are above 20°.

## Friday, April 10, 2020

### Too sneaky

I am getting the impression that the SARS-CoV-2 virus is simply too sneaky to get it under control. Countries all over the world have implemented strickt lock-down measures, but even some countries that were succesful in getting the out-break under control, such as South-Korea, are having problems getting their number below zero. I also heard some report about that in some cases the virus can become active again in someone who has been infected and tested negative afterwards. It also seems that people who had only mild symptoms often do not have high antibody levels, which might make them sensitive to being reinfected after some time. It is also not clear how long imunity will remain for those who have been infected.

## Saturday, April 11, 2020

### Book

At 15:25:44, I bought the book Masters of Rietveld: Dutch Design Education in the 21st Century edited by Mieke Gerritzen, written in English, and published by All Media Foundation in 2008, ISBN:9789081165525, from bookshop Broekhuis for € 24.95. I already had considered to buy this book for many years. An important reason to buy it now is to support the bookshop, because I fear that they are going to a rough time due to much fewer people visiting the shop.

## Wednesday, April 15, 2020

### Dune (2020 film)

In the past days, the first still shots for the upcoming Dune film were released in the article Behold Dune: An Exclusive Look at Timothée Chalamet, Zendaya, Oscar Isaac, and More. What I am most afraid of is that visuals will be more important than the story of the book, which is actually quite tragic: a teenage boy, losing both parents (one through death, the other through alienation), on a very lonely journey, falling into a trap he cannot escape. It is only recently that I begin to understand the deeper story that Frank Herbert, maybe partly unconsciously, wrote into the novel. Yes, on the surface, the story reads a heroic adventure where the main character defeates all his enemies, but on the deeper level, it is just the opposite. This why so many people were deeply disappointed by its sequels, that make it all the clear that the hero is no real hero. Brain Herbert, the elder son of Frank Herbert, said that he tried to preserve his fathers heritage when reviewing the screenplay written by Denis Villeneuve, Eric Roth, and Jon Spaihts, but I am afraid that he is not in touch with the deeper story but only with the surface story. (I agree with the author of The Irony of the Gom Jabbar, that: "Frank passed the Gom Jabbar, Brian did not.") And I am afraid that this is also true for the directory, Villeneuve, and the screenwriters. Denis Villeneuve already made some remarks that this movie was going to be a 'Star Wars for adults'. And now again, he makes Jessica into "warrior priestess" and jokes "It's better than 'space nun.'" The latter characterization is probably more problematic than the first, because Jessica was far from being a nun, as she is deeply in love with Leto and the mother of Paul. Also the organization she is part of, is political, not religious. I am afraid that it will be a long time before the literary qualities of the novel will be recognized.

## Thursday, April 16, 2020

### 3%

Today, it was reported that with 3% of four thousands of people who donated blood in the past weeks, antibodies against the SARS-CoV-2 virus were found. The donors are representative for the population between 18 en 79 years, but not all regions are equally represented. The number is expect to be between 2.5% and 3.5%. Based on these figure, one could conclude that about half a milion people in the Netherlands have had COVID-19. As of today, there are 29,214 people who tested positive and 3,315 of these have died so far. Looking at the death numbers from the national bureau of statistics it might be that the actual number of people who have died might be twice as high. This would be in accordance of a fatality rate of about one or two percent. It is clear that the elderly are hit hardest and among them the men. Below a table by age with in the first two columns the population in thousands (split in male and female), in the third column the number of registered infections, in the fourth column the number of people (having been) hospitalized, and in the last columns the number of casualties.

```          pop.      inf.  hosp.    cas.
m    f                   m    f
0-4     444  422     62     37       0
5-9     470  447     10      2       0
10-14   490  467     40      6       0
15-19   538  514    204     21       0
20-24   548  532    963     40       0
25-29   569  553   1313     84       3
30-34   543  515   1297    102       2
35-39   517  515   1115    147       4
40-44   510  619   1290    217       3
45-49   617  615   1885    436       8
50-54   642  635   2488    656    18   12
55-59   617  615   2762    874    38   15
60-64   546  551   2327   1009    66   30
65-69   492  501   1793   1091   138   65
70-74   445  468   2294   1404   236  131
75-79   283  324   2525   1351   433  196
80-84   181  241   2596   1004   450  281
85-89    92  159   2407    617   410  295
90-94    30   71   1397    183   146  220
95+       6   20    429     27    30   84
```

## Sunday, April 19, 2020

### Books

Today, I got the following books:
• A Sanskrit-English Dictionary written by Sir Monier Monier-Williams, in English, and published by Clarendon Press in 1979, ISBN:019864308X.
• Feeling Good: The New Mood Therapy written by David D. Burns in English and published by HarperCollins in January 2009, ISBN:9780380810338.
• Wolter's Wiskunde in je Pocket written by Erwin Broekema in Dutch and published by Wolters-Noordhoff in 2004, ISBN:9789001970727.
• The Number Devil: A Mathematical Adventure written by Hans Magnus Enzensberger in English, published by Granta Books in 2006, and translated by Michael Henry Heim from the German Der Zahlenteufel: Ein Kopfkissenbuch für alle, die Anst vor der Mathematick haben, ISBN:9781847080530.
• Een gesloten boek: autisme en emoties written by Peter Vermeulen in Dutch and published by Acco in 2005, ISBN:9789033459092.
• Brein bedriegt: als autisme niet op autisme lijkt written by Peter Vermeulen in Dutch and published by Vlaamse Dienst Autisme in 1999, ISBN:9789064451270
• Aspergirls: een wegwijzer voor meisjes en vrouwen met het syndroom van Asperger written by Rudy Simone in Dutch and published by Hogrefe in 2010, ISBN:9789079729289.
• Waarneming en zintuiglijke ervaringen bij mensen met autisme en aspergersyndroom written by Olga Bogdashina in Dutch and published by Garant in 2004, ISBN:9789044115376.
• Waarom niet iedereen mij leuk hoeft te vinden written by Bert van Dijk in Dutch and published by Thema in 2009, ISBN:9789058715913.

## Tuesday, April 21, 2020

### April 28

In the evening, our government announced the measures with respect to COVID-19 for the periode after April 28 as the had promised to do when they announced the intelligent lock-down. Some of the restrictions will be lifted (mainly with respect to primary school children) and some have been extended (mainly with respect to public events that require a permit). They did announce that testing would now also become available for teachers as it has been available for caregivers. The idea to losen restrictions for young children is that they according to the RIVM there is not much evidence that they do spread the virus. Today, the RIVM is some other publication, did state (translated from Dutch by me): "Analysis of clusters of patients shows that spreading also takes place by persons who do not (yet) have any complaints or symptoms." On their website, the still state that there is no evidence for this. And somewhere else they state that although it might be possible, it is still unclear to which extend this constributes to the spread of the virus.

Earlier this week, Professor Marc Bonten complained that the available testing capacity is not optiomal used and that this mainly due to bureaucracy. He remarked that it usually takes four to five days before a test result is reported to the person who was tested. It seems that it is much easier for the government to issue measures for other than to change their own organization. Also the process for developing an app, has been rahter chaotic. The governments asked groups of experts to evaluate proposals, but in the resulting shortlist of seven apps, there were some which violated privacy. The groups also complained that they were not given any guidelines on how to evaluate the proposals.

## Thursday, April 23, 2020

### Book

At 19:08, I collected the book AKI eindexamen fotografie 2001 with a preface by Rutger Ten Broeke in Dutch and published by AKI in 2001 from a private person. I paid € 4.50 for the book.

## Friday, April 24, 2020

### Book

At 16:45:53, I bought the book De Voorstelling: Nederlandse kunst in het Stedelijk Paleis written by Rudi Fuchs and Lien Heyting in Dutch and published by Stedelijk Museum Amsterdam in 2000, ISBN:9789076588155, from charity shop Het Goed for € 2.25.

## Monday, April 27, 2020

### Groot Brunink

Today is Koningsdag, a national holiday to celebrate the birthday of Willem-Alexander of the Netherlands, our king. Due to COVID-19 many of the traditional celebrations have been cancelled. Conny took me on a biking/walking trip to the Groot Brunink estate. We biked to the start of the road called Weustinkhoekweg and parked our bikes there. We walked around for about one and half hour and also took a short break during which Conny took a picture of me. I really enjoyed the beautiful landscape, which is so typical for the area where we live. It really felt like being on a holiday. When biking home, we took a little detour, and Conny showed me the Berktepaal, a stone that marks the border between the two 'markes' of Usselo and Buurse. A 'marke' is a medieval collective of larger farmers who jointly regulated the management and use of their common land. The word marke (related to the English 'mark') is also used to indicate the area that belongs to a village. At home, I read about the history of these border poles and learned some interesting facts about the life as it was here in the past.

## Wednesday, April 29, 2020

### Small chestnut tree

Yesterday afternoon, Annabel noted that the small chestnut tree, that I feared had died, was growing again. It has some bright leaves. The stem seems thicker and has a strange twist a few centimeters above the ground. This is an interesting development. This afternoon, I took a picture, which is displayed on the right.

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