Module s2.util.container.dense_hash_table

A dense hashtable is a particular implementation of a hashtable: one that is meant to minimize memory allocation. It does this by using an array to store all the data. We steal a value from the key space to indicate "empty" array elements (ie indices where no item lives) and another to indicate "deleted" elements.

(Note it is possible to change the value of the delete key on the fly; you can even remove it, though after that point the hashtable is insert_only until you set it again. The empty value however can't be changed.)

To minimize allocation and pointer overhead, we use internal probing, in which the hashtable is a single table, and collisions are resolved by trying to insert again in another bucket. The most cache-efficient internal probing schemes are linear probing (which suffers, alas, from clumping) and quadratic probing, which is what we implement by default.

Type requirements: value_type is required to be Copy Constructible and Default Constructible. It is not required to be (and commonly isn't) Assignable.

You probably shouldn't use this code directly. Use dense_hash_map<> or dense_hash_set<> instead.

You can change the following below:

- HT_OCCUPANCY_PCT -- how full before we double size

- HT_EMPTY_PCT -- how empty before we halve size

- HT_MIN_BUCKETS -- default smallest bucket size

You can also change enlarge_factor (which defaults to HT_OCCUPANCY_PCT), and shrink_factor (which defaults to HT_EMPTY_PCT) with set_resizing_parameters().

How to decide what values to use? shrink_factor's default of .4 * OCCUPANCY_PCT, is probably good. HT_MIN_BUCKETS is probably unnecessary since you can specify (indirectly) the starting number of buckets at construct-time. For enlarge_factor, you can use this chart to try to trade-off expected lookup time to the space taken up. By default, this code uses quadratic probing, though you can change it to linear via JUMP_ below if you really want to.

From http://www.augustana.ca/~mohrj/courses/1999.fall/csc210/lecture_notes/hashing.html 

NUMBER OF PROBES / LOOKUP       Successful            Unsuccessful
Quadratic collision resolution   1 - ln(1-L) - L/2    1/(1-L) - L - ln(1-L)
Linear collision resolution     [1+1/(1-L)]/2         [1+1/(1-L)2]/2

-- enlarge_factor --           0.10  0.50  0.60  0.75  0.80  0.90  0.99
QUADRATIC COLLISION RES.
   probes/successful lookup    1.05  1.44  1.62  2.01  2.21  2.85  5.11
   probes/unsuccessful lookup  1.11  2.19  2.82  4.64  5.81  11.4  103.6
LINEAR COLLISION RES.
   probes/successful lookup    1.06  1.5   1.75  2.5   3.0   5.5   50.5
   probes/unsuccessful lookup  1.12  2.5   3.6   8.5   13.0  50.0  5000.0

Classes

NameDescription
DenseHashTable Hashtable class, used to implement the hashed associative containers hash_set and hash_map.

Structs

NameDescription
DenseHashTableIterator A basic iterator type for finding entries and iterating. We're just an array, but we need to skip over empty and deleted elements.