Chapter 5 - Hash Tables

Open addressing

Collision resolution technique where collisions are resolved by looking for an empty bucket elsewhere in the table

Chaining

Collision resolution technique where each bucket has a list of items (more than one item per bucket)

Empty-after-removal bucket

Had an item removed that caused the bucket to now be empty

Empty-since-start bucket

Has been empty since the hash table was created

probing sequence

Iterating through sequential i values to obtain the desired index in hash table

encryption

alteration of data to hide the original meaning

hash function

computes a bucket index from the item's key

modulo operator, %

computes the integer remainder when dividing two numbers. Is a commonly used hash function

password hashing function

cryptographic hashing function that produces a hash value for a password; databases commonly store hashes instead of actual passwords

hash table

data structure that stores unordered items by mapping (hashing) each item to a location in an array (or vector) -main advantage is that searching, inserting, or removing an item may require only an O(1) runtime

bucket

each hash table array element

cryptograpy

field of study focusing on transmitting data securely

Linear probing

handles a collision by starting at the key's mapped bucket, and then linearly searches subsequent buckets until an empty bucket is found

Quadratic Probing

handles a collision by starting at the key's mapped bucket, and then quadratically searches subsequent buckets until an empty bucket is found: (H+c1∗i+c2∗i^2)mod(tablesize) is used to determine the item's index in the hash table

cryptographic hash function

hash function designed specifically for cryptography; commonly used for encrypting and decrypting data

direct access table

hash table with a direct hash function

modulo hash function

key % N (hash table is size N)

perfect hash function

maps items to buckets with no collisions. If number of items and all possible item keys are know beforehand, perfect hash function can be created -insertion,search, and remove all are O(1)

collision

occurs when an item being inserted into a hash table maps to the same bucket as an existing item in the hash table

double hashing

open-addressing collision resolution technique that uses 2 different hash functions to compute bucket indices. Using hash functions h1 and h2, a key's index in the table is computed with the formula ( h1(key) + i∗h2(key) )mod(tablesize)

decryption

reconstruction of original data from encrypted data

multiplicative string hash

repeatedly multiplies the hash value and adds the ASCII (or Unicode) value of each character in the string

mid-square hash

squares the key, extracts R digits from the result's middle and returns the remainder of the middle digits divide by hash table size, N

key

the value used to map to an index in a hash table. Every key is unique

direct hash function

uses the item's key as the bucket index