• Announcements

    • khawk

      Download the Game Design and Indie Game Marketing Freebook   07/19/17

      GameDev.net and CRC Press have teamed up to bring a free ebook of content curated from top titles published by CRC Press. The freebook, Practices of Game Design & Indie Game Marketing, includes chapters from The Art of Game Design: A Book of Lenses, A Practical Guide to Indie Game Marketing, and An Architectural Approach to Level Design. The GameDev.net FreeBook is relevant to game designers, developers, and those interested in learning more about the challenges in game development. We know game development can be a tough discipline and business, so we picked several chapters from CRC Press titles that we thought would be of interest to you, the GameDev.net audience, in your journey to design, develop, and market your next game. The free ebook is available through CRC Press by clicking here. The Curated Books The Art of Game Design: A Book of Lenses, Second Edition, by Jesse Schell Presents 100+ sets of questions, or different lenses, for viewing a game’s design, encompassing diverse fields such as psychology, architecture, music, film, software engineering, theme park design, mathematics, anthropology, and more. Written by one of the world's top game designers, this book describes the deepest and most fundamental principles of game design, demonstrating how tactics used in board, card, and athletic games also work in video games. It provides practical instruction on creating world-class games that will be played again and again. View it here. A Practical Guide to Indie Game Marketing, by Joel Dreskin Marketing is an essential but too frequently overlooked or minimized component of the release plan for indie games. A Practical Guide to Indie Game Marketing provides you with the tools needed to build visibility and sell your indie games. With special focus on those developers with small budgets and limited staff and resources, this book is packed with tangible recommendations and techniques that you can put to use immediately. As a seasoned professional of the indie game arena, author Joel Dreskin gives you insight into practical, real-world experiences of marketing numerous successful games and also provides stories of the failures. View it here. An Architectural Approach to Level Design This is one of the first books to integrate architectural and spatial design theory with the field of level design. The book presents architectural techniques and theories for level designers to use in their own work. It connects architecture and level design in different ways that address the practical elements of how designers construct space and the experiential elements of how and why humans interact with this space. Throughout the text, readers learn skills for spatial layout, evoking emotion through gamespaces, and creating better levels through architectural theory. View it here. Learn more and download the ebook by clicking here. Did you know? GameDev.net and CRC Press also recently teamed up to bring GDNet+ Members up to a 20% discount on all CRC Press books. Learn more about this and other benefits here.
Sign in to follow this  
Followers 0
Geometrian

C++ unordered_set's .insert slow, even with preallocation

7 posts in this topic

Hi,

I have an array of structs, each containing three elements:

class A { public: unsigned int x, y, z; };
//...
A* long_list; //Has at least 250,000 elements

. . . and I need to find a set of the unique structs in long_list.

I try to construct the set as follows:

unordered_set<A> mapping(/*passing in long_list's length to preallocate*/);
//Now .insert(...) each struct into the set

The problem is that this takes a ridiculously long amount of time (like 30 seconds in debug mode, 0.5 in release).

Also, the following:

namespace std {

template<> struct hash<typename my::A> {
	std::size_t operator()(const my::A& vr) const {
		std::hash<unsigned int> func;
		size_t h1 = func(vr.x);
		size_t h2 = func(vr.y);
		size_t h3 = func(vr.z);
		return h1+h2+h3;
	}
};

}

And:

bool operator==(const A& a, const A& b) {
	if (a.x!=b.x) return false;
	if (a.y!=b.y) return false;
	if (a.z!=b.z) return false;
	return true;
}

Maybe the hashing combination is leading to a lot of collisions? Anyway, why is this so slow?

Thanks,
-G

Edited by Geometrian
0

Share this post


Link to post
Share on other sites
Adding hash values together is a poor way to generate a composite hash. In your case if you have three elements (0, 0, 1), (0, 1, 0) and (1, 0, 0) they will all collide. Since one legal way of implementing std::hash<int> is to just return the integer then you could also get collisions with things like (1, 2, 3), (2, 2, 2) or (6, 0, 0). One way to handle this is to use boost::hash_combine() which will combine hash values in a more or less sane way. If you don't want to use boost, it's hash_combine implementation looks like:
template<typename T> void hash_combine(size_t & seed, T const& v) {
  seed ^= hash_value(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}
(or at least one of it's versions anyways)
0

Share this post


Link to post
Share on other sites

Computing the hash function as:

size_t h  = func(vr.x);
size_t h2 = func(vr.y);
size_t h3 = func(vr.z);
hash_combine(h,h2);
hash_combine(h,h3);
return h;

or:

size_t h = 0u;
hash_combine(h,vr.x);
hash_combine(h,vr.y);
hash_combine(h,vr.z);
return h;

. . . seems to give no noticeable improvement.
Thanks,

Edited by Geometrian
0

Share this post


Link to post
Share on other sites
That's not elements to pre-allocate, that's buckets. It's implementation defined, but very likely it still has to allocate the elements as it goes. And since you will probably have many matches, that's way too many buckets, so it's not going to help at all and could even make it slower.

I don't have a quick fix. You try write or try to find a hash map that pre-allocates elements, or use a custom allocator in unordered_map.
0

Share this post


Link to post
Share on other sites

Oops. Well, removing the "preallocation" doesn't seem to do much. If anything, it's a bit faster, but not enough.

0

Share this post


Link to post
Share on other sites
I could add, writing a hash map is not that much work if all you care about is insert and find, but writing allocators for stl can be a nightmare, so I know which I would do.
0

Share this post


Link to post
Share on other sites
Some possibilities: 1) if you're inserting elements one by one, try using range insert instead. 2) Switch to a pooled allocator like boost::pool.
1

Share this post


Link to post
Share on other sites
I would use something like FNV-1a for the hash.

What kind of container is this coming from? If it starts off in a linked list then it could easily be that much of the time is taken by traversing that list, especially if it's home-grown, or you are removing the items from it as you go.
1

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0