Phil Bagwell, in his 2002 paper on the VList data structure, indicates that you can use a VList to implement a persistent hash table. However, his explanation of how that worked didn't include much detail, and I don't understand it. Can anybody give me a more detailed explanation, or even examples?
Further, it appears to me from what I can see that this data structure, while it may have the same big-O complexity as a Hashtable, will be slower because it does additional lookups. Does anybody care to do a detailed analysis of just how much slower, preferably including cache behaviour? How does the performance relationship between the two change in the case of having no collisions or many?
I had a look at this paper, and it appears very preliminary. The fact that no later version has been published, and that the original appeared in IFL (which is a work-in-progress sort of meeting), suggests that you may be wasting your time.
Hrmm there seem to be a number of issues with the data structures proposed by the paper in question.
Off the cuff, the naive vlists mentioned first seem to need unique references in order to get anything near the time guarantees proposed. You lose the ability for the most part to share tails. You can share the tiny nodes towards the back of the list, but you wind up having to duplicate the largest vlist node the moment you cons something onto the cdr of vlist that is still active. That cost is proportional to the cost of copying the whole list.
With the 2d modifications mentioned later it becomes constant again, but its a pretty large constant, since you wind up at least copying the head of a list of pages (or worse, a vlist) and the first page in your list.
The functional hash list stuff in there didn't seem to make much sense to me to be honest. It was just a brief blurb that seemed to be bolted onto an otherwise unrelated paper, without enough detail to really make out how practical it is.
Related
I was reading this answer
Efficient (and well explained) implementation of a Quadtree for 2D collision detection
and encountered this paragraph
All right, so actually quadtrees are not my favorite data structure for this purpose. I tend to prefer a grid hierarchy, like a coarse grid for the world, a finer grid for a region, and an even finer grid for a sub-region (3 fixed levels of dense grids, and no trees involved), with row-based optimizations so that a row that has no entities in it will be deallocated and turned into a null pointer, and likewise completely empty regions or sub-regions turned into nulls. While this simple implementation of the quadtree running in one thread can handle 100k agents on my i7 at 60+ FPS, I've implemented grids that can handle a couple million agents bouncing off each other every frame on older hardware (an i3). Also I always liked how grids made it very easy to predict how much memory they'll require, since they don't subdivide cells. But I'll try to cover how to implement a reasonably efficient quadtree.
This type of grid seems intuitive, it sort of sounds like a "N-order" grid, where instead of 4 child nodes, you have N child nodes per parent. N^3 can go much further than 4^3, which allows better precision with potentially (I guess) less branching (since there are many less nodes to branch).
I'm a little puzzled because I would intuitively use a single, or maybe 3 std::map with the proper < operator(), to reduce its memory footprint, but I'm not sure it would be so fast, since querying an AABB would mean stacking several accesses that are O(log n).
What exactly are those row-based optimizations he is talking about? Is this type of grid search common?
I have some understanding of a z order curve, and I'm not entirely satisfied with a quadtree.
It's my own quote. But that's based on a common pattern I encountered in my personal experience. Also, terms like "parent" and "child" are ones I'd largely discard when talking about grids. You just got a big 2-dimensional or N-dimensional table/matrix storing stuff. There's not really a hierarchy involved whatsoever -- these data structures are more comparable to arrays than trees.
"Coarse" and "Fine"
On "coarse" and "fine", what I meant there is that "coarse" search queries tend to be cheaper but give more false positives. A coarser grid would be one that is lower in grid resolution (fewer, larger cells). Coarse searches may involve traversing/searching fewer and larger grid cells. For example, say we want to see if an element intersects a point/dot in a gigantic cell (imagine just a 1x1 grid storing everything in the simulation). If the dot intersects the cell, we may get a whole lot of elements returned in that cell but maybe only one or none of them actually intersect the dot.
So a "coarse" query is broad and simple but not very precise at narrowing down the list of candidates (or "suspects"). It may return too many results and still leave a whole lot of processing required left to do to narrow down what actually intersects the search parameter*.
It's like in those detective shows when they search a database for a
possible killer, putting in "white male" might not require much
processing to list the results but might give way too many results to
properly narrow down the suspects. "Fine" would be the opposite and might require more processing of the database but narrow down the result to just one suspect.
This is a crude and flawed analogy but I hope it helps.
Often the key to broadly optimizing spatial indices before we get into things like memory optimizations whether we're talking spatial hashes or quadtrees is to find a nice balance between "coarse" and "fine". Too "fine" and we might spend too much time traversing the data structure (searching many small cells in a uniform grid, or spending too much time in tree traversal for adaptive grids like quadtrees). Too "coarse" and the spatial index might give back too many results to significantly reduce the amount of time required for further processing. For spatial hashes (a data structure I don't personally like very much but they're very popular in gamedev), there's often a lot of thought and experimentation and measuring involved in determining an optimal cell size to use.
With uniform NxM grids, how "coarse" or "fine" they are (big or small cells and high or low grid resolution) not only impacts search times for a query but can also impact insertion and removal times if the elements are larger than a point. If the grid is too fine, a single large or medium-sized element may have to be inserted into many tiny cells and removed from many tiny cells, using lots of extra memory and processing. If it's too coarse, the element may only have to be inserted and removed to/from one large cell but at costs to the data structure's ability to narrow down the number of candidates returned from a search query to a minimum. Without care, going too "fine/granular" can become very bottlenecky in practice and a developer might find his grid structure using gigabytes of RAM for a modest input size. With tree variants like quadtrees, a similar thing can happen if the maximum allowed tree depth is too high a value causing explosive memory use and processing when the leaf nodes of the quadtree store the tiniest cell sizes (we can even start running into floating-point precision bugs that wreck performance if the cells are allowed to be subdivided to too small a size in the tree).
The essence of accelerating performance with spatial indices is often this sort of balancing act. For example, we typically don't want to apply frustum culling to individual polygons being rendered in computer graphics because that's typically not only redundant with what the hardware already does at the clipping stage, but it's also too "fine/granular" and requires too much processing on its own compared to the time required to just request to render one or more polygons. But we might net huge performance improvements with something a bit "coarser", like applying frustum culling to an entire creature or space ship (an entire model/mesh), allowing us to avoid requesting to render many polygons at once with a single test. So I often use the terms, "coarse" and "fine/granular" frequently in these sorts of discussions (until I find better terminology that more people can easily understand).
Uniform vs. Adaptive Grid
You can think of a quadtree as an "adaptive" grid with adaptive grid cell sizes arranged hierarchically (working from "coarse' to "fine" as we drill down from root to leaf in a single smart and adaptive data structure) as opposed to a simple NxM "uniform" grid.
The adaptive nature of the tree-based structures is very smart and can handle a broad range of use cases (although typically requiring some fiddling of maximum tree depth and/or minimum cell size allowed and possibly how many maximum elements are stored in a cell/node before it subdivides). However, it can be more difficult to optimize tree data structures because the hierarchical nature doesn't lend itself so easily to the kind of contiguous memory layout that our hardware and memory hierarchy is so well-suited to traverse. So very often I find data structures that don't involve trees to be easier to optimize in the same sense that optimizing a hash table might be simpler than optimizing a red-black tree, especially when we can anticipate a lot about the type of data we're going to be storing in advance.
Another reason I tend to favor simpler, more contiguous data structures in lots of contexts is that the performance requirements of a realtime simulation often want not just fast frame rates, but consistent and predictable frame rates. The consistency is important because even if a video game has very high frame rates for most of the game but some part of the game causes the frame rates to drop substantially for even a brief period of time, the player may die and game over as a result of it. It was often very important in my case to avoid these types of scenarios and have data structures largely absent pathological worst-case performance scenarios. In general, I find it easier to predict the performance characteristics of lots of simpler data structures that don't involve an adaptive hierarchy and are kind of on the "dumber" side. Very often I find the consistency and predictability of frame rates to be roughly tied to how easily I can predict the data structure's overall memory usage and how stable that is. If the memory usage is wildly unpredictable and sporadic, I often (not always, but often) find the frame rates will likewise be sporadic.
So I often find better results using grids personally, but if it's tricky to determine a single optimal cell size to use for the grid in a particular simulation context, I just use multiple instances of them: one instance with larger cell sizes for "coarse" searches (say 10x10), one with smaller ones for "finer" searches (say 100x100), and maybe even one with even smaller cells for the "finest" searches (say 1000x1000). If no results are returned in the coarse search, then I don't proceed with the finer searches. I get some balance of the benefits of quadtrees and grids this way.
What I did when I used these types of representations in the past is not to store a single element in all three grid instances. That would triple the memory use of an element entry/node into these structures. Instead, what I did was insert the indices of the occupied cells of the finer grids into the coarser grids, as there are typically far fewer occupied cells than there are a total number of elements in the simulation. Only the finest, highest-resolution grid with the smallest cell sizes would store the element. The cells in the finest grid are analogous to the leaf nodes of a quadtree.
The "loose-tight double grid" as I'm calling it in one of the answers to that question is an expansion on this multi-grid idea. The difference is that the finer grid is actually loose and has cell sizes that expand and shrink based on the elements inserted to it, always guaranteeing that a single element, no matter how large or small, needs only be inserted to one cell in the grid. The coarser grid stores the occupied cells of the finer grid leading to two constant-time queries (one in the coarser tight grid, another into the finer loose grid) to return an element list of potential candidates matching the search parameter. It also has the most stable and predictable memory use (not necessarily the minimal memory use because the fine/loose cells require storing an axis-aligned bounding box that expands/shrinks which adds another 16 bytes or so to a cell) and corresponding stable frame rates because one element is always inserted to one cell and doesn't take any additional memory required to store it besides its own element data with the exception of when its insertion causes a loose cell to expand to the point where it has to be inserted to additional cells in the coarser grid (which should be a fairly rare-case scenario).
Multiple Grids For Other Purposes
I'm a little puzzled because I would intuitively use a single, or maybe 3 std::map with the proper operator(), to reduce its memory footprint, but I'm not sure it would be so fast, since querying an AABB would mean stacking several accesses that are O(log n).
I think that's an intuition many of us have and also probably a subconscious desire to just lean on one solution for everything because programming can get quite tedious and repetitive and it'd be ideal to just implement something once and reuse it for everything: a "one-size-fits-all" t-shirt. But a one-sized-fits-all shirt can be poorly tailored to fit our very broad and muscular programmer bodies*. So sometimes it helps to use the analogy of a small, medium, and large size.
This is a very possibly poor attempt at humor on my part to make my long-winded texts less boring to read.
For example, if you are using std::map for something like a spatial hash, then there can be a lot of thought and fiddling around trying to find an optimal cell size. In a video game, one might compromise with something like making the cell size relative to the size of your average human in the game (perhaps a bit smaller or bigger), since lots of the models/sprites in the game might be designed for human use. But it might get very fiddly and be very sub-optimal for teeny things and very sub-optimal for gigantic things. In that case, we might do well to resist our intuitions and desires to just use one solution and use multiple (it could still be the same code but just multiple instances of the same class instance for the data structure constructed with varying parameters).
As for the overhead of searching multiple data structures instead of a single one, that's something best measured and it's worth remembering that the input sizes of each container will be smaller as a result, reducing the cost of each search and very possibly improve locality of reference. It might exceed the benefits in a hierarchical structure that requires logarithmic search times like std::map (or not, best to just measure and compare), but I tend to use more data structures which do this in constant-time (grids or hash tables). In my cases, I find the benefits far exceeding the additional cost of requiring multiple searches to do a single query, especially when the element sizes vary radically or I want some basic thing resembling a hierarchy with 2 or more NxM grids that range from "coarse" to "fine".
Row-Based Optimizations
As for "row-based optimizations", that's very specific to uniform fixed-sized grids and not trees. It refers to using a separate variable-sized list/container per row instead of a single one for the entire grid. Aside from potentially reducing memory use for empty rows that just turn into nulls without requiring an allocated memory block, it can save on lots of processing and improve memory access patterns.
If we store a single list for the entire grid, then we have to constantly insert and remove from that one shared list as elements move around, particles are born, etc. That could lead to more heap allocations/deallocations growing and shrinking the container but also increases the average memory stride to get from one element in that list to the next which will tend to translate to more cache misses as a result of more irrelevant data being loaded into a cache line. Also these days we have so many cores so having a single shared container for the entire grid may reduce the ability to process the grid in parallel (ex: searching one row while simultaneously inserting to another). It can also lead to more net memory use for the structure since if we use a contiguous sequence like std::vector or ArrayList, those can often store the memory capacity of as many as twice the elements required to reduce the time of insertions to amortized constant time by minimizing the need to reallocate and copy the former elements in linear-time by keeping excess capacity.
By associating a separate medium-sized container per grid row or per column instead of gigantic one for the entire grid, we can mitigate these costs in some cases*.
This is the type of thing you definitely measure before and after though to make sure it actually improves overall frame rates, and probably attempt in response to a first attempt storing a single list for the entire grid revealing many non-compulsory cache misses in the profiler.
This might beg the question of why we don't use a separate teeny list container for every single cell in the grid. It's a balancing act. If we store that many containers (ex: a million instances of std::vector for a 1000x1000 grid possibly storing very few or no elements each), it would allow maximum parallelism and minimize the stride to get from one element in a cell to the next one in the cell, but that can be quite explosive in memory use and introduce a lot of extra processing and heap overhead.
Especially in my case, my finest grids might store a million cells or more, but I only require 4 bytes per cell. A variable-sized sequence per cell would typically explode to at least something like 24 bytes or more (typically far more) per cell on 64-bit architectures to store the container data (typically a pointer and a couple of extra integers, or three pointers on top of the heap-allocated memory block), but on top of that, every single element inserted to an empty cell may require a heap allocation and compulsory cache miss and page fault and very frequently due to the lack of temporal locality. So I find the balance and sweet spot to be one list container per row typically among my best-measured implementations.
I use what I call a "singly-linked array list" to store elements in a grid row and allow constant-time insertions and removals while still allowing some degree of spatial locality with lots of elements being contiguous. It can be described like this:
struct GridRow
{
struct Node
{
// Element data
...
// Stores the index into the 'nodes' array of the next element
// in the cell or the next removed element. -1 indicates end of
// list.
int next = -1;
};
// Stores all the nodes in the row.
std::vector<Node> nodes;
// Stores the index of the first removed element or -1
// if there are no removed elements.
int free_element = -1;
};
This combines some of the benefits of a linked list using a free list allocator but without the need to manage separate allocator and data structure implementations which I find to be too generic and unwieldy for my purposes. Furthermore, doing it this way allows us to halve the size of a pointer down to a 32-bit array index on 64-bit architectures which I find to be a big measured win in my use cases when the alignment requirements of the element data combined with the 32-bit index don't require an additional 32-bits of padding for the class/struct which is frequently the case for me since I often use 32-bit or smaller integers and 32-bit single-precision floating-point or 16-bit half-floats.
Unorthodox?
On this question:
Is this type of grid search common?
I am not sure! I tend to struggle a bit with terminology and I'll have to ask people's forgiveness and patience in communication. I started programming from early childhood in the 1980s before the internet was widespread, so I came to rely on inventing a lot of my own techniques and using my own crude terminology as a result. I got my degree in computer science about a decade and a half later when I reached my 20s and corrected some of my terminology and misconceptions but I've had many years just rolling my own solutions. So I am often not sure if other people have come across some of the same solutions or not, and if there are formal names and terms for them or not.
That makes communication with other programmers difficult and very frustrating for both of us at times and I have to ask for a lot of patience to explain what I have in mind. I've made it a habit in meetings to always start off showing something with very promising results which tends to make people more patient with my crude and long-winded explanations of how they work. They tend to give my ideas much more of a chance if I start off by showing results, but I'm often very frustrated with the orthodoxy and dogmatism that can be prevalent in this industry that can sometimes prioritize concepts far more than execution and actual results. I'm a pragmatist at heart so I don't think in terms of "what is the best data structure?" I think in terms of what we can effectively implement personally given our strengths and weaknesses and what is intuitive and counter-intuitive to us and I'm willing to endlessly compromise on the purity of concepts in favor of a simpler and less problematic execution. I just like good, reliable solutions that roll naturally off our fingertips no matter how orthodox or unorthodox they may be, but a lot of my methods may be unorthodox as a result (or not and I might just have yet to find people who have done the same things). I've found this site useful at rare times in finding peers who are like, "Oh, I've done that too! I found the best results if we do this [...]" or someone pointing out like, "What you are proposing is called [...]."
In performance-critical contexts, I kind of let the profiler come up with the data structure for me, crudely speaking. That is to say, I'll come up with some quick first draft (typically very orthodox) and measure it with the profiler and let the profiler's results give me ideas for a second draft until I converge to something both simple and performant and appropriately scalable for the requirements (which may become pretty unorthodox along the way). I'm very happy to abandon lots of ideas since I figure we have to weed through a lot of bad ideas in a process of elimination to come up with a good one, so I tend to cycle through lots of implementations and ideas and have come to become a really rapid prototyper (I have a psychological tendency to stubbornly fall in love with solutions I spent lots of time on, so to counter that I've learned to spend the absolute minimal time on a solution until it's very, very promising).
You can see my exact methodology at work in the very answers to that
question where I iteratively converged through lots of profiling and
measuring over the course of a few days and prototyping from a fairly orthodox quadtree to that
unorthodox "loose-tight double grid" solution that handled the largest
number of agents at the most stable frame rates and was, for me
anyway, much faster and simpler to implement than all the structures
before it. I had to go through lots of orthodox solutions and measure them though to generate the final idea for the unusual loose-tight variant. I always start off with and favor the orthodox solutions and start off inside the box because they're well-documented and understood and just very gently and timidly venture outside, but I do often find myself a bit outside the box when the requirements are steep enough. I'm no stranger to the steepest requirements since in my industry and as a fairly low-level type working on engines, the ability to handle more data at good frame rates often translates not only to greater interactivity for the user but also allows artists to create more detailed content of higher visual quality than ever before. We're always chasing higher and higher visual quality at good frame rates, and that often boils down to a combination of both performance and getting away with crude approximations whenever possible. This inevitably leads to some degree of unorthodoxy with lots of in-house solutions very unique to a particular engine, and each engine tends to have its own very unique strengths and weaknesses as you find comparing something like CryEngine to Unreal Engine to Frostbite to Unity.
For example, I have this data structure I've been using since childhood and I don't know the name of it. It's a straightforward concept and it's just a hierarchical bitset that allows set intersections of potentially millions of elements to be found in as little as a few iterations of simple work as well as traverse millions of elements occupying the set with just a few iterations (less than linear-time requirements to traverse everything in the set just through the data structure itself which returns ranges of occupied elements/set bits instead of individual elements/bit indices). But I have no idea what the name is since it's just something I rolled and I've never encountered anyone talking about it in computer science. I tend to refer to it as a "hierarchical bitset". Originally I called it a "sparse bitset tree" but that seems a tad verbose. It's not a particularly clever concept at all and I wouldn't be surprised or disappointed (actually quite happy) to find someone else discovering the same solution before me but just one I don't find people using or talking about ever. It just expands on the strengths of a regular, flat bitset in rapidly finding set intersections with bitwise OR and rapidly traverse all set bits using FFZ/FFS but reducing the linear-time requirements of both down to logarithmic (with the logarithm base being a number much larger than 2).
Anyway, I wouldn't be surprised if some of these solutions are very unorthodox, but also wouldn't be surprised if they are reasonably orthodox and I've just failed to find the proper name and terminology for these techniques. A lot of the appeal of sites like this for me is a lonely search for someone else who has used similar techniques and to try to find proper names and terms for them often to end in frustration. I'm also hoping to improve on my ability to explain them although I've always been so bad and long-winded here. I find using pictures helps me a lot because I find human language to be incredibly riddled with ambiguities. I'm also fond of deliberately imprecise figurative language which embraces and celebrates the ambiguities such as metaphor and analogy and humorous hyperbole, but I've not found it's the type of thing programmers tend to appreciate so much due to its lack of precision. But I've never found precision to be that important so long as we can convey the meaty stuff and what is "cool" about an idea while they can draw their own interpretations of the rest. Apologies for the whole explanation but hopefully that clears some things up about my crude terminology and the overall methodology I use to arrive at these techniques. English is also not my first language so that adds another layer of convolution where I have to sort of translate my thoughts into English words and struggle a lot with that.
So I've been(so far unsuccessfully) trying to make my red-black tree implementation work consistently with duplicates, but it seems to always be missing that small something, so here I am.
I tried make the tree lean to one side, but It didn't seem to balance it properly(from the color perspective).I'd like to ask how should one go about adding duplicates to a red-black tree?(apart obviously making the node fat, holding or pointing to duplicate key values).
Not really looking for a code review, more interested in suggestions. So basically the methods(taken from Introduction to Algorithms, Third Edition) I use for insert and balancing are these(while rotations are pretty obvious):
If you look at the pseudo-code you wrote here, it is completely agnostic to the question of whether keys are duplicate or not. The code here only looks at the result of comparing keys, and doesn't care if they are identical or not. In fact, unique-key implementations need to go out of their way to make RB-Insert detect duplicate keys. The data structure doesn't care about this naturally, and the algorithms and proofs hold whether there are duplicate keys or not. If you implemented these functions correctly, it should work as is.
I also disagree with the comments advising you to hold what you call "fat nodes". Holding multiple keys is the common implementation of C++'s std::multimap, for example. Not that from a computational complexity point of view, say that you have altogether n keys, but each k are a multiple. Using the "efficient" fat node version, the complexity of the basic find operation will be Θ(log(n / k)) = Θ(log(n) - log(k)); using the multiple key version, the complexity will be Θ(log(n)). In real life cases, probably k << n, which means that the relative difference is negligible.
I have a question about fundamentals in data structures.
I understand that array's access time is faster than a linked list. O(1)- array vs O(N) -linked list
But a linked list beats an array in removing an element since there is no shifting needing O(N)- array vs O(1) -linked list
So my understanding is that if the majority of operations on the data is delete then using a linked list is preferable.
But if the use case is:
delete elements but not too frequently
access ALL elements
Is there a clear winner? In a general case I understand that the downside of using the list is that I access each node which could be on a separate page while an array has better locality.
But is this a theoretical or an actual concern that I should have?
And is the mixed-type i.e. create a linked list from an array (using extra fields) good idea?
Also does my question depend on the language? I assume that shifting elements in array has the same cost in all languages (at least asymptotically)
Singly-linked lists are very useful and can be better performance-wise relative to arrays if you are doing a lot of insertions/deletions, as opposed to pure referencing.
I haven't seen a good use for doubly-linked lists for decades.
I suppose there are some.
In terms of performance, never make decisions without understanding relative performance of your particular situation.
It's fairly common to see people asking about things that, comparatively speaking, are like getting a haircut to lose weight.
Before writing an app, I first ask if it should be compute-bound or IO-bound.
If IO-bound I try to make sure it actually is, by avoiding inefficiencies in IO, and keeping the processing straightforward.
If it should be compute-bound then I look at what its inner loop is likely to be, and try to make that swift.
Regardless, no matter how much I try, there will be (sometimes big) opportunities to make it go faster, and to find them I use this technique.
Whatever you do, don't just try to think it out or go back to your class notes.
Your problem is different from anyone else's, and so is the solution.
The problem with a list is not just the fragmentation, but mostly the data dependency. If you access every Nth element in array you don't have locality, but the accesses may still go to memory in parallel since you know the address. In a list it depends on the data being retrieved, and therefore traversing a list effectively serializes your memory accesses, causing it to be much slower in practice. This of course is orthogonal to asymptotic complexities, and would harm you regardless of the size.
I am looking to implement my own collection class. The characteristics I want are:
Iterable - order is not important
Insertion - either at end or at iterator location, it does not matter
Random Deletion - this is the tricky one. I want to be able to have a reference to a piece of data which is guaranteed to be within the list, and remove it from the list in O(1) time.
I plan on the container only holding custom classes, so I was thinking a doubly linked list that required the components to implement a simple interface (or abstract class).
Here is where I am getting stuck. I am wondering whether it would be better practice to simply have the items in the list hold a reference to their node, or to build the node right into them. I feel like both would be fairly simple, but I am worried about coupling these nodes into a bunch of classes.
I am wondering if anyone has an idea as to how to minimize the coupling, or possibly know of another data structure that has the characteristics I want.
It'd be hard to beat a hash map.
Take a look at tries.
Apparently they can beat hashtables:
Unlike most other algorithms, tries have the peculiar feature that the time to insert, or to delete or to find is almost identical because the code paths followed for each are almost identical. As a result, for situations where code is inserting, deleting and finding in equal measure tries can handily beat binary search trees or even hash tables, as well as being better for the CPU's instruction and branch caches.
It may or may not fit your usage, but if it does, it's likely one of the best options possible.
In C++, this sounds like the perfect fit for std::unordered_set (that's std::tr1::unordered_set or boost::unordered_set to you if you have an older compiler). It's implemented as a hash set, which has the characteristics you describe.
Here's the interface documentation. Note that the hash containers actually offer two sets of iterators, the usual ones and local ones which only go through one bucket.
Many other languages have "hash sets" as well, certainly Java and C#.
I've been working on a problem which I thought people might find interesting (and perhaps someone is aware of a pre-existing solution).
I have a large dataset consisting of a long list of pairs of pointers to objects, something like this:
[
(a8576, b3295),
(a7856, b2365),
(a3566, b5464),
...
]
There are way too many objects to keep in memory at any one time (potentially hundreds of gigabytes), so they need to be stored on disk, but can be cached in memory (probably using an LRU cache).
I need to run through this list processing every pair, which requires that both objects in the pair be loaded into memory (if they aren't already cached there).
So, the question: is there a way to reorder the pairs in the list to maximize the effectiveness of an in-memory cache (in other words: minimize the number of cache misses)?
Notes
Obviously, the re-ordering algorithm should be as fast as possible, and shouldn't depend on being able to have the entire list in memory at once (since we don't have enough RAM for that) - but it could iterate over the list several times if necessary.
If we were dealing with individual objects, not pairs, then the simple answer would be to sort them. This obviously won't work in this situation because you need to consider both elements in the pair.
The problem may be related to that of finding a minimum graph cut, but even if the problems are equivalent, I don't think solutions to min-cut meet
My assumption is that the heuristic would stream the data off the disk, and write it back in chunks in a better order. It may need to iterate over this several times.
Actually it may not just be pairs, it could be triplets, quadruplets, or more. I'm hoping that an algorithm that does this for pairs can be easily generalized.
Your problem is related to a similar one for computer graphics hardware:
When rendering indexed vertices in a triangle mesh, typically the hardware has a cache of most recently transformed vertices (~128 the last time I had to worry about it, but suspect the number is larger these days). Vertices not cached need a relatively expensive transform operation to calculate. "Mesh optimisation" to restructure triangle meshes to optimise cache usage used to be a pretty hot research topic. Googling
vertex cache optimisation
(or optimization :^) might find you some interesting material relevant to your problem. As other posters suggest, I suspect doing this effectively will depend on exploiting any inherent coherence in your data.
Another thing to bear in mind: as an LRU cache becomes overloaded it can be well worth changing to an MRU replacement strategy to at least hold some of the items in memory (rather than turning over the entire cache each pass). I seem to remember John Carmack has written some good material on this subject in connection with Direct3D texture caching strategies.
For start, you could mmap the list. That works if there's enough address space, not memory, e.g. on 64-bit CPUs. This makes it easier to access the elements in order.
You could sort that list according to a minimum distance in cache which considers both elements, which works well if the objects are in a contiguous space. The sorting function could be something like: compare (a, b) to (c, d) = (a - c) + (b - d) (which looks like a Hamming distance). Then you pull in slices of the object store and process according to the list.
EDIT: fixed a mistake in the distance.
Even though you're not just sorting this list, the general pattern of a multiway merge sort might be applicable - that is, consider some kind of (possibly recursive) breakdown of the set into smaller sets that can be dealt with in memory separately, and then a second phase where small chunks of the previously dealt-with sets can all be combined together. Even not knowing the specific nature of what you're doing with the pairs, it's safe to say that many algorithmic problems are made much more straightforward when you're dealing with sorted data (including graph problems, which might be what you have on your hands here).
I think the answer to this question is going to depend very heavily on exactly the access pattern of the pair of objects. As you said, just sorting the pointers would be best in a simple, non-paired case. In a more complex case it may still make sense to sort by one of the halves of the pair if the pattern is such that locality for those values is more important (if, for example, these are key/value pairs and you are doing a lot of searches, locality for the keys is infinitely more important than for the values).
So, really, my answer is that this question can't be answered in a general case.
For storing your structure, what you actually want is probably a B-tree. These are designed for what you're talking about--keeping track of large collections where you don't want to (or can't) keep the whole thing in memory.