Hash table C++ implementation

Requirements

Before running the project, install the following libraries:

• OpenSSL: to generate sha256

sudo yum install openssl
sudo yum install openssl-devel

• Boost: to store and operate with 256 bit integers

sudo yum install boost-devel
sudo yum install boost

To link the libraries with the project, add the following lines to CMakeList.txt

find_package(OpenSSL REQUIRED)
find_package(Boost REQUIRED)

target_link_libraries(
        ${PROJECT_NAME}
        OpenSSL::SSL
        Boost::boost
)

Run the project

git clone https://github.com/ByJuanDiego/hash-table.git
cd hash-table
chmod +x run.zsh
./run.zsh

Member functions

$n :=$ total number of records in the hash table

$e_{avg} :=$ average number of entries in a bucket

$v_{avg} :=$ average number of values on an entry

Member Function	Big $\mathcal{O}$ performance	Big $\Theta$ performance	Notes
bucket_count()	$\mathcal{O}(1)$	Same as $\mathcal{O}$	-
bucket_size(int i)	$\mathcal{O}(1)$	Same as $\mathcal{O}$	-
key_count()	$\mathcal{O}(1)$	Same as $\mathcal{O}$	-
size()	$\mathcal{O}(1)$	Same as $\mathcal{O}$	-
empty()	$\mathcal{O}(1)$	Same as $\mathcal{O}$	-
clear()	$\mathcal{O}(n)$	Same as $\mathcal{O}$	if V is a pointer, records will not be freed
find(K key)	$\mathcal{O}(n)$	$\Theta(e_{avg})$	keeps the array length
insert(V value)	$\mathcal{O}(n)$	$\Theta(e_{avg})$	-
remove(K key)	$\mathcal{O}(n)$	$\Theta(e_{avg} + v_{avg})$	-
search(K key)	$\mathcal{O}(n)$	$\Theta(e_{avg} + v_{avg})$	-
print(std::ostream &os, Print<V> print_value, Print<K> print_key)	$\mathcal{O}(n)$	Same as $\mathcal{O}$	print_value and print_key has default functions for fundamental types

Usage cases

Initialization

using std::string;
using sha = sha2::sha256<string>;
using index_t = std::function<string(transaction *)>;
using equal_t = std::function<bool(string, string)>;

sha hash;
index_t index = [&](const transaction *tx) -> string {
    return tx->emisor;
};
equal_t equal = [&](const string &a, const string &b) -> bool {
    return (a == b);
};
hash_table<string, transaction *, sha, index_t, equal_t> hashTable(index, hash, equal);

Instantiates a hashTable that contains transaction * indexed by emisor

• equal is an optional parameter. By default, it receives an instance of std::equal_to, which works properly for built-in types. Using a non-built-in type as key makes necessary equal parameter or an std::equal_to specialization
• hash is an instance of sha2::sha256, which is well-defined for std::to_string convertable key-types and specialized for std::string usage. To use other key-types a sha2::sha256 specialization is required
• usage of other hash functions such as std::hash is allowed by passing the desire hash function as template type parameter

Querying

std::string key = "juan-diego";
for (const transaction *t: hashTable.search(key)) {
    std::cout << t->to_string() << std::endl;
}

This query returns all the transactions made by juan-diego

Freeing memory

If the value-type is a pointer, the pointed values will not be freed when hash_table::~hash_table is called. This is manual process.

for (transaction *tx: destructor) {
    delete tx;
}

To be implemented

• iterator class for the hash table
• shrink_to_fit private member function to be used to resize the hash table when deleting a certain number of keys