// ----------------------------------------------------------------------------
// -                        Open3D: www.open3d.org                            -
// ----------------------------------------------------------------------------
// Copyright (c) 2018-2023 www.open3d.org
// SPDX-License-Identifier: MIT
// ----------------------------------------------------------------------------
//
// Adapted for Open3D.
// Commit 75e164f61d391979b4829bf2746a5d74b94e95f2 2022-01-21
// Documentation:
// https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h
//
//===- llvm/ADT/SmallVector.h - 'Normally small' vectors --------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file defines the SmallVector class.
///
//===----------------------------------------------------------------------===//

#pragma once

#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <initializer_list>
#include <iterator>
#include <limits>
#include <memory>
#include <new>
#include <type_traits>
#include <utility>

#ifndef LLVM_LIKELY
#define LLVM_LIKELY /* [[likely]] */
#endif
#ifndef LLVM_UNLIKELY
#define LLVM_UNLIKELY /* [[unlikely]] */
#endif
#ifndef LLVM_NODISCARD
#define LLVM_NODISCARD /* [[nodiscard]] */
#endif
#ifndef LLVM_GSL_OWNER
#define LLVM_GSL_OWNER
#endif

namespace open3d {
namespace core {
// from llvm/include/llvm/Support/MemAlloc.h
inline void *safe_malloc(size_t Sz) {
    void *Result = std::malloc(Sz);
    if (Result == nullptr) {
        // It is implementation-defined whether allocation occurs if the space
        // requested is zero (ISO/IEC 9899:2018 7.22.3). Retry, requesting
        // non-zero, if the space requested was zero.
        if (Sz == 0) return safe_malloc(1);
        throw std::bad_alloc();
    }
    return Result;
}

inline void *safe_realloc(void *Ptr, size_t Sz) {
    void *Result = std::realloc(Ptr, Sz);
    if (Result == nullptr) {
        // It is implementation-defined whether allocation occurs if the space
        // requested is zero (ISO/IEC 9899:2018 7.22.3). Retry, requesting
        // non-zero, if the space requested was zero.
        if (Sz == 0) return safe_malloc(1);
        throw std::bad_alloc();
    }
    return Result;
}

template <typename IteratorT>
class iterator_range;

/// This is all the stuff common to all SmallVectors.
///
/// The template parameter specifies the type which should be used to hold the
/// Size and Capacity of the SmallVector, so it can be adjusted.
/// Using 32 bit size is desirable to shrink the size of the SmallVector.
/// Using 64 bit size is desirable for cases like SmallVector<char>, where a
/// 32 bit size would limit the vector to ~4GB. SmallVectors are used for
/// buffering bitcode output - which can exceed 4GB.
template <class Size_T>
class SmallVectorBase {
protected:
    void *BeginX;
    Size_T Size = 0, Capacity;

    /// The maximum value of the Size_T used.
    static constexpr size_t SizeTypeMax() {
        return std::numeric_limits<Size_T>::max();
    }

    SmallVectorBase() = delete;
    SmallVectorBase(void *FirstEl, size_t TotalCapacity)
        : BeginX(FirstEl), Capacity(TotalCapacity) {}

    /// This is a helper for \a grow() that's out of line to reduce code
    /// duplication.  This function will report a fatal error if it can't grow
    /// at least to \p MinSize.
    void *mallocForGrow(size_t MinSize, size_t TSize, size_t &NewCapacity);

    /// This is an implementation of the grow() method which only works
    /// on POD-like data types and is out of line to reduce code duplication.
    /// This function will report a fatal error if it cannot increase capacity.
    void grow_pod(void *FirstEl, size_t MinSize, size_t TSize);

public:
    size_t size() const { return Size; }
    size_t capacity() const { return Capacity; }

    LLVM_NODISCARD bool empty() const { return !Size; }

protected:
    /// Set the array size to \p N, which the current array must have enough
    /// capacity for.
    ///
    /// This does not construct or destroy any elements in the vector.
    void set_size(size_t N) {
        assert(N <= capacity());
        Size = N;
    }
};

template <class T>
using SmallVectorSizeType =
        typename std::conditional<sizeof(T) < 4 && sizeof(void *) >= 8,
                                  uint64_t,
                                  uint32_t>::type;

/// Figure out the offset of the first element.
template <class T, typename = void>
struct SmallVectorAlignmentAndSize {
    alignas(SmallVectorBase<SmallVectorSizeType<T>>) char Base[sizeof(
            SmallVectorBase<SmallVectorSizeType<T>>)];
    alignas(T) char FirstEl[sizeof(T)];
};

/// This is the part of SmallVectorTemplateBase which does not depend on whether
/// the type T is a POD. The extra dummy template argument is used by ArrayRef
/// to avoid unnecessarily requiring T to be complete.
template <typename T, typename = void>
class SmallVectorTemplateCommon
    : public SmallVectorBase<SmallVectorSizeType<T>> {
    using Base = SmallVectorBase<SmallVectorSizeType<T>>;

    /// Find the address of the first element.  For this pointer math to be
    /// valid with small-size of 0 for T with lots of alignment, it's important
    /// that SmallVectorStorage is properly-aligned even for small-size of 0.
    void *getFirstEl() const {
        return const_cast<void *>(reinterpret_cast<const void *>(
                reinterpret_cast<const char *>(this) +
                offsetof(SmallVectorAlignmentAndSize<T>, FirstEl)));
    }
    // Space after 'FirstEl' is clobbered, do not add any instance vars after
    // it.

protected:
    SmallVectorTemplateCommon(size_t Size) : Base(getFirstEl(), Size) {}

    void grow_pod(size_t MinSize, size_t TSize) {
        Base::grow_pod(getFirstEl(), MinSize, TSize);
    }

    /// Return true if this is a smallvector which has not had dynamic
    /// memory allocated for it.
    bool isSmall() const { return this->BeginX == getFirstEl(); }

    /// Put this vector in a state of being small.
    void resetToSmall() {
        this->BeginX = getFirstEl();
        this->Size = this->Capacity =
                0;  // FIXME: Setting Capacity to 0 is suspect.
    }

    /// Return true if V is an internal reference to the given range.
    bool isReferenceToRange(const void *V,
                            const void *First,
                            const void *Last) const {
        // Use std::less to avoid UB.
        std::less<> LessThan;
        return !LessThan(V, First) && LessThan(V, Last);
    }

    /// Return true if V is an internal reference to this vector.
    bool isReferenceToStorage(const void *V) const {
        return isReferenceToRange(V, this->begin(), this->end());
    }

    /// Return true if First and Last form a valid (possibly empty) range in
    /// this vector's storage.
    bool isRangeInStorage(const void *First, const void *Last) const {
        // Use std::less to avoid UB.
        std::less<> LessThan;
        return !LessThan(First, this->begin()) && !LessThan(Last, First) &&
               !LessThan(this->end(), Last);
    }

    /// Return true unless Elt will be invalidated by resizing the vector to
    /// NewSize.
    bool isSafeToReferenceAfterResize(const void *Elt, size_t NewSize) {
        // Past the end.
        if (LLVM_LIKELY(!isReferenceToStorage(Elt))) return true;

        // Return false if Elt will be destroyed by shrinking.
        if (NewSize <= this->size()) return Elt < this->begin() + NewSize;

        // Return false if we need to grow.
        return NewSize <= this->capacity();
    }

    /// Check whether Elt will be invalidated by resizing the vector to NewSize.
    void assertSafeToReferenceAfterResize(const void *Elt, size_t NewSize) {
        assert(isSafeToReferenceAfterResize(Elt, NewSize) &&
               "Attempting to reference an element of the vector in an "
               "operation "
               "that invalidates it");
    }

    /// Check whether Elt will be invalidated by increasing the size of the
    /// vector by N.
    void assertSafeToAdd(const void *Elt, size_t N = 1) {
        this->assertSafeToReferenceAfterResize(Elt, this->size() + N);
    }

    /// Check whether any part of the range will be invalidated by clearing.
    void assertSafeToReferenceAfterClear(const T *From, const T *To) {
        if (From == To) return;
        this->assertSafeToReferenceAfterResize(From, 0);
        this->assertSafeToReferenceAfterResize(To - 1, 0);
    }
    template <class ItTy,
              std::enable_if_t<
                      !std::is_same<std::remove_const_t<ItTy>, T *>::value,
                      bool> = false>
    void assertSafeToReferenceAfterClear(ItTy, ItTy) {}

    /// Check whether any part of the range will be invalidated by growing.
    void assertSafeToAddRange(const T *From, const T *To) {
        if (From == To) return;
        this->assertSafeToAdd(From, To - From);
        this->assertSafeToAdd(To - 1, To - From);
    }
    template <class ItTy,
              std::enable_if_t<
                      !std::is_same<std::remove_const_t<ItTy>, T *>::value,
                      bool> = false>
    void assertSafeToAddRange(ItTy, ItTy) {}

    /// Reserve enough space to add one element, and return the updated element
    /// pointer in case it was a reference to the storage.
    template <class U>
    static const T *reserveForParamAndGetAddressImpl(U *This,
                                                     const T &Elt,
                                                     size_t N) {
        size_t NewSize = This->size() + N;
        if (LLVM_LIKELY(NewSize <= This->capacity())) return &Elt;

        bool ReferencesStorage = false;
        int64_t Index = -1;
        if (!U::TakesParamByValue) {
            if (LLVM_UNLIKELY(This->isReferenceToStorage(&Elt))) {
                ReferencesStorage = true;
                Index = &Elt - This->begin();
            }
        }
        This->grow(NewSize);
        return ReferencesStorage ? This->begin() + Index : &Elt;
    }

public:
    using size_type = size_t;
    using difference_type = ptrdiff_t;
    using value_type = T;
    using iterator = T *;
    using const_iterator = const T *;

    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
    using reverse_iterator = std::reverse_iterator<iterator>;

    using reference = T &;
    using const_reference = const T &;
    using pointer = T *;
    using const_pointer = const T *;

    using Base::capacity;
    using Base::empty;
    using Base::size;

    // forward iterator creation methods.
    iterator begin() { return (iterator)this->BeginX; }
    const_iterator begin() const { return (const_iterator)this->BeginX; }
    iterator end() { return begin() + size(); }
    const_iterator end() const { return begin() + size(); }

    // reverse iterator creation methods.
    reverse_iterator rbegin() { return reverse_iterator(end()); }
    const_reverse_iterator rbegin() const {
        return const_reverse_iterator(end());
    }
    reverse_iterator rend() { return reverse_iterator(begin()); }
    const_reverse_iterator rend() const {
        return const_reverse_iterator(begin());
    }

    size_type size_in_bytes() const { return size() * sizeof(T); }
    size_type max_size() const {
        return std::min(this->SizeTypeMax(), size_type(-1) / sizeof(T));
    }

    size_t capacity_in_bytes() const { return capacity() * sizeof(T); }

    /// Return a pointer to the vector's buffer, even if empty().
    pointer data() { return pointer(begin()); }
    /// Return a pointer to the vector's buffer, even if empty().
    const_pointer data() const { return const_pointer(begin()); }

    reference operator[](size_type idx) {
        assert(idx < size());
        return begin()[idx];
    }
    const_reference operator[](size_type idx) const {
        assert(idx < size());
        return begin()[idx];
    }

    reference front() {
        assert(!empty());
        return begin()[0];
    }
    const_reference front() const {
        assert(!empty());
        return begin()[0];
    }

    reference back() {
        assert(!empty());
        return end()[-1];
    }
    const_reference back() const {
        assert(!empty());
        return end()[-1];
    }
};

/// SmallVectorTemplateBase<TriviallyCopyable = false> - This is where we put
/// method implementations that are designed to work with non-trivial T's.
///
/// We approximate is_trivially_copyable with trivial move/copy construction and
/// trivial destruction. While the standard doesn't specify that you're allowed
/// copy these types with memcpy, there is no way for the type to observe this.
/// This catches the important case of std::pair<POD, POD>, which is not
/// trivially assignable.
template <typename T,
          bool = (std::is_trivially_copy_constructible<T>::value) &&
                 (std::is_trivially_move_constructible<T>::value) &&
                 std::is_trivially_destructible<T>::value>
class SmallVectorTemplateBase : public SmallVectorTemplateCommon<T> {
    friend class SmallVectorTemplateCommon<T>;

protected:
    static constexpr bool TakesParamByValue = false;
    using ValueParamT = const T &;

    SmallVectorTemplateBase(size_t Size) : SmallVectorTemplateCommon<T>(Size) {}

    static void destroy_range(T *S, T *E) {
        while (S != E) {
            --E;
            E->~T();
        }
    }

    /// Move the range [I, E) into the uninitialized memory starting with
    /// "Dest", constructing elements as needed.
    template <typename It1, typename It2>
    static void uninitialized_move(It1 I, It1 E, It2 Dest) {
        std::uninitialized_copy(std::make_move_iterator(I),
                                std::make_move_iterator(E), Dest);
    }

    /// Copy the range [I, E) onto the uninitialized memory starting with
    /// "Dest", constructing elements as needed.
    template <typename It1, typename It2>
    static void uninitialized_copy(It1 I, It1 E, It2 Dest) {
        std::uninitialized_copy(I, E, Dest);
    }

    /// Grow the allocated memory (without initializing new elements), doubling
    /// the size of the allocated memory. Guarantees space for at least one more
    /// element, or MinSize more elements if specified.
    void grow(size_t MinSize = 0);

    /// Create a new allocation big enough for \p MinSize and pass back its size
    /// in \p NewCapacity. This is the first section of \a grow().
    T *mallocForGrow(size_t MinSize, size_t &NewCapacity) {
        return static_cast<T *>(
                SmallVectorBase<SmallVectorSizeType<T>>::mallocForGrow(
                        MinSize, sizeof(T), NewCapacity));
    }

    /// Move existing elements over to the new allocation \p NewElts, the middle
    /// section of \a grow().
    void moveElementsForGrow(T *NewElts);

    /// Transfer ownership of the allocation, finishing up \a grow().
    void takeAllocationForGrow(T *NewElts, size_t NewCapacity);

    /// Reserve enough space to add one element, and return the updated element
    /// pointer in case it was a reference to the storage.
    const T *reserveForParamAndGetAddress(const T &Elt, size_t N = 1) {
        return this->reserveForParamAndGetAddressImpl(this, Elt, N);
    }

    /// Reserve enough space to add one element, and return the updated element
    /// pointer in case it was a reference to the storage.
    T *reserveForParamAndGetAddress(T &Elt, size_t N = 1) {
        return const_cast<T *>(
                this->reserveForParamAndGetAddressImpl(this, Elt, N));
    }

    static T &&forward_value_param(T &&V) { return std::move(V); }
    static const T &forward_value_param(const T &V) { return V; }

    void growAndAssign(size_t NumElts, const T &Elt) {
        // Grow manually in case Elt is an internal reference.
        size_t NewCapacity;
        T *NewElts = mallocForGrow(NumElts, NewCapacity);
        std::uninitialized_fill_n(NewElts, NumElts, Elt);
        this->destroy_range(this->begin(), this->end());
        takeAllocationForGrow(NewElts, NewCapacity);
        this->set_size(NumElts);
    }

    template <typename... ArgTypes>
    T &growAndEmplaceBack(ArgTypes &&... Args) {
        // Grow manually in case one of Args is an internal reference.
        size_t NewCapacity;
        T *NewElts = mallocForGrow(0, NewCapacity);
        ::new ((void *)(NewElts + this->size()))
                T(std::forward<ArgTypes>(Args)...);
        moveElementsForGrow(NewElts);
        takeAllocationForGrow(NewElts, NewCapacity);
        this->set_size(this->size() + 1);
        return this->back();
    }

public:
    void push_back(const T &Elt) {
        const T *EltPtr = reserveForParamAndGetAddress(Elt);
        ::new ((void *)this->end()) T(*EltPtr);
        this->set_size(this->size() + 1);
    }

    void push_back(T &&Elt) {
        T *EltPtr = reserveForParamAndGetAddress(Elt);
        ::new ((void *)this->end()) T(::std::move(*EltPtr));
        this->set_size(this->size() + 1);
    }

    void pop_back() {
        this->set_size(this->size() - 1);
        this->end()->~T();
    }
};

// Define this out-of-line to dissuade the C++ compiler from inlining it.
template <typename T, bool TriviallyCopyable>
void SmallVectorTemplateBase<T, TriviallyCopyable>::grow(size_t MinSize) {
    size_t NewCapacity;
    T *NewElts = mallocForGrow(MinSize, NewCapacity);
    moveElementsForGrow(NewElts);
    takeAllocationForGrow(NewElts, NewCapacity);
}

// Define this out-of-line to dissuade the C++ compiler from inlining it.
template <typename T, bool TriviallyCopyable>
void SmallVectorTemplateBase<T, TriviallyCopyable>::moveElementsForGrow(
        T *NewElts) {
    // Move the elements over.
    this->uninitialized_move(this->begin(), this->end(), NewElts);

    // Destroy the original elements.
    destroy_range(this->begin(), this->end());
}

// Define this out-of-line to dissuade the C++ compiler from inlining it.
template <typename T, bool TriviallyCopyable>
void SmallVectorTemplateBase<T, TriviallyCopyable>::takeAllocationForGrow(
        T *NewElts, size_t NewCapacity) {
    // If this wasn't grown from the inline copy, deallocate the old space.
    if (!this->isSmall()) free(this->begin());

    this->BeginX = NewElts;
    this->Capacity = NewCapacity;
}

/// SmallVectorTemplateBase<TriviallyCopyable = true> - This is where we put
/// method implementations that are designed to work with trivially copyable
/// T's. This allows using memcpy in place of copy/move construction and
/// skipping destruction.
template <typename T>
class SmallVectorTemplateBase<T, true> : public SmallVectorTemplateCommon<T> {
    friend class SmallVectorTemplateCommon<T>;

protected:
    /// True if it's cheap enough to take parameters by value. Doing so avoids
    /// overhead related to mitigations for reference invalidation.
    static constexpr bool TakesParamByValue = sizeof(T) <= 2 * sizeof(void *);

    /// Either const T& or T, depending on whether it's cheap enough to take
    /// parameters by value.
    using ValueParamT =
            typename std::conditional<TakesParamByValue, T, const T &>::type;

    SmallVectorTemplateBase(size_t Size) : SmallVectorTemplateCommon<T>(Size) {}

    // No need to do a destroy loop for POD's.
    static void destroy_range(T *, T *) {}

    /// Move the range [I, E) onto the uninitialized memory
    /// starting with "Dest", constructing elements into it as needed.
    template <typename It1, typename It2>
    static void uninitialized_move(It1 I, It1 E, It2 Dest) {
        // Just do a copy.
        uninitialized_copy(I, E, Dest);
    }

    /// Copy the range [I, E) onto the uninitialized memory
    /// starting with "Dest", constructing elements into it as needed.
    template <typename It1, typename It2>
    static void uninitialized_copy(It1 I, It1 E, It2 Dest) {
        // Arbitrary iterator types; just use the basic implementation.
        std::uninitialized_copy(I, E, Dest);
    }

    /// Copy the range [I, E) onto the uninitialized memory
    /// starting with "Dest", constructing elements into it as needed.
    template <typename T1, typename T2>
    static void uninitialized_copy(
            T1 *I,
            T1 *E,
            T2 *Dest,
            std::enable_if_t<std::is_same<typename std::remove_const<T1>::type,
                                          T2>::value> * = nullptr) {
        // Use memcpy for PODs iterated by pointers (which includes SmallVector
        // iterators): std::uninitialized_copy optimizes to memmove, but we can
        // use memcpy here. Note that I and E are iterators and thus might be
        // invalid for memcpy if they are equal.
        if (I != E)
            memcpy(reinterpret_cast<void *>(Dest), I, (E - I) * sizeof(T));
    }

    /// Double the size of the allocated memory, guaranteeing space for at
    /// least one more element or MinSize if specified.
    void grow(size_t MinSize = 0) { this->grow_pod(MinSize, sizeof(T)); }

    /// Reserve enough space to add one element, and return the updated element
    /// pointer in case it was a reference to the storage.
    const T *reserveForParamAndGetAddress(const T &Elt, size_t N = 1) {
        return this->reserveForParamAndGetAddressImpl(this, Elt, N);
    }

    /// Reserve enough space to add one element, and return the updated element
    /// pointer in case it was a reference to the storage.
    T *reserveForParamAndGetAddress(T &Elt, size_t N = 1) {
        return const_cast<T *>(
                this->reserveForParamAndGetAddressImpl(this, Elt, N));
    }

    /// Copy \p V or return a reference, depending on \a ValueParamT.
    static ValueParamT forward_value_param(ValueParamT V) { return V; }

    void growAndAssign(size_t NumElts, T Elt) {
        // Elt has been copied in case it's an internal reference, side-stepping
        // reference invalidation problems without losing the realloc
        // optimization.
        this->set_size(0);
        this->grow(NumElts);
        std::uninitialized_fill_n(this->begin(), NumElts, Elt);
        this->set_size(NumElts);
    }

    template <typename... ArgTypes>
    T &growAndEmplaceBack(ArgTypes &&... Args) {
        // Use push_back with a copy in case Args has an internal reference,
        // side-stepping reference invalidation problems without losing the
        // realloc optimization.
        push_back(T(std::forward<ArgTypes>(Args)...));
        return this->back();
    }

public:
    void push_back(ValueParamT Elt) {
        const T *EltPtr = reserveForParamAndGetAddress(Elt);
        memcpy(reinterpret_cast<void *>(this->end()), EltPtr, sizeof(T));
        this->set_size(this->size() + 1);
    }

    void pop_back() { this->set_size(this->size() - 1); }
};

/// This class consists of common code factored out of the SmallVector class to
/// reduce code duplication based on the SmallVector 'N' template parameter.
template <typename T>
class SmallVectorImpl : public SmallVectorTemplateBase<T> {
    using SuperClass = SmallVectorTemplateBase<T>;

public:
    using iterator = typename SuperClass::iterator;
    using const_iterator = typename SuperClass::const_iterator;
    using reference = typename SuperClass::reference;
    using size_type = typename SuperClass::size_type;

protected:
    using SmallVectorTemplateBase<T>::TakesParamByValue;
    using ValueParamT = typename SuperClass::ValueParamT;

    // Default ctor - Initialize to empty.
    explicit SmallVectorImpl(unsigned N) : SmallVectorTemplateBase<T>(N) {}

    void assignRemote(SmallVectorImpl &&RHS) {
        this->destroy_range(this->begin(), this->end());
        if (!this->isSmall()) free(this->begin());
        this->BeginX = RHS.BeginX;
        this->Size = RHS.Size;
        this->Capacity = RHS.Capacity;
        RHS.resetToSmall();
    }

public:
    SmallVectorImpl(const SmallVectorImpl &) = delete;

    ~SmallVectorImpl() {
        // Subclass has already destructed this vector's elements.
        // If this wasn't grown from the inline copy, deallocate the old space.
        if (!this->isSmall()) free(this->begin());
    }

    void clear() {
        this->destroy_range(this->begin(), this->end());
        this->Size = 0;
    }

private:
    // Make set_size() private to avoid misuse in subclasses.
    using SuperClass::set_size;

    template <bool ForOverwrite>
    void resizeImpl(size_type N) {
        if (N == this->size()) return;

        if (N < this->size()) {
            this->truncate(N);
            return;
        }

        this->reserve(N);
        for (auto I = this->end(), E = this->begin() + N; I != E; ++I)
            if (ForOverwrite)
                new (&*I) T;
            else
                new (&*I) T();
        this->set_size(N);
    }

public:
    void resize(size_type N) { resizeImpl<false>(N); }

    /// Like resize, but \ref T is POD, the new values won't be initialized.
    void resize_for_overwrite(size_type N) { resizeImpl<true>(N); }

    /// Like resize, but requires that \p N is less than \a size().
    void truncate(size_type N) {
        assert(this->size() >= N && "Cannot increase size with truncate");
        this->destroy_range(this->begin() + N, this->end());
        this->set_size(N);
    }

    void resize(size_type N, ValueParamT NV) {
        if (N == this->size()) return;

        if (N < this->size()) {
            this->truncate(N);
            return;
        }

        // N > this->size(). Defer to append.
        this->append(N - this->size(), NV);
    }

    void reserve(size_type N) {
        if (this->capacity() < N) this->grow(N);
    }

    void pop_back_n(size_type NumItems) {
        assert(this->size() >= NumItems);
        truncate(this->size() - NumItems);
    }

    LLVM_NODISCARD T pop_back_val() {
        T Result = ::std::move(this->back());
        this->pop_back();
        return Result;
    }

    void swap(SmallVectorImpl &RHS);

    /// Add the specified range to the end of the SmallVector.
    template <typename in_iter,
              typename = std::enable_if_t<std::is_convertible<
                      typename std::iterator_traits<in_iter>::iterator_category,
                      std::input_iterator_tag>::value>>
    void append(in_iter in_start, in_iter in_end) {
        this->assertSafeToAddRange(in_start, in_end);
        size_type NumInputs = std::distance(in_start, in_end);
        this->reserve(this->size() + NumInputs);
        this->uninitialized_copy(in_start, in_end, this->end());
        this->set_size(this->size() + NumInputs);
    }

    /// Append \p NumInputs copies of \p Elt to the end.
    void append(size_type NumInputs, ValueParamT Elt) {
        const T *EltPtr = this->reserveForParamAndGetAddress(Elt, NumInputs);
        std::uninitialized_fill_n(this->end(), NumInputs, *EltPtr);
        this->set_size(this->size() + NumInputs);
    }

    void append(std::initializer_list<T> IL) { append(IL.begin(), IL.end()); }

    void append(const SmallVectorImpl &RHS) { append(RHS.begin(), RHS.end()); }

    void assign(size_type NumElts, ValueParamT Elt) {
        // Note that Elt could be an internal reference.
        if (NumElts > this->capacity()) {
            this->growAndAssign(NumElts, Elt);
            return;
        }

        // Assign over existing elements.
        std::fill_n(this->begin(), std::min(NumElts, this->size()), Elt);
        if (NumElts > this->size())
            std::uninitialized_fill_n(this->end(), NumElts - this->size(), Elt);
        else if (NumElts < this->size())
            this->destroy_range(this->begin() + NumElts, this->end());
        this->set_size(NumElts);
    }

    // FIXME: Consider assigning over existing elements, rather than clearing &
    // re-initializing them - for all assign(...) variants.

    template <typename in_iter,
              typename = std::enable_if_t<std::is_convertible<
                      typename std::iterator_traits<in_iter>::iterator_category,
                      std::input_iterator_tag>::value>>
    void assign(in_iter in_start, in_iter in_end) {
        this->assertSafeToReferenceAfterClear(in_start, in_end);
        clear();
        append(in_start, in_end);
    }

    void assign(std::initializer_list<T> IL) {
        clear();
        append(IL);
    }

    void assign(const SmallVectorImpl &RHS) { assign(RHS.begin(), RHS.end()); }

    iterator erase(const_iterator CI) {
        // Just cast away constness because this is a non-const member function.
        iterator I = const_cast<iterator>(CI);

        assert(this->isReferenceToStorage(CI) &&
               "Iterator to erase is out of bounds.");

        iterator N = I;
        // Shift all elts down one.
        std::move(I + 1, this->end(), I);
        // Drop the last elt.
        this->pop_back();
        return (N);
    }

    iterator erase(const_iterator CS, const_iterator CE) {
        // Just cast away constness because this is a non-const member function.
        iterator S = const_cast<iterator>(CS);
        iterator E = const_cast<iterator>(CE);

        assert(this->isRangeInStorage(S, E) &&
               "Range to erase is out of bounds.");

        iterator N = S;
        // Shift all elts down.
        iterator I = std::move(E, this->end(), S);
        // Drop the last elts.
        this->destroy_range(I, this->end());
        this->set_size(I - this->begin());
        return (N);
    }

private:
    template <class ArgType>
    iterator insert_one_impl(iterator I, ArgType &&Elt) {
        // Callers ensure that ArgType is derived from T.
        static_assert(
                std::is_same<
                        std::remove_const_t<std::remove_reference_t<ArgType>>,
                        T>::value,
                "ArgType must be derived from T!");

        if (I == this->end()) {  // Important special case for empty vector.
            this->push_back(::std::forward<ArgType>(Elt));
            return this->end() - 1;
        }

        assert(this->isReferenceToStorage(I) &&
               "Insertion iterator is out of bounds.");

        // Grow if necessary.
        size_t Index = I - this->begin();
        std::remove_reference_t<ArgType> *EltPtr =
                this->reserveForParamAndGetAddress(Elt);
        I = this->begin() + Index;

        ::new ((void *)this->end()) T(::std::move(this->back()));
        // Push everything else over.
        std::move_backward(I, this->end() - 1, this->end());
        this->set_size(this->size() + 1);

        // If we just moved the element we're inserting, be sure to update
        // the reference (never happens if TakesParamByValue).
        static_assert(!TakesParamByValue || std::is_same<ArgType, T>::value,
                      "ArgType must be 'T' when taking by value!");
        if (!TakesParamByValue &&
            this->isReferenceToRange(EltPtr, I, this->end()))
            ++EltPtr;

        *I = ::std::forward<ArgType>(*EltPtr);
        return I;
    }

public:
    iterator insert(iterator I, T &&Elt) {
        return insert_one_impl(I, this->forward_value_param(std::move(Elt)));
    }

    iterator insert(iterator I, const T &Elt) {
        return insert_one_impl(I, this->forward_value_param(Elt));
    }

    iterator insert(iterator I, size_type NumToInsert, ValueParamT Elt) {
        // Convert iterator to elt# to avoid invalidating iterator when we
        // reserve()
        size_t InsertElt = I - this->begin();

        if (I == this->end()) {  // Important special case for empty vector.
            append(NumToInsert, Elt);
            return this->begin() + InsertElt;
        }

        assert(this->isReferenceToStorage(I) &&
               "Insertion iterator is out of bounds.");

        // Ensure there is enough space, and get the (maybe updated) address of
        // Elt.
        const T *EltPtr = this->reserveForParamAndGetAddress(Elt, NumToInsert);

        // Uninvalidate the iterator.
        I = this->begin() + InsertElt;

        // If there are more elements between the insertion point and the end of
        // the range than there are being inserted, we can use a simple approach
        // to insertion.  Since we already reserved space, we know that this
        // won't reallocate the vector.
        if (size_t(this->end() - I) >= NumToInsert) {
            T *OldEnd = this->end();
            append(std::move_iterator<iterator>(this->end() - NumToInsert),
                   std::move_iterator<iterator>(this->end()));

            // Copy the existing elements that get replaced.
            std::move_backward(I, OldEnd - NumToInsert, OldEnd);

            // If we just moved the element we're inserting, be sure to update
            // the reference (never happens if TakesParamByValue).
            if (!TakesParamByValue && I <= EltPtr && EltPtr < this->end())
                EltPtr += NumToInsert;

            std::fill_n(I, NumToInsert, *EltPtr);
            return I;
        }

        // Otherwise, we're inserting more elements than exist already, and
        // we're not inserting at the end.

        // Move over the elements that we're about to overwrite.
        T *OldEnd = this->end();
        this->set_size(this->size() + NumToInsert);
        size_t NumOverwritten = OldEnd - I;
        this->uninitialized_move(I, OldEnd, this->end() - NumOverwritten);

        // If we just moved the element we're inserting, be sure to update
        // the reference (never happens if TakesParamByValue).
        if (!TakesParamByValue && I <= EltPtr && EltPtr < this->end())
            EltPtr += NumToInsert;

        // Replace the overwritten part.
        std::fill_n(I, NumOverwritten, *EltPtr);

        // Insert the non-overwritten middle part.
        std::uninitialized_fill_n(OldEnd, NumToInsert - NumOverwritten,
                                  *EltPtr);
        return I;
    }

    template <typename ItTy,
              typename = std::enable_if_t<std::is_convertible<
                      typename std::iterator_traits<ItTy>::iterator_category,
                      std::input_iterator_tag>::value>>
    iterator insert(iterator I, ItTy From, ItTy To) {
        // Convert iterator to elt# to avoid invalidating iterator when we
        // reserve()
        size_t InsertElt = I - this->begin();

        if (I == this->end()) {  // Important special case for empty vector.
            append(From, To);
            return this->begin() + InsertElt;
        }

        assert(this->isReferenceToStorage(I) &&
               "Insertion iterator is out of bounds.");

        // Check that the reserve that follows doesn't invalidate the iterators.
        this->assertSafeToAddRange(From, To);

        size_t NumToInsert = std::distance(From, To);

        // Ensure there is enough space.
        reserve(this->size() + NumToInsert);

        // Uninvalidate the iterator.
        I = this->begin() + InsertElt;

        // If there are more elements between the insertion point and the end of
        // the range than there are being inserted, we can use a simple approach
        // to insertion.  Since we already reserved space, we know that this
        // won't reallocate the vector.
        if (size_t(this->end() - I) >= NumToInsert) {
            T *OldEnd = this->end();
            append(std::move_iterator<iterator>(this->end() - NumToInsert),
                   std::move_iterator<iterator>(this->end()));

            // Copy the existing elements that get replaced.
            std::move_backward(I, OldEnd - NumToInsert, OldEnd);

            std::copy(From, To, I);
            return I;
        }

        // Otherwise, we're inserting more elements than exist already, and
        // we're not inserting at the end.

        // Move over the elements that we're about to overwrite.
        T *OldEnd = this->end();
        this->set_size(this->size() + NumToInsert);
        size_t NumOverwritten = OldEnd - I;
        this->uninitialized_move(I, OldEnd, this->end() - NumOverwritten);

        // Replace the overwritten part.
        for (T *J = I; NumOverwritten > 0; --NumOverwritten) {
            *J = *From;
            ++J;
            ++From;
        }

        // Insert the non-overwritten middle part.
        this->uninitialized_copy(From, To, OldEnd);
        return I;
    }

    void insert(iterator I, std::initializer_list<T> IL) {
        insert(I, IL.begin(), IL.end());
    }

    template <typename... ArgTypes>
    reference emplace_back(ArgTypes &&... Args) {
        if (LLVM_UNLIKELY(this->size() >= this->capacity()))
            return this->growAndEmplaceBack(std::forward<ArgTypes>(Args)...);

        ::new ((void *)this->end()) T(std::forward<ArgTypes>(Args)...);
        this->set_size(this->size() + 1);
        return this->back();
    }

    SmallVectorImpl &operator=(const SmallVectorImpl &RHS);

    SmallVectorImpl &operator=(SmallVectorImpl &&RHS);

    bool operator==(const SmallVectorImpl &RHS) const {
        if (this->size() != RHS.size()) return false;
        return std::equal(this->begin(), this->end(), RHS.begin());
    }
    bool operator!=(const SmallVectorImpl &RHS) const {
        return !(*this == RHS);
    }

    bool operator<(const SmallVectorImpl &RHS) const {
        return std::lexicographical_compare(this->begin(), this->end(),
                                            RHS.begin(), RHS.end());
    }
    bool operator>(const SmallVectorImpl &RHS) const { return RHS < *this; }
    bool operator<=(const SmallVectorImpl &RHS) const { return !(*this > RHS); }
    bool operator>=(const SmallVectorImpl &RHS) const { return !(*this < RHS); }
};

template <typename T>
void SmallVectorImpl<T>::swap(SmallVectorImpl<T> &RHS) {
    if (this == &RHS) return;

    // We can only avoid copying elements if neither vector is small.
    if (!this->isSmall() && !RHS.isSmall()) {
        std::swap(this->BeginX, RHS.BeginX);
        std::swap(this->Size, RHS.Size);
        std::swap(this->Capacity, RHS.Capacity);
        return;
    }
    this->reserve(RHS.size());
    RHS.reserve(this->size());

    // Swap the shared elements.
    size_t NumShared = this->size();
    if (NumShared > RHS.size()) NumShared = RHS.size();
    for (size_type i = 0; i != NumShared; ++i) std::swap((*this)[i], RHS[i]);

    // Copy over the extra elts.
    if (this->size() > RHS.size()) {
        size_t EltDiff = this->size() - RHS.size();
        this->uninitialized_copy(this->begin() + NumShared, this->end(),
                                 RHS.end());
        RHS.set_size(RHS.size() + EltDiff);
        this->destroy_range(this->begin() + NumShared, this->end());
        this->set_size(NumShared);
    } else if (RHS.size() > this->size()) {
        size_t EltDiff = RHS.size() - this->size();
        this->uninitialized_copy(RHS.begin() + NumShared, RHS.end(),
                                 this->end());
        this->set_size(this->size() + EltDiff);
        this->destroy_range(RHS.begin() + NumShared, RHS.end());
        RHS.set_size(NumShared);
    }
}

template <typename T>
SmallVectorImpl<T> &SmallVectorImpl<T>::operator=(
        const SmallVectorImpl<T> &RHS) {
    // Avoid self-assignment.
    if (this == &RHS) return *this;

    // If we already have sufficient space, assign the common elements, then
    // destroy any excess.
    size_t RHSSize = RHS.size();
    size_t CurSize = this->size();
    if (CurSize >= RHSSize) {
        // Assign common elements.
        iterator NewEnd;
        if (RHSSize)
            NewEnd = std::copy(RHS.begin(), RHS.begin() + RHSSize,
                               this->begin());
        else
            NewEnd = this->begin();

        // Destroy excess elements.
        this->destroy_range(NewEnd, this->end());

        // Trim.
        this->set_size(RHSSize);
        return *this;
    }

    // If we have to grow to have enough elements, destroy the current elements.
    // This allows us to avoid copying them during the grow.
    // FIXME: don't do this if they're efficiently moveable.
    if (this->capacity() < RHSSize) {
        // Destroy current elements.
        this->clear();
        CurSize = 0;
        this->grow(RHSSize);
    } else if (CurSize) {
        // Otherwise, use assignment for the already-constructed elements.
        std::copy(RHS.begin(), RHS.begin() + CurSize, this->begin());
    }

    // Copy construct the new elements in place.
    this->uninitialized_copy(RHS.begin() + CurSize, RHS.end(),
                             this->begin() + CurSize);

    // Set end.
    this->set_size(RHSSize);
    return *this;
}

template <typename T>
SmallVectorImpl<T> &SmallVectorImpl<T>::operator=(SmallVectorImpl<T> &&RHS) {
    // Avoid self-assignment.
    if (this == &RHS) return *this;

    // If the RHS isn't small, clear this vector and then steal its buffer.
    if (!RHS.isSmall()) {
        this->assignRemote(std::move(RHS));
        return *this;
    }

    // If we already have sufficient space, assign the common elements, then
    // destroy any excess.
    size_t RHSSize = RHS.size();
    size_t CurSize = this->size();
    if (CurSize >= RHSSize) {
        // Assign common elements.
        iterator NewEnd = this->begin();
        if (RHSSize) NewEnd = std::move(RHS.begin(), RHS.end(), NewEnd);

        // Destroy excess elements and trim the bounds.
        this->destroy_range(NewEnd, this->end());
        this->set_size(RHSSize);

        // Clear the RHS.
        RHS.clear();

        return *this;
    }

    // If we have to grow to have enough elements, destroy the current elements.
    // This allows us to avoid copying them during the grow.
    // FIXME: this may not actually make any sense if we can efficiently move
    // elements.
    if (this->capacity() < RHSSize) {
        // Destroy current elements.
        this->clear();
        CurSize = 0;
        this->grow(RHSSize);
    } else if (CurSize) {
        // Otherwise, use assignment for the already-constructed elements.
        std::move(RHS.begin(), RHS.begin() + CurSize, this->begin());
    }

    // Move-construct the new elements in place.
    this->uninitialized_move(RHS.begin() + CurSize, RHS.end(),
                             this->begin() + CurSize);

    // Set end.
    this->set_size(RHSSize);

    RHS.clear();
    return *this;
}

/// Storage for the SmallVector elements.  This is specialized for the N=0 case
/// to avoid allocating unnecessary storage.
template <typename T, unsigned N>
struct SmallVectorStorage {
    alignas(T) char InlineElts[N * sizeof(T)];
};

/// We need the storage to be properly aligned even for small-size of 0 so that
/// the pointer math in \a SmallVectorTemplateCommon::getFirstEl() is
/// well-defined.
template <typename T>
struct alignas(T) SmallVectorStorage<T, 0> {};

/// Forward declaration of SmallVector so that
/// calculateSmallVectorDefaultInlinedElements can reference
/// `sizeof(SmallVector<T, 0>)`.
template <typename T, unsigned N>
class LLVM_GSL_OWNER SmallVector;

/// Helper class for calculating the default number of inline elements for
/// `SmallVector<T>`.
///
/// This should be migrated to a constexpr function when our minimum
/// compiler support is enough for multi-statement constexpr functions.
template <typename T>
struct CalculateSmallVectorDefaultInlinedElements {
    // Parameter controlling the default number of inlined elements
    // for `SmallVector<T>`.
    //
    // The default number of inlined elements ensures that
    // 1. There is at least one inlined element.
    // 2. `sizeof(SmallVector<T>) <= kPreferredSmallVectorSizeof` unless
    // it contradicts 1.
    static constexpr size_t kPreferredSmallVectorSizeof = 64;

    // static_assert that sizeof(T) is not "too big".
    //
    // Because our policy guarantees at least one inlined element, it is
    // possible for an arbitrarily large inlined element to allocate an
    // arbitrarily large amount of inline storage. We generally consider it an
    // antipattern for a SmallVector to allocate an excessive amount of inline
    // storage, so we want to call attention to these cases and make sure that
    // users are making an intentional decision if they request a lot of inline
    // storage.
    //
    // We want this assertion to trigger in pathological cases, but otherwise
    // not be too easy to hit. To accomplish that, the cutoff is actually
    // somewhat larger than kPreferredSmallVectorSizeof (otherwise,
    // `SmallVector<SmallVector<T>>` would be one easy way to trip it, and that
    // pattern seems useful in practice).
    //
    // One wrinkle is that this assertion is in theory non-portable, since
    // sizeof(T) is in general platform-dependent. However, we don't expect this
    // to be much of an issue, because most LLVM development happens on 64-bit
    // hosts, and therefore sizeof(T) is expected to *decrease* when compiled
    // for 32-bit hosts, dodging the issue. The reverse situation, where
    // development happens on a 32-bit host and then fails due to sizeof(T)
    // *increasing* on a 64-bit host, is expected to be very rare.
    static_assert(
            sizeof(T) <= 256,
            "You are trying to use a default number of inlined elements for "
            "`SmallVector<T>` but `sizeof(T)` is really big! Please use an "
            "explicit number of inlined elements with `SmallVector<T, N>` to "
            "make "
            "sure you really want that much inline storage.");

    // Discount the size of the header itself when calculating the maximum
    // inline bytes.
    static constexpr size_t PreferredInlineBytes =
            kPreferredSmallVectorSizeof - sizeof(SmallVector<T, 0>);
    static constexpr size_t NumElementsThatFit =
            PreferredInlineBytes / sizeof(T);
    static constexpr size_t value =
            NumElementsThatFit == 0 ? 1 : NumElementsThatFit;
};

/// This is a 'vector' (really, a variable-sized array), optimized
/// for the case when the array is small.  It contains some number of elements
/// in-place, which allows it to avoid heap allocation when the actual number of
/// elements is below that threshold.  This allows normal "small" cases to be
/// fast without losing generality for large inputs.
///
/// \note
/// In the absence of a well-motivated choice for the number of inlined
/// elements \p N, it is recommended to use \c SmallVector<T> (that is,
/// omitting the \p N). This will choose a default number of inlined elements
/// reasonable for allocation on the stack (for example, trying to keep \c
/// sizeof(SmallVector<T>) around 64 bytes).
///
/// \warning This does not attempt to be exception safe.
///
/// \see https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h
template <typename T,
          unsigned N = CalculateSmallVectorDefaultInlinedElements<T>::value>
class LLVM_GSL_OWNER SmallVector : public SmallVectorImpl<T>,
                                   SmallVectorStorage<T, N> {
public:
    SmallVector() : SmallVectorImpl<T>(N) {}

    ~SmallVector() {
        // Destroy the constructed elements in the vector.
        this->destroy_range(this->begin(), this->end());
    }

    explicit SmallVector(size_t Size, const T &Value = T())
        : SmallVectorImpl<T>(N) {
        this->assign(Size, Value);
    }

    template <typename ItTy,
              typename = std::enable_if_t<std::is_convertible<
                      typename std::iterator_traits<ItTy>::iterator_category,
                      std::input_iterator_tag>::value>>
    SmallVector(ItTy S, ItTy E) : SmallVectorImpl<T>(N) {
        this->append(S, E);
    }

    template <typename RangeTy>
    explicit SmallVector(const iterator_range<RangeTy> &R)
        : SmallVectorImpl<T>(N) {
        this->append(R.begin(), R.end());
    }

    SmallVector(std::initializer_list<T> IL) : SmallVectorImpl<T>(N) {
        this->assign(IL);
    }

    SmallVector(const SmallVector &RHS) : SmallVectorImpl<T>(N) {
        if (!RHS.empty()) SmallVectorImpl<T>::operator=(RHS);
    }

    SmallVector &operator=(const SmallVector &RHS) {
        SmallVectorImpl<T>::operator=(RHS);
        return *this;
    }

    SmallVector(SmallVector &&RHS) : SmallVectorImpl<T>(N) {
        if (!RHS.empty()) SmallVectorImpl<T>::operator=(::std::move(RHS));
    }

    SmallVector(SmallVectorImpl<T> &&RHS) : SmallVectorImpl<T>(N) {
        if (!RHS.empty()) SmallVectorImpl<T>::operator=(::std::move(RHS));
    }

    SmallVector &operator=(SmallVector &&RHS) {
        if (N) {
            SmallVectorImpl<T>::operator=(::std::move(RHS));
            return *this;
        }
        // SmallVectorImpl<T>::operator= does not leverage N==0. Optimize the
        // case.
        if (this == &RHS) return *this;
        if (RHS.empty()) {
            this->destroy_range(this->begin(), this->end());
            this->Size = 0;
        } else {
            this->assignRemote(std::move(RHS));
        }
        return *this;
    }

    SmallVector &operator=(SmallVectorImpl<T> &&RHS) {
        SmallVectorImpl<T>::operator=(::std::move(RHS));
        return *this;
    }

    SmallVector &operator=(std::initializer_list<T> IL) {
        this->assign(IL);
        return *this;
    }
};

template <typename T, unsigned N>
inline size_t capacity_in_bytes(const SmallVector<T, N> &X) {
    return X.capacity_in_bytes();
}

template <typename RangeType>
using ValueTypeFromRangeType =
        typename std::remove_const<typename std::remove_reference<decltype(
                *std::begin(std::declval<RangeType &>()))>::type>::type;

/// Given a range of type R, iterate the entire range and return a
/// SmallVector with elements of the vector.  This is useful, for example,
/// when you want to iterate a range and then sort the results.
template <unsigned Size, typename R>
SmallVector<ValueTypeFromRangeType<R>, Size> to_vector(R &&Range) {
    return {std::begin(Range), std::end(Range)};
}
template <typename R>
SmallVector<ValueTypeFromRangeType<R>,
            CalculateSmallVectorDefaultInlinedElements<
                    ValueTypeFromRangeType<R>>::value>
to_vector(R &&Range) {
    return {std::begin(Range), std::end(Range)};
}

}  // namespace core
}  // namespace open3d

namespace std {

/// Implement std::swap in terms of SmallVector swap.
template <typename T>
inline void swap(open3d::core::SmallVectorImpl<T> &LHS,
                 open3d::core::SmallVectorImpl<T> &RHS) {
    LHS.swap(RHS);
}

/// Implement std::swap in terms of SmallVector swap.
template <typename T, unsigned N>
inline void swap(open3d::core::SmallVector<T, N> &LHS,
                 open3d::core::SmallVector<T, N> &RHS) {
    LHS.swap(RHS);
}

}  // end namespace std