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#beginners
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2020-07-19
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Vachi12:07:40

hi, I have a big csv, is it possible to read lazily the lines in inverse order? from tail to head

jaihindhreddy13:07:10

You can reverse the file with tac file.csv > rev_file.csv and then read rev_file.csv normally. Definitely an option if the file isn't too large.

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noisesmith14:07:06

surely someone has made a "from end buffered line reader" on top of mmap

noisesmith14:07:53

it's inefficient (requires consuming the whole thing and working backward) but doable

noisesmith16:07:29

actually, mmap might be the one way to do this that doesn't use heap space inefficiently (if I'm remembering the API correctly)

noisesmith16:07:17

you can use the memory mapped API to do this without putting the whole contents in heap, it even lets you skip to the end and work toward the front, without consuming what's in between https://howtodoinjava.com/java7/nio/memory-mapped-files-mappedbytebuffer/

drewverlee16:07:44

I'm curious why that would be useful. @vachichng

alexmiller16:07:28

I assume you have something accumulated in time order and you want to process from new to old

noisesmith16:07:45

next question is if anybody hooked that up to read lines in backward order yet

noisesmith16:07:21

clever usage of tac and ProcessBuilder with output to an iostream instead of file might allow similar via the OS(?)

Vachi16:07:52

@drewverlee exactly as Alex said, I have a big cvs of timestamped data that I have to process it windowed by timeframe and the original source has it ordered by recent to old, but the window function needs old to recent

phronmophobic16:07:25

> Reads lines in a file reversely (similar to a BufferedReader, but starting at the last line). Useful for e.g. searching in log files.

noisesmith16:07:40

nice, I'd hoped someone made that

drewverlee16:07:30

But if you have to process the whole thing then the out put will be the same no? The window closing logic should handle any order of time stamped events.

noisesmith16:07:01

many reducing processes aren't associative, ordering can matter a lot

drewverlee16:07:38

Yes, but I think, for instance, onyxs semantics express the window closing trigger this way.

Vachi16:07:58

@drewverlee no, ordering matters because the window function needs to know the oldest timestamp to create sequence

tjb20:07:21

hello again everyone! i have a question: is there a preferred structure in a repo for having both a client (cljs) and api (clj)?

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tjb21:07:33

i currently used lein new shadow-cljs <app-name> +reagent but curious if there is another way to generate both the client template and a plain lein project? maybe i should just lein new my-app` and then lein new shadow-cljs inside of that app?

nick23:07:12

luminus template has really good defaults and the overall structure(clj/cljs/cljc). lein new luminus new-app +cljs +shadow-cljs

tjb23:07:47

@U0113AVHL2W -- thank you so much! ill take a peek

nick12:07:04

haha sorry mate. I recommended you giving luminus a try and looks like you experienced the exactly the same issue I was fighting in the recent Luminus template 😄 Infinite build completed/compiling Referenced issue https://github.com/luminus-framework/luminus/issues/270 You may need to upgrade your shadow-cljs to 2.10.16 or use ~ month old luminus template as a workaround

tjb15:07:06

@U0113AVHL2W thank you a ton! i got it resolved when you responded to me in #shadow-cljs i appreciate your support and guidance on this!

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