double_hashing Namespace Reference

An implementation of hash table using double hashing algorithm. More...

Classes
struct	Entry

Typedefs
using	Entry = Entry

Functions
bool	putProber (const Entry &entry, int key)
bool	searchingProber (const Entry &entry, int key)
void	add (int key)
size_t	hashFxn (int key)
	Hash a key. Uses the STL library's std::hash() function. More...
size_t	otherHashFxn (int key)
	Used for second hash function. More...
int	doubleHash (int key, bool searching)
	Performs double hashing to resolve collisions. More...
void	display ()
void	rehash ()
void	remove (int key)
void	addInfo (int key)
void	removalInfo (int key)

Variables
int	notPresent
std::vector< Entry >	table
int	totalSize
int	tomb = -1
int	size
bool	rehashing

Detailed Description

An implementation of hash table using double hashing algorithm.

Function Documentation

add()

void double_hashing::add

(

int

key

)

Checks for load factor here

Parameters

key	key value to add to the table

                  {
    // auto* entry = new Entry();
    // entry->key = key;
    int index = doubleHash(key, false);
    table[index].key = key;
    // Load factor greater than 0.5 causes resizing
    if (++size / static_cast<double>(totalSize) >= 0.5) {
        rehash();
    }
}

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addInfo()

void double_hashing::addInfo

(

int

key

)

Information about the adding process

Parameters

key	key value to add to table

                      {
    std::cout << "Initial table: ";
    display();
    std::cout << std::endl;
    std::cout << "hash of " << key << " is " << hashFxn(key) << " % "
              << totalSize << " == " << hashFxn(key) % totalSize;
    std::cout << std::endl;
    add(key);
    std::cout << "New table: ";
    display();
}

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display()

void double_hashing::display

(

)

Displays the table

Returns

None

               {
    for (int i = 0; i < totalSize; i++) {
        if (table[i].key == notPresent) {
            std::cout << " Empty ";
        } else if (table[i].key == tomb) {
            std::cout << " Tomb ";
        } else {
            std::cout << " ";
            std::cout << table[i].key;
            std::cout << " ";
        }
    }
    std::cout << std::endl;
}

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doubleHash()

int double_hashing::doubleHash	(	int	key,
		bool	searching
	)

Performs double hashing to resolve collisions.

Parameters

key	key value to apply double-hash on
searching	true to check for conflicts

Returns

Index of key when found

new hash if no conflicts present

                                        {
    int hash = static_cast<int>(hashFxn(key));
    int i = 0;
    Entry entry;
    do {
        int index =
            static_cast<int>(hash + (i * otherHashFxn(key))) % totalSize;
        entry = table[index];
        if (searching) {
            if (entry.key == notPresent) {
                return notPresent;
            }
            if (searchingProber(entry, key)) {
                std::cout << "Found key!" << std::endl;
                return index;
            }
            std::cout << "Found tombstone or equal hash, checking next"
                      << std::endl;
            i++;
        } else {
            if (putProber(entry, key)) {
                if (!rehashing) {
                    std::cout << "Spot found!" << std::endl;
                }
                return index;
            }
            if (!rehashing) {
                std::cout << "Spot taken, looking at next (next index:"
                          << " "
                          << static_cast<int>(hash + (i * otherHashFxn(key))) %
                                 totalSize
                          << ")" << std::endl;
            }
            i++;
        }
        if (i == totalSize * 100) {
            std::cout << "DoubleHash probe failed" << std::endl;
            return notPresent;
        }
    } while (entry.key != notPresent);
    return notPresent;
}

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hashFxn()

size_t double_hashing::hashFxn

(

int

key

)

Hash a key. Uses the STL library's std::hash() function.

Parameters

key	value to hash

Returns

hash value of the key

                        {
    std::hash<int> hash;
    return hash(key);
}

otherHashFxn()

size_t double_hashing::otherHashFxn

(

int

key

)

Used for second hash function.

Parameters

key	key value to hash

Returns

hash value of the key

                             {
    std::hash<int> hash;
    return 1 + (7 - (hash(key) % 7));
}

putProber()

bool double_hashing::putProber	(	const Entry &	entry,
		int	key
	)

Finds empty spot in a vector

Parameters

entry	vector to search in
key	key to search for

Returns

true if key is not present or is a toumb

false is already occupied

                                            {
    if (entry.key == notPresent || entry.key == tomb) {
        return true;
    }
    return false;
}

rehash()

void double_hashing::rehash

(

)

Rehashes the table into a bigger table

Returns

None

              {
    // Necessary so wall of add info isn't printed all at once
    rehashing = true;
    int oldSize = totalSize;
    std::vector<Entry> oldTable(table);
    // Really this should use the next prime number greater than totalSize * 2
    table = std::vector<Entry>(totalSize * 2);
    totalSize *= 2;
    for (int i = 0; i < oldSize; i++) {
        if (oldTable[i].key != -1 && oldTable[i].key != notPresent) {
            size--;  // Size stays the same (add increments size)
            add(oldTable[i].key);
        }
    }
    // delete[] oldTable;
    // oldTable.reset();
 
    rehashing = false;
    std::cout << "Table was rehashed, new size is: " << totalSize << std::endl;
}

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removalInfo()

void double_hashing::removalInfo

(

int

key

)

Information about removal process

Parameters

key	key value to remove from table

                          {
    std::cout << "Initial table: ";
    display();
    std::cout << std::endl;
    std::cout << "hash of " << key << " is " << hashFxn(key) << " % "
              << totalSize << " == " << hashFxn(key) % totalSize;
    std::cout << std::endl;
    remove(key);
    std::cout << "New table: ";
    display();
}

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remove()

void double_hashing::remove

(

int

key

)

Removes key. Leaves tombstone upon removal.

Parameters

key	key value to remove

                     {
    int index = doubleHash(key, true);
    if (index == notPresent) {
        std::cout << "key not found" << std::endl;
    }
    table[index].key = tomb;
    std::cout << "Removal successful, leaving tombstone" << std::endl;
    size--;
}

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searchingProber()

bool double_hashing::searchingProber	(	const Entry &	entry,
		int	key
	)

Looks for a matching key

Parameters

entry	vector to search in
key	key value to search

Returns

true if found

false if not found

                                                  {
    if (entry.key == key) {
        return true;
    }
    return false;
}