Module com.github.akurilov.concurrent

Package it.unimi.dsi.fastutil

Interface Hash

All Known Implementing Classes:

Int2IntOpenHashMap, Int2ObjectOpenHashMap, Object2IntOpenHashMap

public interface Hash

Basic data for all hash-based classes.

Historical note

Warning: the following comments are here for historical reasons, and apply just to the double hash classes that can be optionally generated. The standard fastutil distribution since 6.1.0 uses linear-probing hash tables, and tables are always sized as powers of two.

The classes in fastutil are built around open-addressing hashing implemented via double hashing. Following Knuth's suggestions in the third volume of The Art of Computer Programming, we use for the table size a prime p such that p-2 is also prime. In this way hashing is implemented with modulo p, and secondary hashing with modulo p-2.

Entries in a table can be in three states: FREE, OCCUPIED or REMOVED. The naive handling of removed entries requires that you search for a free entry as if they were occupied. However, fastutil implements two useful optimizations, based on the following invariant:

Let i₀, i₁, …, i_p-1 be the permutation of the table indices induced by the key k, that is, i₀ is the hash of k and the following indices are obtained by adding (modulo p) the secondary hash plus one. If there is a OCCUPIED entry with key k, its index in the sequence above comes before the indices of any REMOVED entries with key k.

When we search for the key k we scan the entries in the sequence i₀, i₁, …, i_p-1 and stop when k is found, when we finished the sequence or when we find a FREE entry. Note that the correctness of this procedure it is not completely trivial. Indeed, when we stop at a REMOVED entry with key k we must rely on the invariant to be sure that no OCCUPIED entry with the same key can appear later. If we insert and remove frequently the same entries, this optimization can be very effective (note, however, that when using objects as keys or values deleted entries are set to a special fixed value to optimize garbage collection).

Moreover, during the probe we keep the index of the first REMOVED entry we meet. If we actually have to insert a new element, we use that entry if we can, thus avoiding to pollute another FREE entry. Since this position comes a fortiori before any REMOVED entries with the same key, we are also keeping the invariant true.

Nested Class Summary

Nested Classes

Modifier and Type	Interface	Description
static interface	Hash.Strategy<K>	A generic hash strategy.

Field Summary

Fields

Modifier and Type	Field	Description
static int	DEFAULT_GROWTH_FACTOR	The default growth factor of a hash table.
static int	DEFAULT_INITIAL_SIZE	The initial default size of a hash table.
static float	DEFAULT_LOAD_FACTOR	The default load factor of a hash table.
static float	FAST_LOAD_FACTOR	The load factor for a (usually small) table that is meant to be particularly fast.
static byte	FREE	The state of a free hash table entry.
static byte	OCCUPIED	The state of a occupied hash table entry.
static int[]	PRIMES	A list of primes to be used as table sizes.
static byte	REMOVED	The state of a hash table entry freed by a deletion.
static float	VERY_FAST_LOAD_FACTOR	The load factor for a (usually very small) table that is meant to be extremely fast.

Field Detail

DEFAULT_INITIAL_SIZE

static final int DEFAULT_INITIAL_SIZE

The initial default size of a hash table.

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Constant Field Values

DEFAULT_LOAD_FACTOR

static final float DEFAULT_LOAD_FACTOR

The default load factor of a hash table.

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Constant Field Values

FAST_LOAD_FACTOR

static final float FAST_LOAD_FACTOR

The load factor for a (usually small) table that is meant to be particularly fast.

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Constant Field Values

VERY_FAST_LOAD_FACTOR

static final float VERY_FAST_LOAD_FACTOR

The load factor for a (usually very small) table that is meant to be extremely fast.

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Constant Field Values

DEFAULT_GROWTH_FACTOR

static final int DEFAULT_GROWTH_FACTOR

The default growth factor of a hash table.

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Constant Field Values

FREE

static final byte FREE

The state of a free hash table entry.

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Constant Field Values

OCCUPIED

static final byte OCCUPIED

The state of a occupied hash table entry.

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Constant Field Values

REMOVED

static final byte REMOVED

The state of a hash table entry freed by a deletion.

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Constant Field Values

PRIMES

static final int[] PRIMES

A list of primes to be used as table sizes. The i-th element is the largest prime p smaller than 2^(i+28)/16 and such that p-2 is also prime (or 1, for the first few entries).