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Programmer tries to explain binary search to the police (startrek.website)

submitted 11 months ago by The_Picard_Maneuver@startrek.website to c/programmer_humor@programming.dev

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[-] acceptable_pumpkin@lemmy.world 55 points 11 months ago* (last edited 11 months ago)

Some security camera systems have this built in. They show snapshots of various times where you choose the total period, say 24 hours. Then you glance through the snapshots that are all displayed at once on the screen and click on the last one where your bike was still there. That will then “zoom in” the timeline and show another set of snapshots, though this time within a smaller total time window. Keep clicking on the last panel with the bike, and it will soon show you the clip of the bike being stolen.

Really helpful to find out when something changed.

[-] dudinax@programming.dev 9 points 11 months ago

Yeah, there's no reason it should take an hour no matter how long the tape is.

[-] justJanne@startrek.website 21 points 11 months ago* (last edited 11 months ago)

If you've got 14 billion years, a theft takes a minute, then you need 53 recursion levels of binary search to find the moment of the theft. (14 billion years can be split into about 7.3e15 1-minute segments, 53 levels of binary search allow you to search through 9e15 segments)

That means OP assumed that it'd take 1 minute to decide whether at a certain still frame the theft had already occured or not, to compute the new offset to seek to, and the time it'd take to actually seek the tape to that point.

Not an unreasonable assumption, but a very conservative estimate. Assuming the footage is on an HDD and you've got an automated system for binary search, I'd actually assume it'd take 5 seconds for each step, meaning finding a 1min theft on 14 billion years of footage would take 5 minutes.

[-] Anemia@lemmy.world 3 points 11 months ago

According to my napkin math it would take longer than an hour if the tape was ~3.3*10^218 sec long (or three million trillion trillion... (18 trillions) ...trillion years). Assuming you have only have two options to choose between but can pick which alternative in in 5 seconds (2^720) and you want to get down to a 1 minute intervall.

So i mean its not impossible to find a tape long enough though it seems unlikely that we would be so off in our estimates of the age of the universe.