On 03/30/2011 03:31 PM, Steven Schveighoffer wrote:
On Tue, 29 Mar 2011 22:20:05 -0400, Nick Sabalausky <a@a.a> wrote:
"spir" <denis.s...@gmail.com> wrote in message
news:mailman.2909.1301443345.4748.digitalmars-d-le...@puremagic.com...
On 03/30/2011 01:24 AM, Nick Sabalausky wrote:
My understanding of hash tables is that they allocate a fixed size array
and
map keys to indicies within the range 0..predefined_length_of_the_AA.
So I've been wondering, how many elements do D's built-in AAs have? And
what's the content of each one, just a single pointer?
Each element is a data structure, often called bucket (typically a link
list), storing (key:value) pairs for which the key, once hashed and
modulo-ed, maps to the given index. That's why the famous O(1) lookup time
for hash tables is very theoretic: the constant part holds average time
for hashing which is very variable, plus another average time for linearly
traversing the bucket. The latter part depends on table load factor (=
number of elements / number of buckets) and proper statistical repartition
of keys into buckets.
The key (!) points are finding a good hash func to "linearize" said
repartition (which depends on actual key-data domain, not only on their
type...), but better ones rapidly eat much time (ones used in practice are
rather simple & fast); and finding optimal load factor, and growth scheme.
In practice, all of this tends to make hash tables an implementation
nightmare (for me). I'd love to find practicle alternatives, but efficient
binary trees also are complex and even more depend on kind of keys, I
guess.
Right, I know that, but that's not what I was asking. Take this hypothetical
implementation:
struct Bucket(TKey, TVal)
{
...
}
enum hashTableSize = ...;
struct Hash(TKey, TVal)
{
Bucket!(TKey, TVal)[hashTableSize] data;
TVal get(TKey key) { ... }
void set(TKey key, TVal value) { ... }
}
I assume that D's AA are something at least vaguely like that. My questions
are:
1. What does D use for "hashTableSize"? Or does "hashTableSize" vary? If it
varies, what's a typical rough ballpark size? (And just out of curiosity, if
it varies, is it decided at compile-time, or does it change even at
runtime?)
It varies. The hash table size is not constant, the load factor is. The load
factor is the number of elements in the hash divided by the number of buckets.
You never want to fill up all the spaces, because the more full you get, the
more chances for collisions there are. Essentially, the tricky part about
hashing is what to do about collisions (two elements are different, but go in
the same bucket).
So what happens is when the load factor exceeds a predefined constant (e.g. in
dcollections the load factor defaults to .75), the table "rehashes", or
increases (usually logarithmically) the size of the array, and re-inserts all
its elements.
I believe there is a minimum array size, and things are increased from there. I
think also you can do a manual "rehash" which should adjust the size of the
array to match a certain load factor (below the maximum).
Yes, there is a rehash method.
In some implementations, hashes will even shrink when the load factor goes
below a minimum (dcollections does not do this to avoid invalidating ranges).
There are a million different ways to implement the basic hash. The most
complex part though, is usually the collision handling. In my algo book, there
are at least 3 ways to handle collisions, and I think there are countless more.
If you look up hashing on wikipedia, you'll get a much better explanation.
2. What is "sizeof(Bucket(TKey, TVal))"? And I mean the shallow size, not
deep size. Is it dependent on TKey or TVal? Or is it just simply a pointer
to the start of a linked list (and therefore "sizeof(size_t)")?
Here is the AA implementation:
https://github.com/D-Programming-Language/druntime/blob/master/src/rt/aaA.d
From that page, you can see that AA is your bucket (note this is runtime
stuff, so there are no templates), and BB is your Hash struct. It looks like BB
has an array of AA pointers.
IIRC, this is because buckets are (minimal) link lists. Cells holding key:value
are list nodes.
-Steve
--
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