== `sqlite/function.hpp`

=== `function_flags`

[source, cpp]
----
enum function_flags
{
  deterministic  = SQLITE_DETERMINISTIC,
  directonly     = SQLITE_DIRECTONLY,
  subtype        = SQLITE_SUBTYPE,
  innocuous      = SQLITE_INNOCUOUS,
  result_subtype = SQLITE_RESULT_SUBTYPE,
  selforder1     = SQLITE_SELFORDER1,
  selforder1     = SQLITE_SELFORDER1
};
----

These function flags can be used in accordance to the
https://www.sqlite.org/c3ref/c_deterministic.html[sqlite documentation].

=== `context`

A context is an object share values between invocations of a scalar function.

.Definition
[source,cpp]
----

template<typename ... Args>
struct context
{
  template<std::size_t N>
  using element = mp11::mp_take_c<mp11::mp_list<Args...>, N>;

  // Set the value in the context at position `Idx`
  template<std::size_t Idx>
  void set(element<Idx> value);

  // Returns the value in the context at position `Idx`. Throws if the value isn't set.
  template<std::size_t Idx>
  auto get() -> element<Idx>  &;

  // Returns the value in the context at position `Idx`. Returns nullptr .value isn't set.
  template<std::size_t Idx>
  auto get_if() -> element<Idx>  *;

  explicit context(sqlite3_context * ctx) noexcept;

  // Set the result through the context, instead of returning it.
  template<typename T>
  auto set_result(T && val);

  // Set the an error through the context, instead of throwing it.
  void set_error(cstring_ref message, int code = SQLITE_ERROR);

  // Returns the connection of the context.
  connection get_connection() const;
};
----


.Example
[source,cpp]
----
extern sqlite::connection conn;

sqlite::create_scalar_function(
conn, "my_sum",
[](sqlite::context<std::size_t> ctx,
   boost::span<sqlite::value, 1u> args) -> std::size_t
{
    auto value = args[0].get_int();
    auto p = ctx.get_if<0>();
    if (p != nullptr) // increment the counter
        return (*p) += value;
    else // set the initial value
        ctx.set<0>(value);
    return value;
});
----


=== `create_scalar_function`


Creates a https://www.sqlite.org/appfunc.html[scalar function].

.Definition
[source,cpp]
----

template<typename Func>
auto create_scalar_function(
    connection & conn,
    cstring_ref name,
    Func && func,
    function_flags flags = {});

template<typename Func>
auto create_scalar_function(
    connection & conn,
    cstring_ref name,
    Func && func,
    function_flags flags,
    system::error_code & ec,
    error_info & ei);
----

conn:: The connection to add the function to.
name:: The name of the function
func:: The function to be added

`func` must take `context<Args...>` as the first and a `span<value, N>` as the second value.
If `N` is not `dynamic_extent` it will be used to deduce the number of arguments for the function.


.Example
[source,cpp]
----
extern sqlite::connection conn;

sqlite::create_function(
    conn, "to_upper",
    [](sqlite::context<> ctx,
       boost::span<sqlite::value, 1u> args) -> std::string
    {
        std::string res;
        auto txt = val[0].get_text();
        res.resize(txt.size());
        std::transform(txt.begin(), txt.end(), res.begin(),
                       [](char c){return std::toupper(c);});
        return value;
    });
----


=== `create_aggregate_function`

An aggregrate function will create a new `Func` for a new `aggregate` from the args tuple and call `step` for every step.
When the aggregation is done `final` is called and the result is returned to sqlite.

[source,cpp]
----
template<typename Func, typename Args = std::tuple<>>
void create_aggregate_function(
connection & conn,
cstring_ref name,
Args && args= {},
function_flags flags = {});

template<typename Func, typename Args = std::tuple<>>
void create_aggregate_function(
connection & conn,
cstring_ref name,
Args && args,
function_flags flags,
system::error_code & ec,
error_info & ei);
----


conn:: The connection to add the function to.
name:: The name of the function

args:: The argument tuple to construct `Func` from.
Func:: The function to be added. It needs to be an object with two functions:
[source,cpp]
----
void step(boost::span<sqlite::value, N> args);
T final();
----

.Example
[source,cpp]
----
  extern sqlite::connection conn;

struct aggregate_func
{
  aggregate_func(std::size_t init) : counter(init) {}
  std::int64_t counter;
  void step(, boost::span<sqlite::value, 1u> val)
  {
    counter += val[0].get_text().size();
  }

  std::int64_t final()
  {
    return counter;
  }
};

sqlite::create_function<aggregate_func>(conn, "char_counter", std::make_tuple(42));
----


=== `create_window_function`

NOTE: This is only available starting with sqlite 3.25.0.

An window function will create a new `Func` for a new `aggregate` and call `step` for every step.
When an element is removed from the window `inverse` is called.
When the aggregation is done `final` is called and the result is returned to sqlite.

[source,cpp]
----
template<typename Func, typename Args = std::tuple<>>
void create_window_function(
    connection & conn,
    cstring_ref name,
    Args && args = {},
    function_flags flags = {});

template<typename Func, typename Args = std::tuple<>>
void create_window_function(
    connection & conn,
    cstring_ref name,
    Args && args,
    function_flags flags,
    system::error_code & ec);
----


conn:: The connection to add the function to.
name:: The name of the function
args:: The arguments to construct Func from.
Func:: The function to be added. It needs to be an object with three functions:
[source,cpp]
----
void step(boost::span<sqlite::value, N> args);
void inverse(boost::span<sqlite::value, N> args);
T final();
----


.Example
[source,cpp]
----
extern sqlite::connection conn;

struct window_func
{
  std::int64_t counter;
  void step(boost::span<sqlite::value, 1u> val)
  {
    counter += val[0].get_text().size();
  }
  void inverse(boost::span<sqlite::value, 1u> val)
  {
    counter -= val[0].get_text().size();
  }

  std::int64_t final()
  {
    return counter;
  }
};

sqlite::create_function(conn, "win_char_counter", aggregate_func{});
----