std::adjacent_difference(3)

NAME
std::adjacent_difference - std::adjacent_difference

Synopsis
   Defined in header <numeric>
   template< class InputIt, class OutputIt >
   OutputIt adjacent_difference( InputIt first, (until C++20)
   InputIt last,
   OutputIt d_first );
   template< class InputIt, class OutputIt >
   constexpr OutputIt adjacent_difference( InputIt (since C++20)
   first, InputIt last,
   OutputIt d_first );
   template< class ExecutionPolicy, class
   ForwardIt1, class ForwardIt2 >
   ForwardIt2 adjacent_difference( (2) (since C++17)
   ExecutionPolicy&& policy, ForwardIt1 first,
   ForwardIt1 last,
   ForwardIt2 d_first );
   template< class InputIt, class OutputIt, class
   BinaryOperation > (1)
   OutputIt adjacent_difference( InputIt first, (until C++20)
   InputIt last,
   OutputIt d_first, BinaryOperation op );
   template< class InputIt, class OutputIt, class
   BinaryOperation >
   constexpr OutputIt adjacent_difference( InputIt (since C++20)
   first, InputIt last, (3)
   OutputIt d_first, BinaryOperation op );
   template< class ExecutionPolicy, class
   ForwardIt1, class ForwardIt2, class
   BinaryOperation >
   ForwardIt2 adjacent_difference( (4) (since C++17)
   ExecutionPolicy&& policy, ForwardIt1 first,
   ForwardIt1 last,
   ForwardIt2 d_first, BinaryOperation op );
   Computes the differences between the second and the first of each adjacent pair of
   elements of the range [first, last) and writes them to the range beginning at
   d_first + 1. An unmodified copy of *first is written to *d_first.
   1,3) First, creates an accumulator acc whose type is InputIt's value type,
   initializes it with *first, and assigns the result to *d_first. Then, for every
   iterator i in [first + 1, last) in order, creates an object val whose type is
   InputIt's value type, initializes it with *i, computes
   val - acc
   (until C++20)
   val - std::move(acc)
   (since C++20) (overload (1)) or
   op(val, acc)
   (until C++20)
   op(val, std::move(acc))
   (since C++20) (overload (3)), assigns the result to *(d_first + (i - first)), and
   move assigns from val to acc.
   first may be equal to d_first.
   2,4) Performs *d_first = *first;. For every d in [1, last - first - 1], assigns
   *(first + d) - *(first + d - 1) (overload (2)) or op(*(first + d), *(first + d - 1))
   (overload (4)) to *(d_first + d). This is executed according to policy. These
   overloads do not participate in overload resolution unless
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) is true.
   The behavior is undefined if the input and output ranges overlap in any way.
   Equivalent operation:
 *(d_first) = *first;
 *(d_first+1) = *(first+1) - *(first);
 *(d_first+2) = *(first+2) - *(first+1);
 *(d_first+3) = *(first+3) - *(first+2);
 ...
   op must not have side effects. (until C++11)
   op must not invalidate any iterators, including the end iterators, or (since C++11)
   modify any elements of the ranges involved.

Parameters
   first, last - the range of elements
   d_first - the beginning of the destination range
   policy - the execution policy to use. See execution policy for details.
                   binary operation function object that will be applied.
                   The signature of the function should be equivalent to the following:
                   Ret fun(const Type1 &a, const Type2 &b);
   op -
                   The signature does not need to have const &.
                   The types Type1 and Type2 must be such that an object of type
                   iterator_traits<InputIt>::value_type can be implicitly converted to
                   both of them. The type Ret must be such that an object of type
                   OutputIt can be dereferenced and assigned a value of type Ret.

Type requirements
   -
   InputIt must meet the requirements of LegacyInputIterator. InputIt's value type must
   be MoveAssignable and constructible from the type of *first
   -
   OutputIt must meet the requirements of LegacyOutputIterator. both acc (the
   accumulated value) and the result of
   val - acc or op(val, acc)
   (until C++20)
   val - std::move(acc) or op(val, std::move(acc))
   (since C++20) must be writable to OutputIt
   -
   ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator. The
   results of *first, *first - *first (for (2)) and op(*first, *first) (for (4)) must
   be writable to ForwardIt2.

Return value
   Iterator to the element past the last element written.

Notes
   If first == last, this function has no effect and will merely return d_first.

Complexity
   Exactly (last - first) - 1 applications of the binary operation

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 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 InputIt, class OutputIt>
   constexpr // since C++20
   OutputIt adjacent_difference(InputIt first, InputIt last,
                        OutputIt d_first)
   {
       if (first == last) return d_first;
       typedef typename std::iterator_traits<InputIt>::value_type value_t;
       value_t acc = *first;
       *d_first = acc;
       while (++first != last) {
           value_t val = *first;
           *++d_first = val - std::move(acc); // std::move since C++20
           acc = std::move(val);
       }
       return ++d_first;
   }

Second version
   template<class InputIt, class OutputIt, class BinaryOperation>
   constexpr // since C++20
   OutputIt adjacent_difference(InputIt first, InputIt last,
                        OutputIt d_first, BinaryOperation op)
   {
       if (first == last) return d_first;
       typedef typename std::iterator_traits<InputIt>::value_type value_t;
       value_t acc = *first;
       *d_first = acc;
       while (++first != last) {
           value_t val = *first;
           *++d_first = op(val, std::move(acc)); // std::move since C++20
           acc = std::move(val);
       }
       return ++d_first;
   }

Example
// Run this code
 #include <numeric>
 #include <vector>
 #include <array>
 #include <iostream>
 #include <functional>
 #include <iterator>
 auto print = [](auto comment, auto const& sequence) {
     std::cout << comment;
     for (const auto& n : sequence)
         std::cout << n << ' ';
     std::cout << '\n';
 };
 int main()
 {
     // Default implementation - the difference b/w two adjacent items
     std::vector v {4, 6, 9, 13, 18, 19, 19, 15, 10};
     print("Initially, v = ", v);
     std::adjacent_difference(v.begin(), v.end(), v.begin());
     print("Modified v = ", v);
     // Fibonacci
     std::array<int, 10> a {1};
     adjacent_difference(begin(a), std::prev(end(a)), std::next(begin(a)), std::plus<> {});
     print("Fibonacci, a = ", a);
 }

Output:
 Initially, v = 4 6 9 13 18 19 19 15 10
 Modified v = 4 2 3 4 5 1 0 -4 -5
 Fibonacci, a = 1 1 2 3 5 8 13 21 34 55

See also
   partial_sum computes the partial sum of a range of elements
               (function template)
   accumulate sums up a range of elements
               (function template)