std::search_n

Defined in header `<algorithm>`
template< class ForwardIt, class Size, class T > ForwardIt search_n( ForwardIt first, ForwardIt last, Size count, const T& value );	(1)
template< class ExecutionPolicy, class ForwardIt, class Size, class T > ForwardIt search_n( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Size count, const T& value );	(2)	(since C++17)
template< class ForwardIt, class Size, class T, class BinaryPredicate > ForwardIt search_n( ForwardIt first, ForwardIt last, Size count, const T& value, BinaryPredicate p );	(3)
template< class ExecutionPolicy, class ForwardIt, class Size, class T, class BinaryPredicate > ForwardIt search_n( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Size count, const T& value, BinaryPredicate p );	(4)	(since C++17)

Searches the range [first, last) for the first sequence of count identical elements, each equal to the given value value.

1) Elements are compared using operator==.

3) Elements are compared using the given binary predicate p.

2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Parameters

first, last	-	the range of elements to examine
count	-	the length of the sequence to search for
value	-	the value of the elements to search for
policy	-	the execution policy to use. See execution policy for details.
p	-	binary predicate which returns true if the elements should be treated as equal. The signature of the predicate function should be equivalent to the following: bool pred(const Type1 &a, const Type2 &b); The signature does not need to have const &, but the function must not modify the objects passed to it. The type Type1 must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to Type1. The type Type2 must be such that an object of type T can be implicitly converted to Type2.
Type requirements
- `ForwardIt` must meet the requirements of `ForwardIterator`.

Return value

Iterator to the beginning of the found sequence in the range [first, last). If no such sequence is found, last is returned.

Complexity

At most last - first applications of the predicate.

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the three standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

First version

template<class ForwardIt, class Size, class T>
ForwardIt search_n(ForwardIt first, ForwardIt last,
                    Size count, const T& value)
{
    for(; first != last; ++first) {
        if (!(*first == value)) {
            continue;
        }
 
        ForwardIt candidate = first;
        Size cur_count = 0;
 
        while (true) {
            ++cur_count;
            if (cur_count == count) {
                // success
                return candidate;
            }
            ++first;
            if (first == last) {
                // exhausted the list
                return last;
            }
            if (!(*first == value)) {
                // too few in a row
                break;
            }
        }
    }
    return last;
}

Second version

template<class ForwardIt, class Size, class T, class BinaryPredicate>
ForwardIt search_n(ForwardIt first, ForwardIt last,
                    Size count, const T& value, BinaryPredicate p)
{
    for(; first != last; ++first) {
        if (!p(*first, value)) {
            continue;
        }
 
        ForwardIt candidate = first;
        Size cur_count = 0;
 
        while (true) {
            ++cur_count;
            if (cur_count == count) {
                // success
                return candidate;
            }
            ++first;
            if (first == last) {
                // exhausted the list
                return last;
            }
            if (!p(*first, value)) {
                // too few in a row
                break;
            }
        }
    }
    return last;
}

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