Ask HN: What's the most elegant piece of code you've seen?

[mc42]

Lots of people have spent years writing programs spanning platforms, servers, services, and languages. However, efficient and elegant code is far and few between.

What code has stood out to you for being elegant and efficient? Why or why not?

[nostrademons]

Jeff Dean's original implementation of MapReduce.

It was, IIRC, only 3 C++ classes and just a few hundred lines of code. It outsourced much of the distribution, task running, and disk-access tasks to other Google infrastructure, and only focused on running the computation, collecting results for each key, and distributing to the reducers.

The current (as of ~2012, so not that current anymore) version of MapReduce is much faster and more reliable, but there's a certain elegance to starting a trillion-dollar industry with a few hundred lines of code.

There was another doozy, also by Jeff Dean, in the current (again, as of 2012) MapReduce code. It was an external sorting algorithm, and like most external sorts, it worked by writing a bunch of whole-machine-RAM sized temporary files and then performing an N-way merge. But how did it sort the machine's RAM? Using the STL qsort() function, of course! But how do you sort ~64GB of data efficiently using a standard-library function? He'd written a custom comparator that compared whole records at a time, using IIRC compiler intrinsics that compiled down into SIMD instructions and did some sort of Duff's-Device like unrolling to account for varying key lengths. It was a very clever mix of stock standard library functions with highly-optimized, specialized code.

[StephanTLavavej]

Minor correction: qsort() is CRT, std::sort() is STL.

[vram22]

I'd read somewhere [1] that the built-in Python sort function has a lot of good / clever optimizations too, though maybe not of the same kinds that you describe, i.e. may not be at machine language level. Tim Peters did a lot of that, per what I read, though others may have too.

[1] Think I read it in the Python Cookbook, 2nd Edition, which is a very good book, BTW.

[ganduG]

Do you have a link to the code?

[Chris_Newton]

On a larger scale than most suggestions so far, I’ve always been impressed with SQLite. The developers have managed to create a useful-in-the-real-world tool, small and efficient enough for even quite demanding applications like embedded systems work.

The original source code was solid and well documented in the parts I’ve seen, but the remarkable thing to me is the way they distribute it: it comes as a single ANSI C file and a single matching header, which they refer to as the “amalgamation”. It therefore works just about anywhere, has no packaging or dependency hell issues, can be be incorporated into any build process in moments, and can be statically linked and fully optimised.

[weeksie]

Love that source code. I dug around in it quite a bit when I was teaching myself about B-tree implementations about ten years ago.

[bosky101]

Joe Armstrong - the creator of erlang would turn his cluster of 1000's of machines to change behaviour. The code is really 5 lines.

    My favorite Erlang program
    21 Nov 2013
    The other day I got a mail from Dean Galvin from Rowan    
    University. Dean was doing an Erlang project so he asked 
    “What example program would best exemplify Erlang”.
    ...
    The Universal Server
    Normally servers do something. An HTTP server responds to HTTP requests
    and FTP server responds to FTP request and so on. But what about a Universal 
    Server?
    surely we can generalize the idea of a server and make a universal server that 
    which we can later tell to become a specific sever.

    Here’s my universal server:

    universal_server() ->
        receive
           {become, F} ->
               F()
        end.
    
    ...then I set up a gossip algorithm to flood the network with become messages. 
    Then I had an empty network that in a few seconds would become anything 
    I wanted it to do.

a process in erlang, is nothing but a tail-recursive function. the moment it stops being so - it dies. so here it morphes into F; which can be passed in.

more at http://joearms.github.io/2013/11/21/My-favorite-erlang-progr...

[wasd]

I love the story of the fast inverse square root. A bizzare piece of code from quake 3 shows up on usenet with a magic constant that calculates the inverse square root faster than table lookups and approximately four times faster than regular floating point division. Inverse square roots are used to compute angles of incidence and reflection for lighting and shading in computer graphics. Author unknown but was once thought as of Carmack.

https://en.m.wikipedia.org/wiki/Fast_inverse_square_root

[matt_wulfeck]

I would say that the actual source code there is extremely ugly. It may be an elegant solution, but there's no way I would want to crawl around in that repo:

    x2 = number * 0.5F;
	y  = number;
	i  = * ( long * ) &y;                               // evil floating point bit level hacking
	i  = 0x5f3759df - ( i >> 1 );               
    // what the fuck? 
	y  = * ( float * ) &i;

[dividuum]

That has to be LuaJIT. The amount of knowledge hidden in that code base is mind blowing. It contains a state of the art tracing JIT compiler, optimized for at least 5 different platforms, a custom assembler (dynasm) which is used for implementing hand optimized code for the interpreter in 6 different architectures. Additionally it has one of the best FFI implementations I've ever seen: Just parse a C-header file at runtime and you can call into C code with zero overhead. All of that written by a single person (Mike Pall). If you haven't had a look, you should: https://github.com/LuaJIT/LuaJIT/tree/v2.1

[PeCaN]

I was going to say LuaJIT also. JIT compilers have a habit of looking rather like chicken scratch, but LuaJIT is pretty readable (for a JIT).

Specifically, the way it manages traces (lj_trace.c, lj_record.c) is really elegant. It's not much code, but it's the best available tracing JIT compiler and manages traces quite elegantly IMO.

Mike Pall is legitimately a genius.

[danielvf]

The Redis source code is hand down the best C I've ever seen. And it's not just the code that's beautiful - the tests are too.

https://github.com/antirez/redis/blob/unstable/src/dict.c

[tebeka]

Peter Norvig's Spell Checker http://norvig.com/spell-correct.html

A lot of other code he writes as well.

[krat0sprakhar]

I really love this blog post by Norvig. Last Christmas I was looking for a trivial project to try Clojure and had a lot of fun working through this. Highly recommended - https://github.com/prakhar1989/clj-spellchecker

[biot]

Particularly "Solving Every Sudoku Puzzle": http://www.norvig.com/sudoku.html

[anonfunction]

Also one of my favorites, and also like another reply to this comment I ported it[1] to a language I was learning, Golang.

1. https://github.com/montanaflynn/toy-spelling-corrector

[d13]

Commodore 64's Basic random maze generator:

10 PRINT CHR$(205.5+RND(1)); : GOTO 10

https://www.youtube.com/watch?v=m9joBLOZVEo

[pkaye]

Somebody managed to write a whole book about this random maze generator. http://10print.org/

[agildehaus]

It prints either \ or /, not sure how amazing that is. It also isn't a maze, considering it's unlikely any of those paths leads to an exit.

[austinjp]

I've got to say, that is truly wonderful.

[lokedhs]

The Solaris kernel source. I got a deep appreciation for the elegance of it when I was working for Sun a long time ago. I can enjoy analysing that code base, which is opposite to the feeling I get when looking at the Linux code.

It's not overly clever, but it's incredibly clear and easy to understand.

[jibalt]

Much of that code was inherited from Bell Labs UNIX, much of which came from Ritchie and Thompson.

[gaze]

Jeff Bonwick is one of the best systems programmers ever

[jamessantiago]

Doom 3 was touted[1] as having some "exceptional beauty." Naming, spacing of properties, consistency, and how multiple parameterized calls were formatted are quite nice. You can see an example on github[2], but for some reason the spacing is a bit off in the web view. I'd recommend cloning a local copy and taking a look.

[1] http://kotaku.com/5975610/the-exceptional-beauty-of-doom-3s-...

[2] https://github.com/id-Software/DOOM-3/blob/master/neo/game/a...

[Splines]

> but for some reason the spacing is a bit off in the web view.

It looks like Github doesn't explicitly set tab-size so it defaults to 8. 4 seems to work better.

[jessedhillon]

Fast inverse square root[0] is something that I encountered in the mid-2000s in the Q3A source code. It took me a really long time to understand it, and I eventually had to show it to some professors before I really understood what was going on and why this worked.

That's really an example of how arbitrary human thought processes are. When you release the constraint that your code has to have some human-comprehensible analog, you might arrive at interesting results.

[0] https://en.wikipedia.org/wiki/Fast_inverse_square_root

[pkaye]

Most of that is fairly straightforward. It is using newton's method to calculate the inverse square root. But to get that with one or two iterations, you need a good estimate to start with. The square root of the floating point exponent is half the value. Knowing how the floating point number is packed, we know a right shift is equal to divide by two and negate it. What remains is how the shift affects the mantissa and if some correction factor is needed. This could have been gotten by least square optimization to minimize the error.

[afrancis]

It has been a while since I looked at it, but I was impressed by Russ Cox's implementation of channels in the LibThread library of Plan 9. All channel operations including blocking on several channels and selecting one that is ready, are described by Alt structures, which under the hood implement a simple but elegant algorithm that appears in Rob Pike's "The Implementation of Newsqueak." I feel you want to understand roughly how Go channels works, read that paper and look at LibThread's channel implementation.

A second choice would not be so much code, but an algorithm: Thompson constructions for regular expressions.

[frou_dh]

Yeah, the blocking select on multiple channels inside an infinite for { } struck me as the coolest construct in Go. It's like a little nerve centre.

[Radim]

For me, the memorable pieces of code are those that made me "want to become a programmer", to choose the path I chose, way back in the day.

Unsurprisingly, these were games:

1) A C64 "SnakeByte-like" game, whose exact name I forgot. It was written entirely in BASIC 2.0 (so you could list and read the source code), and with me having no C64 manual, and no English, it was a true revelation. So much fun and beauty emerging from such a concise, approachable program!

2) An ancient five-in-a-row implementation, I think in BASIC again or maybe Pascal. I remember the shock after seeing how simple the code was, compared to its (surprisingly good) playing strength and speed. My github user name, "piskvorky", is an echo of this old experience :-)

The underlying appeal seems to be a combination of simple, elegant rules giving rise to complex and fun behaviour. That, to me, is elegance.

[FrankyHollywood]

This question was asked to some top programmers and computer scientists. The answers are collected in this book:

http://www.worldofbooks.com/catalog/product/view/id/2366045/...

[SixSigma]

The Plan9 Source code

https://github.com/0intro/plan9

> A Professor of Computer Science gave a paper on how he uses Linux to teach his undergraduates about operating systems. Someone in the audience asked 'why use Linux rather than Plan 9?' and the professor answered: Plan 9 looks like it was written by experts; Linux looks like something my students could aspire to write.

But see also the HN thread :

Code which every programmer must read before dying

https://news.ycombinator.com/item?id=2466129

[eps]

I worked for a networking software developer at some point in the past and they were considering licensing a VPN stack from the SSH company [1]. That thing was in C and the sample code included was stunningly beautiful as was the documentation - well modularized, consistent, good naming notation, but above all it was concise. I think I still have a CD with the SDK demo, I can pull up some code from there if anyone's interested.

[1] https://en.wikipedia.org/wiki/SSH_Communications_Security

[rajathagasthya]

I was reading some of the Python standard library code and one piece stood out for me in the heapq.py module.

Merge sorted iterables using min heap:

  def merge(*iterables):
      '''Merge multiple sorted inputs into a single sorted output.
      >>> list(merge([1,3,5,7], [0,2,4,8], [5,10,15,20], [], [25]))
      [0, 1, 2, 3, 4, 5, 5, 7, 8, 10, 15, 20, 25]
      '''
      _heappop, _heapreplace, _StopIteration = heappop, heapreplace, StopIteration
      _len = len

      h = []
      h_append = h.append
      for itnum, it in enumerate(map(iter, iterables)):
          try:
              next = it.next
              h_append([next(), itnum, next])
          except _StopIteration:
              pass
      heapify(h)

      while _len(h) > 1:
          try:
              while 1:
                  v, itnum, next = s = h[0]
                  yield v
                  s[0] = next()               # raises StopIteration when exhausted
                  _heapreplace(h, s)          # restore heap condition
          except _StopIteration:
              _heappop(h)                     # remove empty iterator
      if h:
          # fast case when only a single iterator remains
          v, itnum, next = h[0]
          yield v
          for v in next.__self__:
              yield v

If I'm not wrong, this is written by Raymond Hettinger and it's always a pleasure to read/use his code.

[hayd]

The scope lifting (I forget what this performance hack is called), `_len = len` etc., is unsatisfying.

[sssilver]

Naive factorial implementation in Haskell

factorial 0 = 1

factorial n = n * factorial (n - 1)

[agumonkey]

fib, with its tail biting zip felt even better.

one of my favorite intermediate recursive function is powerset

[davidjnelson]

I really like the code for redux. It's only a few hundred lines of code, but has spawned an ecosystem of plugins and has over 13,000 github stars already.

https://github.com/rackt/redux/tree/master/src

[joncampbelldev]

Indeed its only 99 lines of code if you strip out comments etc. https://twitter.com/dan_abramov/status/646299844487872512 https://gist.github.com/gaearon/ffd88b0e4f00b22c3159

A truly elegant solution to state management.

[visof]

quicksort [] = []

quicksort (p:xs) = (quicksort lesser) ++ [p] ++ (quicksort greater)

    where

        lesser = filter (< p) xs

        greater = filter (>= p) xs

[alisey]

Keep in mind that this algorithm is not in-place, so it wouldn't be fair to say that it's more 'elegant' than imperative quicksort implementations.