Hashing in Data Structure: In computer science, hashing is a technique that is used to store, search, and retrieve data in an efficient manner. It is an important concept in data structures and algorithms and is widely used in various applications such as databases, cryptography, and computer networking. Hashing is a process of converting large data into small data in a fixed size called hash code, which represents the original data. In this article, we will discuss the definition, features, and various aspects of hashing in data structures.

## Definition of Hashing:

Hashing is a technique that is used to map large data into a smaller fixed-size data representation, called a hash code or hash value. The hash value is used to represent the original data in a unique way. The process of computing the hash value from the original data is known as hashing. The hash function takes an input data of arbitrary length and produces a fixed-size output, called a hash code, which is a unique representation of the input data.

## Here are some key features of hashing:

Fast access: One of the key features of hashing is its ability to provide fast access to data. Hashing allows for constant-time access to data, which means that the time required to retrieve data from a hash table is independent of the size of the data set.

### Efficient storage:

Hashing is an efficient method of storing data. Unlike other data structures such as arrays and linked lists, which require a fixed amount of memory to store each data element, hashing only requires memory proportional to the number of data elements stored.

### Easy insertion and deletion:

Another important feature of hashing is its ability to handle insertions and deletions of data with ease. Hashing allows for constant-time insertion and deletion of data, which means that the time required to add or remove data from a hash table is independent of the size of the data set.

Collision handling: In hashing, collisions can occur when two different keys produce the same hash value. An effective hashing algorithm must handle collisions effectively to ensure that data is not lost. There are various techniques for handling collisions, including chaining and open addressing.

### Deterministic:

The same input data should always produce the same hash code. This is a fundamental requirement for any hashing function.

### Secure:

In some applications, hashing is used to provide secure data storage and authentication. In such cases, the hashing function should be designed to be secure and resistant to attacks such as brute force attacks and hash collisions.

### Versatile:

Hashing is a versatile data structure that can be used in a wide range of applications. It can be used for data indexing, searching, and sorting, as well as for cryptographic purposes.

**Types of hashing in data structure**

There are several types of hashing in data structure, each with its own advantages and disadvantages. The three most common types are:

### Division hashing:

This is the simplest form of hashing, where the key is divided by the table size and the remainder is used as the index of the table. For example, if the table size is 10 and the key is 35, the index would be 35 % 10 = 5. Division hashing is easy to implement, but can lead to clustering if the key distribution is not uniform.

### Multiplication hashing:

In this method, the key is multiplied by a constant value between 0 and 1 and the fractional part is used as the index of the table. The constant value is usually chosen to be a prime number. Multiplication hashing is less prone to clustering than division hashing and is efficient for large tables.

### Universal hashing:

This is a randomized hashing method that uses a family of hash functions instead of a single hash function. The hash function is chosen randomly from the family of hash functions for each key, which reduces the likelihood of collisions. Universal hashing is highly resistant to malicious input and is suitable for security applications.

## Other types of hashing include:

### Folding hashing:

This method involves dividing the key into equal-sized parts and adding them together to generate the hash code. For example, if the key is 123456, it can be divided into 12, 34, and 56 and added together to get a hash code of 102.

### Perfect hashing:

In this method, the hash function is designed to guarantee that there are no collisions in the hash table. It is useful for applications where the set of keys is fixed and known in advance.

### Cuckoo hashing:

This is a hash table algorithm that uses two hash functions to map keys to table indexes. If there is a collision, the key is moved to the alternate index, and the process is repeated until a free slot is found. Cuckoo hashing has a worst-case time complexity of O(1) for all operations, but it requires a larger table size than other methods.

** The hash function must have the following properties:**

### Deterministic:

The same input data should always produce the same hash code.

This property ensures that the hash function is predictable and consistent. If two identical values are hashed, they should produce the same hash code every time. This property is essential because it enables us to store and retrieve data using its hash code. If the hash function is not deterministic, we would not be able to accurately retrieve data from the hash table.

### Efficient:

The hash function should be computationally efficient and fast.

This property is crucial because hash tables are often used to store large amounts of data. If the hash function is slow or inefficient, it can significantly impact the performance of the application. A good hash function should be designed to minimize collisions while being computationally efficient.

### Uniform:

The hash function should produce a uniform distribution of hash codes, i.e., the probability of two different inputs producing the same hash code should be minimal.

This property is important because it ensures that the hash table is evenly distributed, and the probability of collisions is minimal. In other words, if we have n elements in the hash table, the probability of two elements colliding should be 1/n. A hash function that produces a non-uniform distribution of hash codes can result in many collisions, leading to poor performance.

In addition to these properties, a good hash function should also be:

### Secure:

A secure hash function should make it difficult to reverse engineer the input data from the hash code. This property is particularly important for cryptographic applications where data needs to be protected from unauthorized access.

### Resilient to collisions:

While collisions are inevitable in any hash function, a good hash function should be designed to minimize the number of collisions. In addition, it should have a mechanism for resolving collisions, such as using a linked list or open addressing.

** Performance Parameters Of Hashing:**

The performance of a hash function is critical because it determines the efficiency and effectiveness of the hash table. The following are some of the key performance parameters of hashing:

### Collision resolution

Collisions occur when two different keys produce the same hash value. Collisions can be resolved using various techniques, such as separate chaining, open addressing, or rehashing. The collision resolution technique used can significantly impact the performance of the hash table. For example, separate chaining can result in better performance for large numbers of collisions, but it can have a higher overhead due to the linked lists used to store the data.

### Load factor

The load factor is the ratio of the number of elements stored in the hash table to the size of the table. A high load factor can result in more collisions and reduce the performance of the hash table. A good hash function should be designed to handle high load factors without impacting performance significantly.

### Hash function complexity

The complexity of the hash function can significantly impact the performance of the hash table. The hash function should be efficient and deterministic to ensure that it can handle a large amount of data without impacting performance. A more complex hash function can result in better distribution of the keys, but it can also be slower and more computationally expensive.

### Size of the hash table

The size of the hash table can impact the performance of the hash function. A larger hash table can reduce the probability of collisions, but it can also result in increased memory usage and slower lookup times. A smaller hash table can result in more collisions and reduced performance.

### Type of data being hashed

The type of data being hashed can impact the performance of the hash function. For example, hashing integers or fixed-size strings can be more efficient than hashing variable-length strings or complex data structures. Therefore, the type of data being hashed should be considered when designing the hash function.

In summary, the performance of hashing is influenced by several parameters, including collision resolution, load factor, hash function complexity, size of the hash table, and type of data being hashed. It is essential to consider these parameters when designing a hash function to ensure that the hash table can handle a large amount of data efficiently and effectively.

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