A C++ template library for embedded applications
MIT licensed
Designed and
maintained by
John Wellbelove
Support the development
of the ETL
flat_set / flat_multiset

A fixed capacity set based on a sorted vector.
The container is an associative lookup table with O(N) insertion and erase, and O(log N) search.
This container is best used for tables that are occasionally updated and spend most of their time being searched.
The interface is most similar to std::set.
Uses std::less as the default key comparison method.

etl::flat_set<typename T, const size_t SIZE, TCompare = std::less>

Inherits from etl::iflat_set<T, TCompare>
etl::iflat_set may be used as a size independent pointer or reference type for any etl::flat_set instance.
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Template deduction guides
C++17 and above

template <typename T, typename... Ts>
etl::flat_set(T, Ts...)
  -> etl::flat_set<etl::enable_if_t<(etl::is_same_v<T, Ts> && ...), T>, 1U + sizeof...(Ts)>;

Example
etl::flat_set data{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

Defines data as an flat_set of int, of length 10, containing the supplied data.
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Member types

key_type                T
value_type              T
size_type               std::size_t
difference_type         std::ptrdiff_t
reference               T&
const_reference         const T&
rvalue_reference        T&&
pointer                 T*
const_pointer           const T*
iterator                Random access iterator
const_iterator          Constant random access iterator
reverse_iterator        reverse_iterator<iterator>
const_reverse_iterator  reverse_iterator<const_iterator>

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Static Constants

MAX_SIZE  The maximum size of the flat set.

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Constructor

etl::flat_set<T, SIZE, TCompare>();

etl::flat_set(const flat_set& other)
etl::flat_set(flat_set&&)

template <typename TIterator>
etl::flat_set<T, SIZE, TCompare>(TIterator begin, TIterator end);

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Iterators

iterator begin()
const_iterator begin() const
const_iterator cbegin() const

Returns an iterator to the beginning of the set.
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iterator end()
const_iterator end() const
const_iterator cend() const

Returns an iterator to the end of the set.
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iterator rbegin()
const_iterator rbegin() const
const_iterator crbegin() const

Returns a reverse iterator to the beginning of the set.
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iterator rend()
const_iterator rend() const
const_iterator crend() const

Returns a reverse iterator to the end of the set.

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Capacity

bool empty() const

Returns true if the size of the set is zero, otherwise false.
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bool full() const

Returns true if the size of the lookup is SIZE, otherwise false.
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size_t size() const

Returns the size of the lookup.
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size_t max_size() const

Returns the maximum possible size of the set.
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size_t available() const

Returns the remaining available capacity in the set.

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Modifiers

flat_set& operator = (const flat_set& rhs)
flat_set& operator = (flat_set&& rhs)

Copies or moves the data from another flat set.
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pair<iterator, bool> insert(const_reference value)

iterator insert(iterator position, const_reference value)

pair<iterator, bool> insert(rvalue_reference value)

iterator insert(iterator position, rvalue_reference  value)

template <typename TIterator>
void insert(TIterator first, TIterator last)

Inserts values in to the set.
If the set is full then emits an etl::flat_set_full. If asserts or exceptions are not enabled then undefined behaviour
occurs.
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pair<iterator, bool> emplace((parameter_t value))

Inserts a value into the map by constructing directly into storage.
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C++03
template <typename T1>
pair<iterator, bool> emplace(const T1& value1)

template <typename T1, typename T2>
pair<iterator, bool> emplace(const T1& value1, const T2& value2)

template <typename T1, typename T2, typename T3>
pair<iterator, bool> emplace(const T1& value1, const T2& value2, const T3& value3)

template <typename T1, typename T2, typename T3, typename T4>
pair<iterator, bool> emplace(const T1& value1, const T2& value2, const T3& value3, const T4& value4)

Inserts values into the map by constructing directly into storage. The value is constructed from the set of 'value'
parameters.

C++11
template <typename ... Args>
pair<iterator, bool> emplace(Args && ... args)
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size_t erase(parameter_t key)
void erase(iterator i_element)
void erase(iterator first, iterator last)

Erases values in the set.
Iterator parameters are not checked for validity.
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void clear();
Clears the lookup to a size of zero.

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Search

iterator find(parameter_t key)
const_iterator find(parameter_t key) const

iterator lower_bound(parameter_t key)
const_iterator lower_bound(parameter_t key) const

iterator upper_bound(parameter_t key)
const_iterator upper_bound(parameter_t key) const

pair<iterator, iterator> equal_range(parameter_t key)
pair<const_iterator, const_iterator> equal_range(parameter_t key) const

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Non-member functions

Lexicographically comparisons

==  true if the contents of the sets are equal, otherwise false.
!=  true if the contents of the sets are not equal, otherwise false.

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Technical stuff

Flat sets are usually implemented internally as a sorted vector of values. Whilst this makes searching fast, it can have a
detrimental effect when items are inserted into a container that stores complex, non-trivial values.
As Inserting requires that all of the items above the insert position must be shifted, this can become an expensive
operation for larger containers.

To improve insertion performance ETL flat sets are implemented as vectors of pointers to values, sorted by value. An
insertion will involve a copy of a range of pointers; an operation that can be made very fast.

The downside is that access to an item via an iterator will involve one indirection and the overhead of the container will
be one pointer per item. A normal flat set implementation does not have this overhead.
flat_set.h / flat_multiset.h