skia2/include/private/SkTemplates.h

/*
 * Copyright 2006 The Android Open Source Project
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#ifndef SkTemplates_DEFINED
#define SkTemplates_DEFINED

#include "include/core/SkTypes.h"
#include "include/private/SkMalloc.h"
#include "include/private/SkTLogic.h"

#include <string.h>
#include <array>
#include <cstddef>
#include <memory>
#include <new>
#include <type_traits>
#include <utility>

/** \file SkTemplates.h

    This file contains light-weight template classes for type-safe and exception-safe
    resource management.
*/

/**
 *  Marks a local variable as known to be unused (to avoid warnings).
 *  Note that this does *not* prevent the local variable from being optimized away.
 */
template<typename T> inline void sk_ignore_unused_variable(const T&) { }

/**
 *  Returns a pointer to a D which comes immediately after S[count].
 */
template <typename D, typename S> static D* SkTAfter(S* ptr, size_t count = 1) {
    return reinterpret_cast<D*>(ptr + count);
}

/**
 *  Returns a pointer to a D which comes byteOffset bytes after S.
 */
template <typename D, typename S> static D* SkTAddOffset(S* ptr, size_t byteOffset) {
    // The intermediate char* has the same cv-ness as D as this produces better error messages.
    // This relies on the fact that reinterpret_cast can add constness, but cannot remove it.
    return reinterpret_cast<D*>(reinterpret_cast<sknonstd::same_cv_t<char, D>*>(ptr) + byteOffset);
}

// TODO: when C++17 the language is available, use template <auto P>
template <typename T, T* P> struct SkFunctionWrapper {
    template <typename... Args>
    auto operator()(Args&&... args) const -> decltype(P(std::forward<Args>(args)...)) {
        return P(std::forward<Args>(args)...);
    }
};

/** \class SkAutoTCallVProc

    Call a function when this goes out of scope. The template uses two
    parameters, the object, and a function that is to be called in the destructor.
    If release() is called, the object reference is set to null. If the object
    reference is null when the destructor is called, we do not call the
    function.
*/
template <typename T, void (*P)(T*)> class SkAutoTCallVProc
    : public std::unique_ptr<T, SkFunctionWrapper<std::remove_pointer_t<decltype(P)>, P>> {
    using inherited = std::unique_ptr<T, SkFunctionWrapper<std::remove_pointer_t<decltype(P)>, P>>;
public:
    using inherited::inherited;
    SkAutoTCallVProc(const SkAutoTCallVProc&) = delete;
    SkAutoTCallVProc(SkAutoTCallVProc&& that) : inherited(std::move(that)) {}

    operator T*() const { return this->get(); }
};

/** Allocate an array of T elements, and free the array in the destructor
 */
template <typename T> class SkAutoTArray  {
public:
    SkAutoTArray() {}
    /** Allocate count number of T elements
     */
    explicit SkAutoTArray(int count) {
        SkASSERT(count >= 0);
        if (count) {
            fArray.reset(new T[count]);
        }
        SkDEBUGCODE(fCount = count;)
    }

    SkAutoTArray(SkAutoTArray&& other) : fArray(std::move(other.fArray)) {
        SkDEBUGCODE(fCount = other.fCount; other.fCount = 0;)
    }
    SkAutoTArray& operator=(SkAutoTArray&& other) {
        if (this != &other) {
            fArray = std::move(other.fArray);
            SkDEBUGCODE(fCount = other.fCount; other.fCount = 0;)
        }
        return *this;
    }

    /** Reallocates given a new count. Reallocation occurs even if new count equals old count.
     */
    void reset(int count = 0) { *this = SkAutoTArray(count); }

    /** Return the array of T elements. Will be NULL if count == 0
     */
    T* get() const { return fArray.get(); }

    /** Return the nth element in the array
     */
    T&  operator[](int index) const {
        SkASSERT((unsigned)index < (unsigned)fCount);
        return fArray[index];
    }

    /** Aliases matching other types, like std::vector. */
    const T* data() const { return fArray.get(); }
    T* data() { return fArray.get(); }

private:
    std::unique_ptr<T[]> fArray;
    SkDEBUGCODE(int fCount = 0;)
};

/** Wraps SkAutoTArray, with room for kCountRequested elements preallocated.
 */
template <int kCountRequested, typename T> class SkAutoSTArray {
public:
    SkAutoSTArray(SkAutoSTArray&&) = delete;
    SkAutoSTArray(const SkAutoSTArray&) = delete;
    SkAutoSTArray& operator=(SkAutoSTArray&&) = delete;
    SkAutoSTArray& operator=(const SkAutoSTArray&) = delete;

    /** Initialize with no objects */
    SkAutoSTArray() {
        fArray = nullptr;
        fCount = 0;
    }

    /** Allocate count number of T elements
     */
    SkAutoSTArray(int count) {
        fArray = nullptr;
        fCount = 0;
        this->reset(count);
    }

    ~SkAutoSTArray() {
        this->reset(0);
    }

    /** Destroys previous objects in the array and default constructs count number of objects */
    void reset(int count) {
        T* start = fArray;
        T* iter = start + fCount;
        while (iter > start) {
            (--iter)->~T();
        }

        SkASSERT(count >= 0);
        if (fCount != count) {
            if (fCount > kCount) {
                // 'fArray' was allocated last time so free it now
                SkASSERT((T*) fStorage != fArray);
                sk_free(fArray);
            }

            if (count > kCount) {
                fArray = (T*) sk_malloc_throw(count, sizeof(T));
            } else if (count > 0) {
                fArray = (T*) fStorage;
            } else {
                fArray = nullptr;
            }

            fCount = count;
        }

        iter = fArray;
        T* stop = fArray + count;
        while (iter < stop) {
            new (iter++) T;
        }
    }

    /** Return the number of T elements in the array
     */
    int count() const { return fCount; }

    /** Return the array of T elements. Will be NULL if count == 0
     */
    T* get() const { return fArray; }

    T* begin() { return fArray; }

    const T* begin() const { return fArray; }

    T* end() { return fArray + fCount; }

    const T* end() const { return fArray + fCount; }

    /** Return the nth element in the array
     */
    T&  operator[](int index) const {
        SkASSERT(index < fCount);
        return fArray[index];
    }

    /** Aliases matching other types, like std::vector. */
    const T* data() const { return fArray; }
    T* data() { return fArray; }
    size_t size() const { return fCount; }

private:
#if defined(SK_BUILD_FOR_GOOGLE3)
    // Stack frame size is limited for SK_BUILD_FOR_GOOGLE3. 4k is less than the actual max, but some functions
    // have multiple large stack allocations.
    static const int kMaxBytes = 4 * 1024;
    static const int kCount = kCountRequested * sizeof(T) > kMaxBytes
        ? kMaxBytes / sizeof(T)
        : kCountRequested;
#else
    static const int kCount = kCountRequested;
#endif

    int     fCount;
    T*      fArray;
    // since we come right after fArray, fStorage should be properly aligned
    char    fStorage[kCount * sizeof(T)];
};

/** Manages an array of T elements, freeing the array in the destructor.
 *  Does NOT call any constructors/destructors on T (T must be POD).
 */
template <typename T,
          typename = std::enable_if_t<std::is_trivially_default_constructible<T>::value &&
                                      std::is_trivially_destructible<T>::value>>
class SkAutoTMalloc  {
public:
    /** Takes ownership of the ptr. The ptr must be a value which can be passed to sk_free. */
    explicit SkAutoTMalloc(T* ptr = nullptr) : fPtr(ptr) {}

    /** Allocates space for 'count' Ts. */
    explicit SkAutoTMalloc(size_t count)
        : fPtr(count ? (T*)sk_malloc_throw(count, sizeof(T)) : nullptr) {}

    SkAutoTMalloc(SkAutoTMalloc&&) = default;
    SkAutoTMalloc& operator=(SkAutoTMalloc&&) = default;

    /** Resize the memory area pointed to by the current ptr preserving contents. */
    void realloc(size_t count) {
        fPtr.reset(count ? (T*)sk_realloc_throw(fPtr.release(), count * sizeof(T)) : nullptr);
    }

    /** Resize the memory area pointed to by the current ptr without preserving contents. */
    T* reset(size_t count = 0) {
        fPtr.reset(count ? (T*)sk_malloc_throw(count, sizeof(T)) : nullptr);
        return this->get();
    }

    T* get() const { return fPtr.get(); }

    operator T*() { return fPtr.get(); }

    operator const T*() const { return fPtr.get(); }

    T& operator[](int index) { return fPtr.get()[index]; }

    const T& operator[](int index) const { return fPtr.get()[index]; }

    /** Aliases matching other types, like std::vector. */
    const T* data() const { return fPtr.get(); }
    T* data() { return fPtr.get(); }

    /**
     *  Transfer ownership of the ptr to the caller, setting the internal
     *  pointer to NULL. Note that this differs from get(), which also returns
     *  the pointer, but it does not transfer ownership.
     */
    T* release() { return fPtr.release(); }

private:
    std::unique_ptr<T, SkFunctionWrapper<void(void*), sk_free>> fPtr;
};

template <size_t kCountRequested,
          typename T,
          typename = std::enable_if_t<std::is_trivially_default_constructible<T>::value &&
                                      std::is_trivially_destructible<T>::value>>
class SkAutoSTMalloc {
public:
    SkAutoSTMalloc() : fPtr(fTStorage) {}

    SkAutoSTMalloc(size_t count) {
        if (count > kCount) {
            fPtr = (T*)sk_malloc_throw(count, sizeof(T));
        } else if (count) {
            fPtr = fTStorage;
        } else {
            fPtr = nullptr;
        }
    }

    SkAutoSTMalloc(SkAutoSTMalloc&&) = delete;
    SkAutoSTMalloc(const SkAutoSTMalloc&) = delete;
    SkAutoSTMalloc& operator=(SkAutoSTMalloc&&) = delete;
    SkAutoSTMalloc& operator=(const SkAutoSTMalloc&) = delete;

    ~SkAutoSTMalloc() {
        if (fPtr != fTStorage) {
            sk_free(fPtr);
        }
    }

    // doesn't preserve contents
    T* reset(size_t count) {
        if (fPtr != fTStorage) {
            sk_free(fPtr);
        }
        if (count > kCount) {
            fPtr = (T*)sk_malloc_throw(count, sizeof(T));
        } else if (count) {
            fPtr = fTStorage;
        } else {
            fPtr = nullptr;
        }
        return fPtr;
    }

    T* get() const { return fPtr; }

    operator T*() {
        return fPtr;
    }

    operator const T*() const {
        return fPtr;
    }

    T& operator[](int index) {
        return fPtr[index];
    }

    const T& operator[](int index) const {
        return fPtr[index];
    }

    /** Aliases matching other types, like std::vector. */
    const T* data() const { return fPtr; }
    T* data() { return fPtr; }

    // Reallocs the array, can be used to shrink the allocation.  Makes no attempt to be intelligent
    void realloc(size_t count) {
        if (count > kCount) {
            if (fPtr == fTStorage) {
                fPtr = (T*)sk_malloc_throw(count, sizeof(T));
                memcpy((void*)fPtr, fTStorage, kCount * sizeof(T));
            } else {
                fPtr = (T*)sk_realloc_throw(fPtr, count, sizeof(T));
            }
        } else if (count) {
            if (fPtr != fTStorage) {
                fPtr = (T*)sk_realloc_throw(fPtr, count, sizeof(T));
            }
        } else {
            this->reset(0);
        }
    }

private:
    // Since we use uint32_t storage, we might be able to get more elements for free.
    static const size_t kCountWithPadding = SkAlign4(kCountRequested*sizeof(T)) / sizeof(T);
#if defined(SK_BUILD_FOR_GOOGLE3)
    // Stack frame size is limited for SK_BUILD_FOR_GOOGLE3. 4k is less than the actual max, but some functions
    // have multiple large stack allocations.
    static const size_t kMaxBytes = 4 * 1024;
    static const size_t kCount = kCountRequested * sizeof(T) > kMaxBytes
        ? kMaxBytes / sizeof(T)
        : kCountWithPadding;
#else
    static const size_t kCount = kCountWithPadding;
#endif

    T*          fPtr;
    union {
        uint32_t    fStorage32[SkAlign4(kCount*sizeof(T)) >> 2];
        T           fTStorage[1];   // do NOT want to invoke T::T()
    };
};

//////////////////////////////////////////////////////////////////////////////////////////////////

/**
 *  Pass the object and the storage that was offered during SkInPlaceNewCheck, and this will
 *  safely destroy (and free if it was dynamically allocated) the object.
 */
template <typename T> void SkInPlaceDeleteCheck(T* obj, void* storage) {
    if (storage == obj) {
        obj->~T();
    } else {
        delete obj;
    }
}

/**
 *  Allocates T, using storage if it is large enough, and allocating on the heap (via new) if
 *  storage is not large enough.
 *
 *      obj = SkInPlaceNewCheck<Type>(storage, size);
 *      ...
 *      SkInPlaceDeleteCheck(obj, storage);
 */
template<typename T, typename... Args>
T* SkInPlaceNewCheck(void* storage, size_t size, Args&&... args) {
    return (sizeof(T) <= size) ? new (storage) T(std::forward<Args>(args)...)
                               : new T(std::forward<Args>(args)...);
}

template <int N, typename T> class SkAlignedSTStorage {
public:
    SkAlignedSTStorage() {}
    SkAlignedSTStorage(SkAlignedSTStorage&&) = delete;
    SkAlignedSTStorage(const SkAlignedSTStorage&) = delete;
    SkAlignedSTStorage& operator=(SkAlignedSTStorage&&) = delete;
    SkAlignedSTStorage& operator=(const SkAlignedSTStorage&) = delete;

    /**
     * Returns void* because this object does not initialize the
     * memory. Use placement new for types that require a constructor.
     */
    void* get() { return fStorage; }
    const void* get() const { return fStorage; }
private:
    alignas(T) char fStorage[sizeof(T)*N];
};

using SkAutoFree = std::unique_ptr<void, SkFunctionWrapper<void(void*), sk_free>>;

template<typename C, std::size_t... Is>
constexpr auto SkMakeArrayFromIndexSequence(C c, std::index_sequence<Is...> is)
-> std::array<decltype(c(std::declval<typename decltype(is)::value_type>())), sizeof...(Is)> {
    return {{ c(Is)... }};
}

template<size_t N, typename C> constexpr auto SkMakeArray(C c)
-> std::array<decltype(c(std::declval<typename std::index_sequence<N>::value_type>())), N> {
    return SkMakeArrayFromIndexSequence(c, std::make_index_sequence<N>{});
}

#endif