VE281 — Programming Assignment 2

Starting from:

~~$35~~

$29

1 Introduction

In this project, you are asked to implement a STL-like hash table data structure.

In the Standard Template Library, hash table is implemented as std::unordered_map[1], which is introduced in the C++11 standard. It is an associative container that contains key-value pairs with unique keys. Search, insertion, and removal of elements have average constant-time complexity. It is named “unordered” because there already exists an ordered data structure of key-value pairs based on RB-tree, std::map.

Internally, the elements are not sorted in any particular order, but organized into buckets. Which bucket an element is placed into depends entirely on the hash of its key. This allows fast access to individual elements, since once the hash is computed, it refers to the exact bucket the element is placed into.

The C++ standard does not mandates whether unordered_map use separate chaining or open addressing for collision resolution. Actually, different compilers use different policies: g++ uses separate chaining and LLVM/Clang uses open addressing. In this project, you’re going to use separate chaining with std::forward_list[2], which is easier to implement and maintain.

You will also have a basic knowledge on iterators in C++: what are iterators and how they work. You can further try to use range-based for loops[3] (a feature introduced in C++11) to make use of iterators.

2 Programming Assignment

2.1 The HashTable Template

2.1.1 Overview

In the project starter code, we define a template class for you:

• template<

• typename Key, typename Value,

• typename Hash = std::hash<Key>,

• typename KeyEqual = std::equal_to<Key>

• >

• class HashTable;

Here Key and Value are the types of the key-value pair stored in the hash table. Hash is the type of a function object, whose instance returns the hash value of a key. The standard library defines a class template std::hash<T>, which can be used as the default. Similarly, KeyEqual is another type of a function object, whose instance returns whether two

Key objects are equal. It is used to determine which key-value pair should be returned when there is a hash collision, and it also help ensure the keys are unique in the hash table.

If you want to define your own hash function and key-equal function, you can refer to the following definitions:

• template<typename Key>

• struct hash<Key> {

• size_t operator()(const Key &key) const;

• }

• template<typename Key>

• struct equal_to<Key> {

• bool operator()(const Key &lhs, const Key &rhs) const;

• }

2.1.2 Number of Buckets

Basically, you will use the “Hashing by Modulo” scheme. Suppose should put a key into the i -th bucket where i = hash(key ) mod choose the hash table size n as a large prime number in ideal.

n is the number of buckets in the hash table, you n. As we’ve discussed in the lecture, you should

The number of buckets in your hash table should be dynamic (like a std::vector, but not exactly the same). A vector in C++ doubles its size when it is full (in most implementations), but this strategy may not be applied to a hash table because you should choose a prime number as the new size. Here we define the following strategy to choose a new hash table size before a rehash.

• First we define a list of prime numbers, each doubles its predecessor approximately. The list is taken from the g++ source code, and is provided with you as the file hash_prime.hpp.

• When a hash table is considered full (its load factor exceeds a preset maximum value), use the next prime as the new number of buckets and perform a rehash.

Furthermore, the maximum load factor value can also be changed in runtime; you can set a minimum number of buckets manually (i.e., to prevent rehashes if you’ve already know the data size) despite the load factor is small. One important thing is that whenever the number of buckets is changed, you need to do a rehash. You will fulfill all these requirements in the function findMinimumBucketSize. Please read its description carefully before implementing it. Though not mandatory, you are encouraged to use binary search to find the nearest prime. You can use the standard function std::lower_bound to perform the binary search so that you don’t need to implement it by yourself.

2.1.3 Internal Data Structures

We’ve already defined the internal data structures for you in this project. The HashNode is a key-value pair. The HashNodeList is a single directional linked list in each bucket, here we use std::forward_list to implement it. The HashTableData is a std::vector of these lists, representing the buckets in the hash table.

• class HashTable {

• public:

• typedef std::pair<const Key, Value> HashNode;

4 typedef std::forward_list<HashNode> HashNodeList;

• typedef std::vector<HashNodeList> HashTableData;

• protected:

• HashTableData buckets;

• size_t tableSize;

• double maxLoadFactor;

10 Hash hash;

11 KeyEqual keyEqual;

12 }

tableSize means the number of key-value pairs in the hash table. You can also add your own private / protected variables (or attributes) after these if necessary, but mostly the defined ones are enough. Here we use the protected keyword so that your HashTable class can be further inherited and overwritten for testing.

2.1.4 Iterators

We’re introducing iterators in this section. First of all, why you need to use iterators in this project? Supposing you want to find a key-value pair in the hash table, and then you may erase it from the hash table if it satisfies certain conditions. There are three implementations to achieved this:

(1) Use the find method to find a key-value pair, use the erase method to erase it.

(2) Use the find method to find a pointer to a key-value pair, use the erase method which accepts a pointer to erase it.

(3) Use the find method to find an iterator to a key-value pair, use the erase method which accepts an iterator to erase it.

In the first implementation, two lookups of the key is performed.

In the second implementation, only one lookup of the key is performed. However, the user can access the internal data structure of the hash table with the pointer, which is dangerous and violates the rule of packaging in Object-Oriented Programming.

In the third implementation, only one lookup of the key is performed as well, and the iterator only provides a restricted access to the key-value pair, so it’s a better solution.

In this project, we’ve already provided you with a completed iterator. You are going to use the iterator in the implementation of other methods in the hash table.

• class Iterator {

• private:

• typedef typename HashTableData::iterator VectorIterator;

• typedef typename HashNodeList::iterator ListIterator;

• const HashTable &hashTable;

• VectorIterator bucketIt;

• ListIterator listItBefore;

• bool endFlag = false;

10 }

Here bucketIt means which bucket the iterator is in, listItBefore means the iterator pointer the “before” the key-value pair in the linked list.

We’ll give a simple explanation of the “before” iterator. If you want to erase a node in a linked list, you need to link the previous node and the next node. However, the list is single directional, so that you can’t get the previous node in O(1) time unless you’ve saved it. If you always use a “before” iterator, the deletion of node will be possible, and you can access the current node by “next” of the “before” iterator. This is a built-in functionality of the std::forward_list[2] class. The class also provides a before_begin method, which is different from other STL containers. You can check the documentation for more information.

Since the list is single directional (in order to save memory), the iterator only supports single directional iteration. The operator ++ is already overloaded for you. The iterator doesn’t have a const version, you can implement it if you’re interested in iterators, but it’s not mandatory. You can see the detail implementation of the iterator in the starter code.

We also defines a variable

• typename HashTableData::iterator firstBucketIt;

in the hash table. It provides an O(1) access to the the first key-value pair in the hash table. You need to update its value whenever an insert, erase or rehash operation is applied to the hash table.

What’s more, the hash table supports basic iteration and range-for loops[3] with the help of iterators and two methods:

begin and end. More specifically, with the C++98 standard you can iterate the hash table by:

• for (auto it = hashTable.begin(); it != hashTable.end(); ++it) {

• std::cout << it->first << " " << it->second << std::endl;

• }

With C++11, you can use the following as an alternative:

• for (auto &item : hashTable) {

• std::cout << item.first << " " << item.second << std::endl;

• }

With C++17, you can even do it in a more graceful way (similar to python and some other modern languages):

• for (auto &[key, value] : hashTable) {

• std::cout << key << " " << value << std::endl;

• }

The order of the output of the above code is arbitrary and is dependent on implementation. We’ll not test the internal order of the key-value pairs in your hash table (returned by iteration), since they’re meant to be “unordered”.

2.2 Comparison of Hash Table and Linked List

We also ask you to study the performances of hash table and linked list. Basically, you will compare these classes:

• HashTable

• std::unordered_map

• std::list or std::forward_list (their performances should be similar)

You can design your own comparison metric. For example, you can insert n key-value pairs into all these classes, then run k find, update, insert or erase operations. In a linked list, you can use linear search, it should be very simple to implement these operations with std::list or std::forward_list.

Write about one page on the report about your findings, plots and explanations.

2.2.1 Hints

• The performance of programs on Windows is usually not stable, so you should do the experiments on a Unix based system.

• You may want to write another program to do this study.

• You can use the C++11 pseudo-random number generation library to generate more randomized numbers (instead of using std::rand).

• You can use the C++11 chrono library to get more accurate runtime of functions than std::clock.

• (Optional) You can use GNU Perf (only available on Linux) to find the bottleneck of your implementation.

• Although the major factor that affects the runtime is the size of the input array, however, the runtime for an algorithm may also weakly depend on the detailed input array. Thus, for each size, you should generate a number of arrays of that size and obtain the mean runtime on all these arrays. Also, for fair comparison, the same set of arrays should be applied to all the data structures.

• You should try at least 5 representative sizes.

2.3 Study of Combining Hash Functions

In this part, you are going to do some literature study on how to combine two hash functions. More specifically, if the type of the key of the hash table is std::pair<T1, T2> and the hash functions are already defined for T1 and T2, how to combine the results of these two hash functions and write a new function hash<std::pair<T1, T2>>.

The definition and a trivial implementation (which is not good) of the combined hash function (object) is provided:

• struct PairHash {

• template<typename T1, typename T2>

• size_t operator()(const pair<T1, T2> &p) const {

• // TODO: this implementation is not good, improve it!

• return std::hash<T1>()(p.first) ^ std::hash<T2>()(p.second);

• }

• };

Improve the implementation of the PairHash above in your report. Make sure it can compile and work with HashTable and std::unordered_map. You should also give a brief explanation of one paragraph about why you choose this implementation. You can refer to any source (i.e., paper, open-source code), but you need to reference it in your report.

3 Implementation Requirements and Restrictions

3.1 Requirements

• You must make sure that your code compiles successfully on a Linux operating system with g++ and the options

-std=c++1z -Wconversion -Wall -Werror -Wextra -pedantic.

• You should not change the definitions of the functions and variables, as well as the public or protected keywords for them in hashtable.hpp.

• You can define helper functions and new variables, don’t forget to mark them as private or protected.

• You should only hand in one file hashtable.hpp.

• You can use any header file defined in the C++17 standard. You can use cppreference as a reference.

You only need to implement the methods (functions) marked with “TODO” in the file hashtable.hpp. Here is a list of the methods (functions):

• Copy Constructor and Assignment Constructor

• findMinimumBucketSize

• find

• insert

• erase

• operator[]

• rehash

Please refer to the descriptions of these functions in the starter code.

3.2 Memory Leak

Hint: You’re not going to use any dynamic memory allocation functions (new, malloc, etc.) directly in this project, thus it’s not possible to have memory leak in your program. This section is only for your reference.

You may not leak memory in any way. To help you see if you are leaking memory, you may wish to call valgrind, which can tell whether you have any memory leaks. (You need to install valgrind first if your system does not have this program.) The command to check memory leak is:

valgrind --leak-check=full <COMMAND>

You should replace <COMMAND> with the actual command you use to issue the program under testing. For example, if you want to check whether running program

./main < input.txt

causes memory leak, then <COMMAND> should be “./main < input.txt”. Thus, the command will be

valgrind --leak-check=full ./main < input.txt

4 Grading

Your program will be graded along five criteria:

4.1 Functional Correctness

Functional Correctness is determined by running a variety of test cases against your program, checking your solution using our automatic testing program.

4.2 Implementation Constraints

We will grade Implementation Constraints to see if you have met all of the implementation requirements and restrictions. In this project, we will also check whether your program has memory leak. For those programs that behave correctly but have memory leaks, we will deduct some points.

4.3 General Style

General Style refers to the ease with which TAs can read and understand your program, and the cleanliness and elegance of your code. Part of your grade will also be determined by the performance of your algorithm.

4.4 Performance

We will test your program with some large test cases. If your program is not able to finish within a reasonable amount of time, you will lose the performance score for those test cases.

4.5 Report on the performance study

Finally, we will also read your report and grade it based on the quality of your performance study.

5 Acknowledgement

The programming assignment is co-authored by Yihao Liu, an alumni of JI and the chief architect of JOJ.

References

[1] std::unordered_map - cppreference : https://en.cppreference.com/w/cpp/container/unordered_map

[2] std::forward_list - cppreference : https://en.cppreference.com/w/cpp/container/forward_list

[3] Range-based for loop - cppreference: https://en.cppreference.com/w/cpp/language/range-for

Appendix

hash_prime.hpp

• // adopted from

,→ /usr/include/c++/10.2.0/ext/pb_ds/detail/resize_policy/hash_prime_size_policy_imp.hpp

• #include <utility>

• namespace HashPrime {

enum {

num_distinct_sizes_32_bit = 30,

num_distinct_sizes_64_bit = 62,

num_distinct_sizes = sizeof(std::size_t) != 8 ?

num_distinct_sizes_32_bit : num_distinct_sizes_64_bit,

};

// Originally taken from the SGI implementation; acknowledged in the docs.

// Further modified (for 64 bits) from tr1's hashtable.

static constexpr

std::size_t g_a_sizes[num_distinct_sizes_64_bit] = {

/* 0

5ul,

/* 1

11ul,

/* 2

23ul,

/* 3

47ul,

/* 4

97ul,

/* 5

199ul,

/* 6

409ul,

/* 7

823ul,

/* 8

1741ul,

/* 9

3469ul,

/* 10

6949ul,

/* 11

14033ul,

/* 12

28411ul,

/* 13

57557ul,

/* 14

116731ul,

/* 15

236897ul,

/* 16

480881ul,

/* 17

976369ul,

/* 18

1982627ul,

/* 19

4026031ul,

/* 20

8175383ul,

/* 21

16601593ul,

/* 22

33712729ul,

/* 23

68460391ul,

/* 24

139022417ul,

/* 25

282312799ul,

/* 26

573292817ul,

/* 27

1164186217ul,

2364114217ul,

4294967291ul,

46 /* 30*/ (std::size_t) 8589934583ull,

47 /* 31*/ (std::size_t) 17179869143ull,

48 /* 32*/ (std::size_t) 34359738337ull,

49 /* 33*/ (std::size_t) 68719476731ull,

50 /* 34*/ (std::size_t) 137438953447ull,

51 /* 35*/ (std::size_t) 274877906899ull,

52 /* 36*/ (std::size_t) 549755813881ull,

53 /* 37*/ (std::size_t) 1099511627689ull,

54 /* 38*/ (std::size_t) 2199023255531ull,

55 /* 39*/ (std::size_t) 4398046511093ull,

56 /* 40*/ (std::size_t) 8796093022151ull,

57 /* 41*/ (std::size_t) 17592186044399ull,

58 /* 42*/ (std::size_t) 35184372088777ull,

59 /* 43*/ (std::size_t) 70368744177643ull,

60 /* 44*/ (std::size_t) 140737488355213ull,

61 /* 45*/ (std::size_t) 281474976710597ull,

62 /* 46*/ (std::size_t) 562949953421231ull,

63 /* 47*/ (std::size_t) 1125899906842597ull,

64 /* 48*/ (std::size_t) 2251799813685119ull,

65 /* 49*/ (std::size_t) 4503599627370449ull,

66 /* 50*/ (std::size_t) 9007199254740881ull,

67 /* 51*/ (std::size_t) 18014398509481951ull,

68 /* 52*/ (std::size_t) 36028797018963913ull,

69 /* 53*/ (std::size_t) 72057594037927931ull,

70 /* 54*/ (std::size_t) 144115188075855859ull,

71 /* 55*/ (std::size_t) 288230376151711717ull,

72 /* 56*/ (std::size_t) 576460752303423433ull,

73 /* 57*/ (std::size_t) 1152921504606846883ull,

74 /* 58*/ (std::size_t) 2305843009213693951ull,

75 /* 59*/ (std::size_t) 4611686018427387847ull,

76 /* 60*/ (std::size_t) 9223372036854775783ull,

77 /* 61*/ (std::size_t) 18446744073709551557ull,

78 };

80 }

hashtable.hpp

• #include "hash_prime.hpp"

• #include <exception>

• #include <functional>

• #include <vector>

• #include <forward_list>

• /**

• * The Hashtable class

10 * The time complexity of functions are based on n and k

11 * n is the size of the hashtable

12 * k is the length of Key

* @tparam Key

key type

* @tparam Value

data type

* @tparam Hash

function object, return the hash value of a key

* @tparam KeyEqual

function object, return whether two keys are the same

17 */

18 template<

19 typename Key, typename Value,

20 typename Hash = std::hash<Key>,

21 typename KeyEqual = std::equal_to<Key>

22 >

23 class HashTable {

24 public:

25 typedef std::pair<const Key, Value> HashNode;

26 typedef std::forward_list<HashNode> HashNodeList;

27 typedef std::vector<HashNodeList> HashTableData;

/**

• A single directional iterator for the hashtable

• ! DO NOT NEED TO MODIFY THIS !

class Iterator {

private:

typedef typename HashTableData::iterator VectorIterator; typedef typename HashNodeList::iterator ListIterator;

38 const HashTable *hashTable;

39 VectorIterator bucketIt;// an iterator of the buckets

40 ListIterator listItBefore; // a before iterator of the list, here we use ,→ "before" for quick erase and insert

41 bool endFlag = false;// whether it is an end iterator

/**

• Increment the iterator

• Time complexity: Amortized O(1)

void increment() {

if (bucketIt == hashTable->buckets.end()) {

endFlag = true;

return;

}

auto newListItBefore = listItBefore;

++newListItBefore;

if (newListItBefore != bucketIt->end()) {

if (++newListItBefore != bucketIt->end()) {

// use the next element in the current forward_list ++listItBefore;

return;

}

while (++bucketIt != hashTable->buckets.end()) { if (!bucketIt->empty()) {

// use the first element in a new forward_list listItBefore = bucketIt->before_begin(); return;

}

endFlag = true;

}

explicit Iterator(HashTable *hashTable) : hashTable(hashTable) { bucketIt = hashTable->buckets.begin();

listItBefore = bucketIt->before_begin(); endFlag = bucketIt == hashTable->buckets.end();

}

77 Iterator(HashTable *hashTable, VectorIterator vectorIt, ListIterator ,→ listItBefore) :

78 hashTable(hashTable), bucketIt(vectorIt), listItBefore(listItBefore) {

79 endFlag = bucketIt == hashTable->buckets.end();

80 }

public:

friend class HashTable;

Iterator() = delete;

Iterator(const Iterator &) = default;

Iterator &operator=(const Iterator &) = default;

91 Iterator &operator++() {

100

101

increment();

return *this;

}

Iterator operator++(int) {

Iterator temp = *this;

increment();

return temp;

}

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bool operator==(const Iterator &that) const {

if (endFlag && that.endFlag) return true;

if (bucketIt != that.bucketIt) return false;

return listItBefore == that.listItBefore;

}

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bool operator!=(const Iterator &that) const { if (endFlag && that.endFlag) return false; if (bucketIt != that.bucketIt) return true; return listItBefore != that.listItBefore;

}

HashNode *operator->() {

auto listIt = listItBefore;

++listIt;

return &(*listIt);

}

120 HashNode &operator*() {

121 auto listIt = listItBefore;

122 ++listIt;

123 return *listIt;

124 }

125 };

126

127

protected:

// DO NOT USE

,→private HERE!

128

static constexpr

double DEFAULT_LOAD_FACTOR = 0.5;

// default

,→maximum load

factor is 0.5

129 static constexpr size_t DEFAULT_BUCKET_SIZE = HashPrime::g_a_sizes[0]; // default

,→number of buckets is 5

130

131

HashTableData buckets;

// buckets,

,→of singly linked lists

132

typename HashTableData::iterator firstBucketIt;

// help get

,→begin iterator in O(1)

time

133

134

size_t tableSize;

// number of

,→

elements

135

double maxLoadFactor;

// maximum

,→

load factor

136

Hash hash;

// hash

,→

function instance

137

KeyEqual keyEqual;

// key equal

,→

function instance

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/**

• Time Complexity: O(k)

• @param key

• @param bucketSize

• @return the hash value of key with a new bucket size

inline size_t hashKey(const Key &key, size_t bucketSize) const { return hash(key) % bucketSize;

}

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/**

• Time Complexity: O(k)

• @param key

• @return the hash value of key with current bucket size

inline size_t hashKey(const Key &key) const {

return hash(key) % buckets.size();

}

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/**

• Find the minimum bucket size for the hashtable

• The minimum bucket size must satisfy all of the following requirements:

• - It is not less than (i.e. greater or equal to) the parameter bucketSize

• - It is greater than floor(tableSize / maxLoadFactor)

• - It is a (prime) number defined in HashPrime (hash_prime.hpp)

• - It is minimum if satisfying all other requirements

• Time Complexity: O(1)

• @throw std::range_error if no such bucket size can be found

• @param bucketSize lower bound of the new number of buckets

size_t findMinimumBucketSize(size_t bucketSize) const { // TODO: implement this function

}

173

174

175

// TODO: define your helper functions here if necessary

176 public:

177 HashTable() :

178 buckets(DEFAULT_BUCKET_SIZE), tableSize(0), ,→ maxLoadFactor(DEFAULT_LOAD_FACTOR),

179 hash(Hash()), keyEqual(KeyEqual()) {

180 firstBucketIt = buckets.end();

181 }

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explicit HashTable(size_t bucketSize) :

tableSize(0), maxLoadFactor(DEFAULT_LOAD_FACTOR), hash(Hash()), keyEqual(KeyEqual()) {

bucketSize = findMinimumBucketSize(bucketSize); buckets.resize(bucketSize); firstBucketIt = buckets.end();

}

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HashTable(const HashTable &that) {

// TODO: implement this function

}

HashTable &operator=(const HashTable &that) {

// TODO: implement this function

};

~HashTable() = default;

Iterator begin() {

if (firstBucketIt != buckets.end()) {

return Iterator(this, firstBucketIt, firstBucketIt->before_begin());

}

return end();

}

Iterator end() {

return Iterator(this, buckets.end(), buckets.begin()->before_begin());

}

/**

• Find whether the key exists in the hashtable

• Time Complexity: Amortized O(k)

• @param key

• @return whether the key exists in the hashtable

bool contains(const Key &key) {

return find(key) != end();

}

/**

223 * Find the value in hashtable by key

224 * If the key exists, iterator points to the corresponding value, and it.endFlag =

,→ false

225 * Otherwise, iterator points to the place that the key were to be inserted, and

,→ it.endFlag = true

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• Time Complexity: Amortized O(k)

• @param key

• @return a pair (success, iterator of the value)

Iterator find(const Key &key) {

// TODO: implement this function

}

234 /**

235 * Insert value into the hashtable according to an iterator returned by find

236 * the function can be only be called if no other write actions are done to the

,→ hashtable after the find

237 * If the key already exists, overwrite its value

238 * firstBucketIt should be updated

239 * If load factor exceeds maximum value, rehash the hashtable

240 * Time Complexity: O(k)

241 * @param it an iterator returned by find

242 * @param key

243 * @param value

244 * @return whether insertion took place (return false if the key already exists)

245 */

246 bool insert(const Iterator &it, const Key &key, const Value &value) {

247 // TODO: implement this function

248 }

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/**

• Insert <key, value> into the hashtable

• If the key already exists, overwrite its value

• firstBucketIt should be updated

• If load factor exceeds maximum value, rehash the hashtable

• Time Complexity: Amortized O(k)

• @param key

• @param value

• @return whether insertion took place (return false if the key already exists)

bool insert(const Key &key, const Value &value) { // TODO: implement this function

}

/**

• Erase the key if it exists in the hashtable, otherwise, do nothing

• DO NOT rehash in this function

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• firstBucketIt should be updated

• Time Complexity: Amortized O(k)

• @param key

• @return whether the key exists

bool erase(const Key &key) {

// TODO: implement this function

}

276 /**

277 * Erase the key at the input iterator

278 * If the input iterator is the end iterator, do nothing and return the input

,→ iterator directly

279 * firstBucketIt should be updated

280 * Time Complexity: O(1)

281 * @param it

282 * @return the iterator after the input iterator before the erase

283 */

284 Iterator erase(const Iterator &it) {

285 // TODO: implement this function

286 }

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/**

• Get the reference of value by key in the hashtable

• If the key doesn't exist, create it first (use default constructor of Value)

• firstBucketIt should be updated

• If load factor exceeds maximum value, rehash the hashtable

• Time Complexity: Amortized O(k)

• @param key

• @return reference of value

Value &operator[](const Key &key) {

// TODO: implement this function

}

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/**

• Rehash the hashtable according to the (hinted) number of buckets

• The bucket size after rehash need not be same as the parameter bucketSize

• Instead, findMinimumBucketSize is called to get the correct number

• firstBucketIt should be updated

• Do nothing if the bucketSize doesn't change

• Time Complexity: O(nk)

• @param bucketSize lower bound of the new number of buckets

void rehash(size_t bucketSize) {

bucketSize = findMinimumBucketSize(bucketSize); if (bucketSize == buckets.size()) return;

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// TODO: implement this function

}

/**

• @return the number of elements in the hashtable

size_t size() const { return tableSize; }

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/**

• @return the number of buckets in the hashtable

size_t bucketSize() const { return buckets.size(); }

/**

• @return the current load factor of the hashtable

double loadFactor() const { return (double) tableSize / (double) buckets.size(); }

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/**

• @return the maximum load factor of the hashtable

double getMaxLoadFactor() const { return maxLoadFactor; }

336 /**

337 * Set the max load factor

338 * @throw std::range_error if the load factor is too small

339 * @param loadFactor

340 */

341 void setMaxLoadFactor(double loadFactor) {

342 if (loadFactor <= 1e-9) {

343 throw std::range_error("invalid load factor!");

344 }

345 maxLoadFactor = loadFactor;

346 rehash(buckets.size());

347 }

348

349 };

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