private/base/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/private/base/SkAlign.h"
#include "include/private/base/SkAssert.h"
#include "include/private/base/SkDebug.h"
#include "include/private/base/SkMalloc.h"
#include "include/private/base/SkTLogic.h"
#include "include/private/base/SkTo.h"

#include <array>
#include <cstddef>
#include <cstdint>
#include <cstring>
#include <memory>
#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&) { }

/**
 * This is a general purpose absolute-value function.
 * See SkAbs32 in (SkSafe32.h) for a 32-bit int specific version that asserts.
 */
template <typename T> static inline T SkTAbs(T value) {
    if (value < 0) {
        value = -value;
    }
    return value;
}

/**
 *  Returns a pointer to a D which comes immediately after S[count].
 */
template <typename D, typename S> inline 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> inline D* SkTAddOffset(S* ptr, ptrdiff_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);
}

template <typename T, T* P> struct SkOverloadedFunctionObject {
    template <typename... Args>
    auto operator()(Args&&... args) const -> decltype(P(std::forward<Args>(args)...)) {
        return P(std::forward<Args>(args)...);
    }
};

template <auto F> using SkFunctionObject =
    SkOverloadedFunctionObject<std::remove_pointer_t<decltype(F)>, F>;

/** \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, SkFunctionObject<P>> {
    using inherited = std::unique_ptr<T, SkFunctionObject<P>>;
public:
    using inherited::inherited;
    SkAutoTCallVProc(const SkAutoTCallVProc&) = delete;
    SkAutoTCallVProc(SkAutoTCallVProc&& that) : inherited(std::move(that)) {}

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


namespace skia_private {
/** Allocate an array of T elements, and free the array in the destructor
 */
template <typename T> class AutoTArray  {
public:
    AutoTArray() {}
    // Allocate size number of T elements
    explicit AutoTArray(size_t size) {
        fSize = check_size_bytes_too_big<T>(size);
        fData.reset(size > 0 ? new T[size] : nullptr);
    }

    // TODO: remove when all uses are gone.
    explicit AutoTArray(int size) : AutoTArray(SkToSizeT(size)) {}

    AutoTArray(AutoTArray&& other) : fData(std::move(other.fData)) {
        fSize = std::exchange(other.fSize, 0);
    }
    AutoTArray& operator=(AutoTArray&& other) {
        if (this != &other) {
            fData = std::move(other.fData);
            fSize = std::exchange(other.fSize, 0);
        }
        return *this;
    }

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

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

    T&  operator[](size_t index) const {
        return fData[sk_collection_check_bounds(index, fSize)];
    }

    const T* data() const { return fData.get(); }
    T* data() { return fData.get(); }

    size_t size() const { return fSize; }
    bool empty() const { return fSize == 0; }
    size_t size_bytes() const { return sizeof(T) * fSize; }

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

    // It's safe to use fItemArray + fSize because if fItemArray is nullptr then adding 0 is
    // valid and returns nullptr. See [expr.add] in the C++ standard.
    T* end() {
        if (fData == nullptr) {
            SkASSERT(fSize == 0);
        }
        return fData + fSize;
    }
    const T* end() const {
        if (fData == nullptr) {
            SkASSERT(fSize == 0);
        }
        return fData + fSize;
    }

private:
    std::unique_ptr<T[]> fData;
    size_t fSize = 0;
};

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

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

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

    ~AutoSTArray() {
        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 {
        return fArray[sk_collection_check_bounds(index, fCount)];
    }

    /** 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;
    alignas(T) 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 AutoTMalloc  {
public:
    /** Takes ownership of the ptr. The ptr must be a value which can be passed to sk_free. */
    explicit AutoTMalloc(T* ptr = nullptr) : fPtr(ptr) {}

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

    AutoTMalloc(AutoTMalloc&&) = default;
    AutoTMalloc& operator=(AutoTMalloc&&) = 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, SkOverloadedFunctionObject<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 AutoSTMalloc {
public:
    AutoSTMalloc() : fPtr(fTStorage) {}

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

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

    ~AutoSTMalloc() {
        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()
    };
};

using UniqueVoidPtr = std::unique_ptr<void, SkOverloadedFunctionObject<void(void*), sk_free>>;

}  // namespace skia_private

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