[Overview][Constants][Types][Classes][Index] | Reference for unit 'DynHashArray' (#lcl)

# Reference for unit 'DynHashArray'

Module of managing dynamic sets or associative arrays

`uses`
`System,`
`Classes,`
`sysutils,`
`LCLProc;` |  | Useful lower level helper functions and classes

## Overview

Extends the functionality of Pascal by offering alternative data structure : set like array (the order is not kept) with fast find/delete. Its size can change automatically during the execution of the program. The price is that it is somewhat slower than a number indexes access of an array.

This unit defines TDynHashArray, which is very similar to a TList, since it also stores pointer/objects.

It supports Add, Remove, Contains, First, Count and Clear.

Because of the hashing nature the operations adding, removing and finding is done in constant time on average.

    Inner structure:
          There are three parts:
            1. The array itself (FItems). Every entry is a pointer to the first
               TDynHashArrayItem of a list with the same hash index. The first item
               of every same index list is the list beginning and its IsOverflow
               flag is set to false. All other items are overflow items.
               To get all items with the same hash index, do a FindHashItem. Then
               search through all "Next" items until Next is nil or its IsOverflow
               flag is set to false.
            2. The items beginning with FFirstItem is a 2-way-connected list of
               TDynHashArrayItem. This list contains all used items.
            3. To reduce GetMem/FreeMem calls, free items are cached.

      Issues:
        The maximum capacity is the PrimeNumber. You can store more items, but the
        performance decreases. The best idea is to provide your own hash function.

        Important: Items in the TDynHashArray must not change their key.
          When changing the key of an item, remove it and add it after the change.

The latest version of this document can be found at lazarus-ccr.sourceforge.net.