src/libaudcore/templates.h

/*
 * templates.h
 * Copyright 2014-2019 John Lindgren
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions, and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions, and the following disclaimer in the documentation
 *    provided with the distribution.
 *
 * This software is provided "as is" and without any warranty, express or
 * implied. In no event shall the authors be liable for any damages arising from
 * the use of this software.
 */

#ifndef LIBAUDCORE_TEMPLATES_H
#define LIBAUDCORE_TEMPLATES_H

#include <new>
#include <type_traits>
#include <utility>

// #undef POSIX and Windows macros to avoid name conflicts
#undef abs
#undef min
#undef max

namespace aud
{

// Utility functions
// =================

// minimum of two numbers
template<class T>
constexpr T min(T a, T b)
{
    return a < b ? a : b;
}

// maximum of two numbers
template<class T>
constexpr T max(T a, T b)
{
    return a > b ? a : b;
}

// make sure a number is within the given range
template<class T>
constexpr T clamp(T x, T low, T high)
{
    return min(max(x, low), high);
}

// absolute value
template<class T>
constexpr T abs(T x)
{
    return x < 0 ? -x : x;
}

// change the sign of x to the sign of s
template<class T>
constexpr T chsign(T x, T s)
{
    return (x < 0) ^ (s < 0) ? -x : x;
}

// integer division with rounding
template<class T>
constexpr T rdiv(T x, T y)
{
    return (x + chsign(y / 2, x)) / y;
}

// convert integer from one scale to another, with rounding
template<class T>
constexpr T rescale(T x, T old_scale, T new_scale)
{
    return rdiv(x * new_scale, old_scale);
}

// number of characters needed to represent an integer (including minus sign)
template<class T>
constexpr T n_digits(T x)
{
    return x < 0 ? 1 + n_digits(-x) : x < 10 ? 1 : 1 + n_digits(x / 10);
}

// number of elements in an array
template<class T, int N>
constexpr int n_elems(const T (&)[N])
{
    return N;
}

// Casts for storing various data in a void pointer
// ================================================

template<class T>
inline void * to_ptr(T t)
{
    union
    {
        void * v;
        T t;
    } u = {nullptr};
    static_assert(sizeof u == sizeof(void *),
                  "Type cannot be stored in a pointer");
    u.t = t;
    return u.v;
}

template<class T>
inline T from_ptr(void * v)
{
    union
    {
        void * v;
        T t;
    } u = {v};
    static_assert(sizeof u == sizeof(void *),
                  "Type cannot be stored in a pointer");
    return u.t;
}

// Move-assignment implemented via move-constructor
// ================================================

template<class T>
T & move_assign(T & a, T && b)
{
    if (&a != &b)
    {
        a.~T();
        new (&a) T(std::move(b));
    }
    return a;
}

// Function wrappers (or "casts") for interaction with C-style APIs
// ================================================================

template<class T>
void delete_obj(void * obj)
{
    (void)sizeof(T);
    delete (T *)obj;
}

template<class T>
void delete_typed(T * obj)
{
    (void)sizeof(T);
    delete obj;
}

template<class T, void (*func)(void *)>
void typed_func(T * obj)
{
    func(obj);
}

template<class T, void (T::*func)()>
static void obj_member(void * obj)
{
    (((T *)obj)->*func)();
}
template<class T, void (T::*func)() const>
static void obj_member(void * obj)
{
    (((T *)obj)->*func)();
}

// Class which provides scope-based "ownership" of an object
// (similar to std::unique_lock, but more flexible)
// =========================================================

template<class T, void (T::*acquire)(), void (T::*release)()>
class owner
{
public:
    explicit owner(T * obj = nullptr) : m_obj(obj)
    {
        if (m_obj)
            (m_obj->*acquire)();
    }
    ~owner()
    {
        if (m_obj)
            (m_obj->*release)();
    }

    owner(owner && b) : m_obj(b.m_obj) { b.m_obj = nullptr; }
    owner & operator=(owner && b) { return move_assign(*this, std::move(b)); }

private:
    T * m_obj;
};

// Wrapper class allowing enumerations to be used as array indexes;
// the enumeration must begin with zero and have a "count" constant
// ================================================================

template<class T, class V>
struct array
{
    // cannot use std::forward here; it is not constexpr until C++14
    template<class... Args>
    constexpr array(Args &&... args) : vals{static_cast<Args &&>(args)...}
    {
    }

    // Due to GCC bug #63707, the forwarding constructor given above cannot be
    // used to initialize the array when V is a class with no copy constructor.
    // As a very limited workaround, provide a second constructor which can be
    // used to initialize the array to default values in this case.
    constexpr array() : vals() {}

    constexpr const V & operator[](T t) const { return vals[(int)t]; }
    constexpr const V * begin() const { return vals; }
    constexpr const V * end() const { return vals + (int)T::count; }
    V & operator[](T t) { return vals[(int)t]; }
    V * begin() { return vals; }
    V * end() { return vals + (int)T::count; }

private:
    V vals[(int)T::count];
};

// Wrapper class allowing enumerations to be used in range-based for loops
// =======================================================================

template<class T, T first = (T)0, T last = (T)((int)T::count - 1)>
struct range
{
    struct iter
    {
        T v;
        constexpr T operator*() const { return v; }
        constexpr bool operator!=(iter other) const { return v != other.v; }
        void operator++() { v = (T)((int)v + 1); }
    };

    static constexpr iter begin() { return {first}; }
    static constexpr iter end() { return {(T)((int)last + 1)}; }
};

// Replacement for std::allocator::construct, which also supports aggregate
// initialization.  For background, see:
//     http://cplusplus.github.io/LWG/lwg-active.html#2089
// ========================================================================

// class constructor proxy
template<class T, bool aggregate>
struct construct_base
{
    template<class... Args>
    static T * make(void * loc, Args &&... args)
    {
        return new (loc) T(std::forward<Args>(args)...);
    }
};

// aggregate constructor proxy
template<class T>
struct construct_base<T, true>
{
    template<class... Args>
    static T * make(void * loc, Args &&... args)
    {
        return new (loc) T{std::forward<Args>(args)...};
    }
};

// generic constructor proxy
template<class T>
struct construct
{
    template<class... Args>
    static T * make(void * loc, Args &&... args)
    {
        constexpr bool aggregate = !std::is_constructible<T, Args &&...>::value;
        return construct_base<T, aggregate>::make(loc,
                                                  std::forward<Args>(args)...);
    }
};

// Convert an integer constant to a string at compile-time; can be used for
// #defines, enums, constexpr calculations, etc.
// ========================================================================

// "metaprogramming" string type: each different string is a unique type
template<char... args>
struct metastring
{
    char data[sizeof...(args) + 1] = {args..., '\0'};
};

// recursive number-printing template, general case (three or more digits)
template<int size, int x, char... args>
struct numeric_builder
{
    typedef typename numeric_builder<size - 1, x / 10, '0' + abs(x) % 10,
                                     args...>::type type;
};

// special case for two digits; minus sign is handled here
template<int x, char... args>
struct numeric_builder<2, x, args...>
{
    typedef metastring <
        x<0 ? '-' : '0' + x / 10, '0' + abs(x) % 10, args...> type;
};

// special case for one digit (positive numbers only)
template<int x, char... args>
struct numeric_builder<1, x, args...>
{
    typedef metastring<'0' + x, args...> type;
};

// convenience wrapper for numeric_builder
template<int x>
class numeric_string
{
private:
    // generate a unique string type representing this number
    typedef typename numeric_builder<n_digits(x), x>::type type;

    // declare a static string of that type (instantiated later at file scope)
    static constexpr type value{};

public:
    // pointer to the instantiated string
    static constexpr const char * str = value.data;
};

// instantiate numeric_string::value as needed for different numbers
template<int x>
constexpr typename numeric_string<x>::type numeric_string<x>::value;

// Functions for creating/copying/destroying objects en masse;
// these will be nullptr for basic types (use memset/memcpy instead)
// =================================================================

typedef void (*FillFunc)(void * data, int len);
typedef void (*CopyFunc)(const void * from, void * to, int len);
typedef void (*EraseFunc)(void * data, int len);

template<class T>
static constexpr FillFunc fill_func()
{
    return std::is_trivial<T>::value ? (FillFunc) nullptr : //
               [](void * data, int len) {
                   T * iter = (T *)data;
                   T * end = (T *)((char *)data + len);
                   while (iter < end)
                       new (iter++) T();
               };
}

template<class T>
static constexpr CopyFunc copy_func()
{
    return std::is_trivial<T>::value ? (CopyFunc) nullptr : //
               [](const void * from, void * to, int len) {
                   const T * src = (const T *)from;
                   T * dest = (T *)to;
                   T * end = (T *)((char *)to + len);
                   while (dest < end)
                       new (dest++) T(*src++);
               };
}

template<class T>
static constexpr EraseFunc erase_func()
{
    return std::is_trivial<T>::value ? (EraseFunc) nullptr : //
               [](void * data, int len) {
                   T * iter = (T *)data;
                   T * end = (T *)((char *)data + len);
                   while (iter < end)
                       (*iter++).~T();
               };
}

} // namespace aud

#endif // LIBAUDCORE_TEMPLATES_H