Donald Knuth's The Art of Computer Programming has, in its first volume, a lengthy section on simulating an elevator. It is a single regular elevator (nothing "special" going on as in the post here), but even so, as he tries to make things precise, you realize how much detail is involved, and get some appreciation for the task of programming.
It occupies about 15 pages (plus several pages of exercises and solutions). Knuth started working on TAOCP when he was a PhD student at Caltech:
> The program developed below simulates the elevator system in the Mathematics building of the California Institute of Technology. The results of such a simulation will perhaps be of use only to people who make reasonably frequent visits to Caltech; and even for them, it may be simpler just to try using the elevator several times instead of writing a computer program. […]
> The algorithm we will now study may not reflect the elevator’s true principles of operation, but it is believed to be the simplest set of rules that explain all the phenomena observed during several hours of experimentation by the author during the writing of this section. […]
> The elevator system described above is quite complicated by comparison with other algorithms we have seen in this book, but the choice of a real-life system is more typical of a simulation problem than any cooked-up “textbook example” would ever be.
It ends with:
> It is hoped that some reader will learn as much about simulation from the example above as the author learned about elevators while the example was being prepared.
And one of the exercises adds:
> It is perhaps significant to note that although the author had used the elevator system for years and thought he knew it well, it wasn’t until he attempted to write this section that he realized there were quite a few facts about the elevator’s system of choosing directions that he did not know. He went back to experiment with the elevator six separate times, each time believing he had finally achieved a complete understanding of its modus operandi. (Now he is reluctant to ride it for fear that some new facet of its operation will appear, contradicting the algorithms given.) We often fail to realize how little we know about a thing until we attempt to simulate it on a computer.
On one of the better teams I’ve worked with the water-cooler activity was trying to come up with an algorithm to describe the behaviour of the Coca Cola machine in the kitchen.
It was one of those clear plexiglass front machines where after punching in the coordinates of the item you wanted, a mechanical arm would move to the coordinates, take a drink, and place it in the delivery bay at the bottom.
While it would always get the drink you wanted, it would rarely go to the exact coordinates you specified. Ie, if there were three columns filled with coke, and you punched in the coordinates for the one on the far right, the arm may take a can from the center, or left column.
We would often wager a can of coke (“if I can’t find anything wrong in your PR, I’ll buy you a coke”), so we were perhaps drinking more soft drink than was medically advisable, but in our defence:
a) the machine was really cheap (AUD$1 or $1.5)
b) it was an excellent 10min break game
Eventually we thought they had it figured out we would gather and make our predictions, but occasionally there would be an upset that would throw a wrench into the model.
We got to a point where we just couldn’t reconcile the machine behaviour with any kind of coherent set of rules, then one morning we saw the delivery guy stocking it.
After chatting with him we learned our algorithm was more or less correct, but the internal state of the machine was prone to getting out of sync with the actual stock levels, so it would make the “wrong” choice near the end of the refill cycle. Then he gave the engineer talking to him a free energy drink (source of stock problems right there haha)
While we were no Knuths, I love that these kinds of games are so universal among engineers/devs. In fact, if I can get someone to tell me a similar story of theirs in an interview (for a technical
role) I’m much more likely to consider them for the position. Curiosity is a powerful trait for a developer.
…and the (simple) algorithm for the machine is:
Take from the column with the most stock, if there are multiple columns with the same stock level, take from the column furthest to the left.
> We often fail to realize how little we know about a thing until we attempt to simulate it on a computer.
"Programming is a good medium for expressing poorly understood and sloppily formulated ideas" is definitely on the list of my favourite programming quotes.
> We often fail to realize how little we know about a thing until we attempt to simulate it on a computer.
I'll often write out with pen and paper of how I think the flow of a model will go, it's always fascinating how much that diagram changes once I actually build the model. There are both drastic simplifications and increases in nuance based on experience with the system being modeled and where divergences occur.
Hm. I've always thought that the algorithm of operation of a single elevator is fairly simple? While passing each floor, if a button of that floor was pressed (is lit up), or if the button of the direction it's going on that floor is pressed, then stop. If the elevator is not in use and someone presses any call button, go to that floor. That's mostly it. At least that's how I have that algorithm in my head.
Now, if there are multiple elevators but a single set of up/down buttons for them on each floor, that's where things get really complex really fast.
This is one of my favourite talks ever. I couldn't have given less of a shit about the topic and wasn't expecting to watch more than 5 minutes of it. Nek minnut the video is ending and I wanted another hour of it haha.
I found a "mode" in an elevator in India that I have not seen anywhere else: if a destination floor number is highlighted, and you press it twice, it gets cancelled. I have used this magic power just once, when an obnoxious man got on and insisted on having a loud conversation on his phone. Since he wasn't paying attention, I double-tapped his floor number and made him skip. I happened to get off at the floor before his, so the elevator went down to the first floor after dropping me off. Small pleasures....
In South Korea, pretty much all elevators have this enabled. Pressing a floor button a second time cancels the request. It's pretty useful (if not abused).
This could be a nice "learning" trick too: sometimes, you see people pressing angrily on buttons many times when they are in a hurry, this teaches them to cool down, don't press the button many times, it won't help.
My favorite interview question is "how do elevators work"?
0. If they immediately blurt out something like "I know how elevators work", probably don't hire them.
1. You can find out what areas they're most interested in. Do they jump straight to the physical mechanics? The programming? The UI? The abstraction that they're solving a problem?
2. Eventually, they reach a point where they just have to say "I don't know". The rabbit hole really just keeps going with elevators. You could know this question is coming and we will still easily reach the knowledge boundary.
3. If they "don't know" you can ask them to guess how a system might work. Or give them time to research it and follow up with you later.
I did a design interview which involved drawing up an elevator control panel for an apartment building with 1000 floors. Me, trying to leverage my sense of humour I guess (oops) designed a system in which the presumed wealthiest users at the top were instantly recognized with a face scan and never had to see a panel of buttons. The rest were forced to use a basic number pad. Needless to say I didn’t get the job. Doesn’t help that when you google my name my father’s extensive work as a labour organizer tends to spook my potential boss…
The elevator in the apartment building I currently live in has a quirk which adds ~2s to the travel. If you know this quirk you can save the ~2s on each use.
So if you just pick the floor then it waits 2 seconds before closing the door. Obviously the elevator waits for the door to get fully shut before it starts moving.
If you first press the "shut door" button and then the floor button, then the door shuts within any delays, which means the elevator starts moving 2 seconds earlier.
Interesting. Mine has an even longer delay than 2s (3-4s?) so I always press the close-doors button which removes the delay, but it never occurred to me I could shave some additional milliseconds off by pressing the close-doors button before the floor selection button.
One time when working as a pentester, we were doing redteaming (read: breaking into target buildings, physically). Well, they were doing redteaming; I always wanted to, but never quite got the opportunity.
One of the ideas thrown around for achieving the objective was to somehow get ahold of an elevator key, stop it, and hide in there until the building closed.
I don't know if they actually did that, but it would've been hilarious to see them pop out like a scoobie doo villain and jack into an ethernet port while the janitor has no idea what's going on.
There’s a legendary defcon talk about elevators where the speaker describes doing exactly that on several occasions. Searching “Defcon Elevator” on YouTube should pull up the video in question.
Slightly off-topic, but I'd definitely recommend Colson Whitehead's _The Intuitionist_[1] for anyone interested in the intersection of elevators and speculative mysteries. Never thought I'd get the chance to post this to an HN thread...
One of my first thoughts too. I preferred John Henry Days, but whatever. As for The Intuitionist -- the whole thing is kind of a troll. Like, the premise is absurd. Of course, that's sort of the point.
"pet mode" reminded about my cat many years ago - we lived on the 5th floor of an apartment building, and going outside the cat would walk the stairs down, yet coming back the cat would sit near elevator on the ground floor until somebody would come to use the elevator and everybody knew that "the white cat rides to the 5th floor" so they would let him out there.
> Does not infect other passengers if the animal has an infection
I think this must be a mistranslation. There are very few infectious diseases that a human can get from being around a house pet. I think they mean allergies.
I had read somewhere that holding the boor close button along with your desired floor button will override the que in many elevators, taking you directly to that floor. I successfully tested the button combo in the elevator in my building. It came in handy when a kid pressed all the buttons and then stuck his tongue out at me.
Another dumb story from "back in my day". When I was in college way back in 1976, me and my Best friend back then both had graduated early and had to live in the dorm the first year until we were 18. The dorm they made us stay in was 6 stories and had a crappy old elevator. Being a couple of goofy hacker types with nothing better to do on a weekend, we figured out how to get up on top of the elevator and to use the alternate control panel up there. We sat up there and could control it, people would get on, we could listen to them talk, stop the elevator between floors and they would get all scared, we would flicker the lights on and off, make it go to the wrong floor and so forth. Pretty fun stuff for a couple of nerds.
We got bored with that pretty quickly until, 4 gals form the girls floor decided they were going to camp out in there and set the Guinness record for the longest stay in an elevator. Some of the bigger dudes knew that we had hacked the elevator and asked us if we wanted to mess them up. Well... Of course. So we got on top and were listen to them yakkedy yak. Then we took it to between the 6th and 5th floors and locked it in place, then opened the upper doors on the 6th floor where a bunch of guys with big trash cans full of water were waiting.
We thought they were just going to douse them good. So we opened the ceiling trap door to the elevator with the lights off and those gals were screaming and squealing like crazy. Then come dude tossed a string of fire crackers in there followed by two giant Garbage cans of water right after that. Must have been at least a couple hundred gallons of water, who knows.
Then we set the thing to go to the first floor and jumped off. Waiting on the first floor where a bunch of people who were in on it with cameras to capture the whole things as the doors opened it was like the seas parting and flooding out as these poor drenched young ladies came floating out. Picture was on the front page of the school paper "Guinness Elevator Record Attempt Drenched" or something.
They called the cops and all that but no one squealed on us and it was all in good fun but to teach everyone a lesson they shut down the elevators for 3 weeks. Young people don't care, worth a climb of the stairs for all the fun!
BTW, I ended up marrying one of the gals on the elevator and yes I told her I was in on it, later. She loved that for some reason :)
Since we’re talking about elevators, it would appear anyone can call the elevator’s emergency phone line. Which is good, but also has unintended consequences: https://vm.tiktok.com/ZMdXyQ5ra/
There was a story of a guy who got stuck in an elevator and called for help, but the installer had gone bust and the number had been assigned to a psychiatrist in another county. At first the therapist claimed he couldn't do anything, but when the man pleaded for help, the therapist replied "Well, how does it feel to be trapped in an elevator."...
Depends on model and implementation. Usually these days the alarm phone will not pick up calls from any number (allow list used), but this looks like older car, so this might be retrofitted and/or maybe some software rules were not set during installation.
I wondered why, during the pandemic, it wasn't common to let elevators "air out" while they were idle. Given what we know about COVID-19 and how it spreads with aerosols. From looking at this list, it appears that that's not a mode they would have by default.
As someone who works in the elevator industry this list is pretty odd and is missing many modes that I would consider default modes.
However to get back to "airing the elevator out". Do you mean just not closing doors after serving a call? Maybe having someone timeout after serving a call to not move? Or actually installing fans to blow out the air?
I work in an office building that's less than 5 years old. During the pandemic the elevators started returning to the ground floor and opening their doors. I wonder if the system is configured via the control panel in the elevator or if there's some other interface.
My university had an elevator with a special three-button keypress that took you to a dark sub-basement full of asbestos warnings and terrifyingly dark that wasn't listed on the display.
The thinking elevator,
so the makers proudly say,
will optimize its program
in an almost human way.
And truly, the resemblance
is uncomfortably strong:
it isn't merely thinking,
it is even thinking wrong.
I ran into https://www.youtube.com/watch?v=ZUvGfuLlZus (Elevator Hacking: From the Pit to the Penthouse) awhile back, and it was so interesting to me that I ended up watching the entire 2 hours in one sitting.
[+] [-] svat|4 years ago|reply
It occupies about 15 pages (plus several pages of exercises and solutions). Knuth started working on TAOCP when he was a PhD student at Caltech:
> The program developed below simulates the elevator system in the Mathematics building of the California Institute of Technology. The results of such a simulation will perhaps be of use only to people who make reasonably frequent visits to Caltech; and even for them, it may be simpler just to try using the elevator several times instead of writing a computer program. […]
> The algorithm we will now study may not reflect the elevator’s true principles of operation, but it is believed to be the simplest set of rules that explain all the phenomena observed during several hours of experimentation by the author during the writing of this section. […]
> The elevator system described above is quite complicated by comparison with other algorithms we have seen in this book, but the choice of a real-life system is more typical of a simulation problem than any cooked-up “textbook example” would ever be.
It ends with:
> It is hoped that some reader will learn as much about simulation from the example above as the author learned about elevators while the example was being prepared.
And one of the exercises adds:
> It is perhaps significant to note that although the author had used the elevator system for years and thought he knew it well, it wasn’t until he attempted to write this section that he realized there were quite a few facts about the elevator’s system of choosing directions that he did not know. He went back to experiment with the elevator six separate times, each time believing he had finally achieved a complete understanding of its modus operandi. (Now he is reluctant to ride it for fear that some new facet of its operation will appear, contradicting the algorithms given.) We often fail to realize how little we know about a thing until we attempt to simulate it on a computer.
[+] [-] rablackburn|4 years ago|reply
On one of the better teams I’ve worked with the water-cooler activity was trying to come up with an algorithm to describe the behaviour of the Coca Cola machine in the kitchen.
It was one of those clear plexiglass front machines where after punching in the coordinates of the item you wanted, a mechanical arm would move to the coordinates, take a drink, and place it in the delivery bay at the bottom.
While it would always get the drink you wanted, it would rarely go to the exact coordinates you specified. Ie, if there were three columns filled with coke, and you punched in the coordinates for the one on the far right, the arm may take a can from the center, or left column.
We would often wager a can of coke (“if I can’t find anything wrong in your PR, I’ll buy you a coke”), so we were perhaps drinking more soft drink than was medically advisable, but in our defence:
a) the machine was really cheap (AUD$1 or $1.5)
b) it was an excellent 10min break game
Eventually we thought they had it figured out we would gather and make our predictions, but occasionally there would be an upset that would throw a wrench into the model.
We got to a point where we just couldn’t reconcile the machine behaviour with any kind of coherent set of rules, then one morning we saw the delivery guy stocking it.
After chatting with him we learned our algorithm was more or less correct, but the internal state of the machine was prone to getting out of sync with the actual stock levels, so it would make the “wrong” choice near the end of the refill cycle. Then he gave the engineer talking to him a free energy drink (source of stock problems right there haha)
While we were no Knuths, I love that these kinds of games are so universal among engineers/devs. In fact, if I can get someone to tell me a similar story of theirs in an interview (for a technical role) I’m much more likely to consider them for the position. Curiosity is a powerful trait for a developer.
…and the (simple) algorithm for the machine is: Take from the column with the most stock, if there are multiple columns with the same stock level, take from the column furthest to the left.
[+] [-] jhgb|4 years ago|reply
"Programming is a good medium for expressing poorly understood and sloppily formulated ideas" is definitely on the list of my favourite programming quotes.
[+] [-] iaw|4 years ago|reply
I'll often write out with pen and paper of how I think the flow of a model will go, it's always fascinating how much that diagram changes once I actually build the model. There are both drastic simplifications and increases in nuance based on experience with the system being modeled and where divergences occur.
[+] [-] grishka|4 years ago|reply
Now, if there are multiple elevators but a single set of up/down buttons for them on each floor, that's where things get really complex really fast.
[+] [-] pram|4 years ago|reply
[+] [-] lozf|4 years ago|reply
You can catch it at:
https://archive.org/details/Defcon22_Talk29/DEF+CON+22+Hacki...
or
https://www.youtube.com/watch?v=oHf1vD5_b5I [01:00:15]
[+] [-] rozab|4 years ago|reply
https://youtu.be/rnmcRTnTNC8
[+] [-] BigJono|4 years ago|reply
[+] [-] 1024core|4 years ago|reply
[+] [-] 63|4 years ago|reply
[+] [-] graup|4 years ago|reply
[+] [-] gregoriol|4 years ago|reply
[+] [-] grensley|4 years ago|reply
0. If they immediately blurt out something like "I know how elevators work", probably don't hire them.
1. You can find out what areas they're most interested in. Do they jump straight to the physical mechanics? The programming? The UI? The abstraction that they're solving a problem?
2. Eventually, they reach a point where they just have to say "I don't know". The rabbit hole really just keeps going with elevators. You could know this question is coming and we will still easily reach the knowledge boundary.
3. If they "don't know" you can ask them to guess how a system might work. Or give them time to research it and follow up with you later.
[+] [-] flycaliguy|4 years ago|reply
[+] [-] praptak|4 years ago|reply
So if you just pick the floor then it waits 2 seconds before closing the door. Obviously the elevator waits for the door to get fully shut before it starts moving.
If you first press the "shut door" button and then the floor button, then the door shuts within any delays, which means the elevator starts moving 2 seconds earlier.
[+] [-] AnssiH|4 years ago|reply
[+] [-] royjacobs|4 years ago|reply
[+] [-] simlevesque|4 years ago|reply
[+] [-] matsemann|4 years ago|reply
A game where you program the logic of elevators to move people around efficiently. Surprisingly complicated.
[+] [-] FooBarBizBazz|4 years ago|reply
[+] [-] sillysaurusx|4 years ago|reply
One of the ideas thrown around for achieving the objective was to somehow get ahold of an elevator key, stop it, and hide in there until the building closed.
I don't know if they actually did that, but it would've been hilarious to see them pop out like a scoobie doo villain and jack into an ethernet port while the janitor has no idea what's going on.
[+] [-] 542458|4 years ago|reply
[+] [-] bombcar|4 years ago|reply
[+] [-] geephroh|4 years ago|reply
[1] https://en.wikipedia.org/wiki/The_Intuitionist
[+] [-] FooBarBizBazz|4 years ago|reply
[+] [-] splatzone|4 years ago|reply
[+] [-] trhway|4 years ago|reply
[+] [-] teruakohatu|4 years ago|reply
> Does not infect other passengers if the animal has an infection
I think this must be a mistranslation. There are very few infectious diseases that a human can get from being around a house pet. I think they mean allergies.
[+] [-] user-the-name|4 years ago|reply
[+] [-] DonHopkins|4 years ago|reply
[+] [-] ortusdux|4 years ago|reply
[+] [-] teameat|4 years ago|reply
We got bored with that pretty quickly until, 4 gals form the girls floor decided they were going to camp out in there and set the Guinness record for the longest stay in an elevator. Some of the bigger dudes knew that we had hacked the elevator and asked us if we wanted to mess them up. Well... Of course. So we got on top and were listen to them yakkedy yak. Then we took it to between the 6th and 5th floors and locked it in place, then opened the upper doors on the 6th floor where a bunch of guys with big trash cans full of water were waiting.
We thought they were just going to douse them good. So we opened the ceiling trap door to the elevator with the lights off and those gals were screaming and squealing like crazy. Then come dude tossed a string of fire crackers in there followed by two giant Garbage cans of water right after that. Must have been at least a couple hundred gallons of water, who knows.
Then we set the thing to go to the first floor and jumped off. Waiting on the first floor where a bunch of people who were in on it with cameras to capture the whole things as the doors opened it was like the seas parting and flooding out as these poor drenched young ladies came floating out. Picture was on the front page of the school paper "Guinness Elevator Record Attempt Drenched" or something.
They called the cops and all that but no one squealed on us and it was all in good fun but to teach everyone a lesson they shut down the elevators for 3 weeks. Young people don't care, worth a climb of the stairs for all the fun!
BTW, I ended up marrying one of the gals on the elevator and yes I told her I was in on it, later. She loved that for some reason :)
[+] [-] MockObject|4 years ago|reply
[+] [-] smabie|4 years ago|reply
[+] [-] banana_giraffe|4 years ago|reply
Wiki lists a "Riot mode", which is just amazing if it's a thing.
[+] [-] nextlevelwizard|4 years ago|reply
[+] [-] dqv|4 years ago|reply
[+] [-] sam_goody|4 years ago|reply
[+] [-] nextlevelwizard|4 years ago|reply
[+] [-] llampx|4 years ago|reply
[+] [-] nextlevelwizard|4 years ago|reply
However to get back to "airing the elevator out". Do you mean just not closing doors after serving a call? Maybe having someone timeout after serving a call to not move? Or actually installing fans to blow out the air?
[+] [-] galago|4 years ago|reply
[+] [-] mmastrac|4 years ago|reply
[+] [-] _squared_|4 years ago|reply
[+] [-] exikyut|4 years ago|reply
When you did this, did the floor indicator show anything different?
And where (what country) was this?
[+] [-] vintermann|4 years ago|reply
[+] [-] rightbyte|4 years ago|reply
[+] [-] jackbeck|4 years ago|reply
[+] [-] mpd|4 years ago|reply