# Poco

# template < class Value, class HashFunc = Hash < Value >>

# class LinearHashTable

Library: Foundation
Package: Hashing
Header: Poco/LinearHashTable.h

## Description

This class implements a linear hash table.

In a linear hash table, the available address space grows or shrinks dynamically. A linear hash table thus supports any number of insertions or deletions without lookup or insertion performance deterioration.

Linear hashing was discovered by Witold Litwin in 1980 and described in the paper LINEAR HASHING: A NEW TOOL FOR FILE AND TABLE ADDRESSING.

For more information on linear hashing, see <http://en.wikipedia.org/wiki/Linear_hash>.

The LinearHashTable is not thread safe.

Value must support comparison for equality.

Find, insert and delete operations are basically O(1) with regard to the total number of elements in the table, and O(N) with regard to the number of elements in the bucket where the element is stored. On average, every bucket stores one element; the exact number depends on the quality of the hash function. In most cases, the maximum number of elements in a bucket should not exceed 3.

## Member Summary

Member Functions: begin, bucketAddress, bucketAddressForHash, buckets, calcSize, clear, count, empty, end, erase, find, insert, merge, operator =, size, split, swap

## Nested Classes

### class ConstIterator

### class Iterator

## Types Aliases

### Bucket

using Bucket = std::vector < Value >;

### BucketIterator

using BucketIterator = typename Bucket::iterator;

### BucketVec

using BucketVec = std::vector < Bucket >;

### BucketVecIterator

using BucketVecIterator = typename BucketVec::iterator;

### ConstPointer

using ConstPointer = const Value *;

### ConstReference

using ConstReference = const Value &;

### Hash

using Hash = HashFunc;

### Pointer

using Pointer = Value *;

### Reference

using Reference = Value &;

### ValueType

using ValueType = Value;

## Constructors

### LinearHashTable

LinearHashTable(
std::size_t initialReserve = 64
);

Creates the LinearHashTable, using the given initialReserve.

### LinearHashTable

LinearHashTable(
const LinearHashTable & table
);

Creates the LinearHashTable by copying another one.

## Destructor

### ~LinearHashTable

~LinearHashTable() = default;

Destroys the LinearHashTable.

## Member Functions

### begin

ConstIterator begin() const;

Returns an iterator pointing to the first entry, if one exists.

### begin

Iterator begin();

Returns an iterator pointing to the first entry, if one exists.

### buckets

std::size_t buckets() const;

Returns the number of allocated buckets.

### clear

void clear();

Erases all elements.

### count

std::size_t count(
const Value & value
) const;

Returns the number of elements with the given value, with is either 1 or 0.

### empty

bool empty() const;

Returns true iff the table is empty.

### end

ConstIterator end() const;

Returns an iterator pointing to the end of the table.

### end

Iterator end();

Returns an iterator pointing to the end of the table.

### erase

void erase(
Iterator it
);

Erases the element pointed to by it.

### erase

void erase(
const Value & value
);

Erases the element with the given value, if it exists.

### find

ConstIterator find(
const Value & value
) const;

Finds an entry in the table.

### find

Iterator find(
const Value & value
);

Finds an entry in the table.

### insert

std::pair < Iterator, bool > insert(
const Value & value
);

Inserts an element into the table.

If the element already exists in the table, a pair(iterator, false) with iterator pointing to the existing element is returned. Otherwise, the element is inserted an a pair(iterator, true) with iterator pointing to the new element is returned.

### operator =

LinearHashTable & operator = (
const LinearHashTable & table
);

Assigns another LinearHashTable.

### size

std::size_t size() const;

Returns the number of elements in the table.

### swap

void swap(
LinearHashTable & table
) noexcept;

Swaps the LinearHashTable with another one.

### bucketAddress

std::size_t bucketAddress(
const Value & value
) const;

### bucketAddressForHash

std::size_t bucketAddressForHash(
std::size_t hash
);

### calcSize

static std::size_t calcSize(
std::size_t initialSize
);

### merge

void merge();

### split

void split();