cesium-native/CesiumGltf/include/CesiumGltf/AccessorView.h

#pragma once

#include <CesiumGltf/Accessor.h>
#include <CesiumGltf/Model.h>

#include <cstddef>
#include <stdexcept>

namespace CesiumGltf {

/**
 * @brief Indicates the status of an accessor view.
 *
 * The {@link AccessorView} constructor always completes successfully. However,
 * it may not always reflect the actual content of the {@link Accessor}, but
 * instead indicate that its {@link AccessorView::size} is 0. This enumeration
 * provides the reason.
 */
enum class AccessorViewStatus {
  /**
   * @brief This accessor is valid and ready to use.
   */
  Valid,

  /**
   * @brief The accessor index does not refer to a valid accessor.
   */
  InvalidAccessorIndex,

  /**
   * @brief The accessor's bufferView index does not refer to a valid
   * bufferView.
   */
  InvalidBufferViewIndex,

  /**
   * @brief The accessor's bufferView's buffer index does not refer to a valid
   * buffer.
   */
  InvalidBufferIndex,

  /**
   * @brief The accessor is too large to fit in its bufferView.
   */
  BufferViewTooSmall,

  /**
   * @brief The accessor's bufferView is too large to fit in its buffer.
   */
  BufferTooSmall,

  /**
   * @brief The `sizeof(T)` does not match the accessor's
   * {@link Accessor::computeBytesPerVertex}.
   */
  WrongSizeT,

  /**
   * @brief The `AccessorSpec:type` is invalid.
   */
  InvalidType,

  /**
   * @brief The {@link AccessorSpec::componentType} is invalid.
   */
  InvalidComponentType,

  /**
   * @brief The {@link BufferView::byteStride} is negative, which is invalid.
   *
   */
  InvalidByteStride,
};

/**
 * @brief A view on the data of one accessor of a glTF asset.
 *
 * It provides the actual accessor data like an array of elements.
 * The type of the accessor elements is determined by the template
 * parameter. Instances are usually from an {@link Accessor},
 * and the {@link operator[]()} can be used to access the elements:
 *
 * @snippet TestAccessorView.cpp createFromAccessorAndRead
 *
 * @tparam T The type of the elements in the accessor.
 */
template <class T> class AccessorView final {
private:
  const std::byte* _pData;
  int64_t _stride;
  int64_t _offset;
  int64_t _size;
  AccessorViewStatus _status;

public:
  /**
   * @brief The type of the elements in the accessor.
   */
  typedef T value_type;

  /**
   * @brief Construct a new instance not pointing to any data.
   *
   * The new instance will have a {@link size} of 0 and a {@link status} of
   * `AccessorViewStatus::InvalidAccessorIndex`.
   *
   * @param status The status of the new accessor. Defaults to
   * {@link AccessorViewStatus::InvalidAccessorIndex}.
   */
  AccessorView(
      AccessorViewStatus status = AccessorViewStatus::InvalidAccessorIndex)
      : _pData(nullptr), _stride(0), _offset(0), _size(0), _status(status) {}

  /**
   * @brief Creates a new instance from low-level parameters.
   *
   * The provided parameters are not validated in any way, and so this overload
   * can easily be used to access invalid memory.
   *
   * @param pData The raw data buffer from which to read.
   * @param stride The stride, in bytes, between successive elements.
   * @param offset The offset from the start of the buffer to the first element.
   * @param size The total number of elements.
   */
  AccessorView(
      const std::byte* pData,
      int64_t stride,
      int64_t offset,
      int64_t size)
      : _pData(pData),
        _stride(stride),
        _offset(offset),
        _size(size),
        _status(AccessorViewStatus::Valid) {}

  /**
   * @brief Creates a new instance from a given model and {@link Accessor}.
   *
   * If the accessor cannot be viewed, the construct will still complete
   * successfully without throwing an exception. However, {@link size} will
   * return 0 and
   * {@link status} will indicate what went wrong.
   *
   * @param model The model to access.
   * @param accessor The accessor to view.
   */
  AccessorView(const Model& model, const Accessor& accessor) noexcept
      : AccessorView() {
    this->create(model, accessor);
  }

  /**
   * @brief Creates a new instance from a given model and accessor index.
   *
   * If the accessor cannot be viewed, the construct will still complete
   * successfully without throwing an exception. However, {@link size} will
   * return 0 and
   * {@link status} will indicate what went wrong.
   *
   * @param model The model to access.
   * @param accessorIndex The index of the accessor to view in the model's
   * {@link Model::accessors} list.
   */
  AccessorView(const Model& model, int32_t accessorIndex) noexcept
      : AccessorView() {
    const Accessor* pAccessor = Model::getSafe(&model.accessors, accessorIndex);
    if (!pAccessor) {
      this->_status = AccessorViewStatus::InvalidAccessorIndex;
      return;
    }

    this->create(model, *pAccessor);
  }

  /**
   * @brief Provides the specified accessor element.
   *
   * @param i The index of the element.
   * @returns The constant reference to the accessor element.
   * @throws A `std::range_error` if the given index is negative
   * or not smaller than the {@link size} of this accessor.
   */
  const T& operator[](int64_t i) const {
    if (i < 0 || i >= this->_size) {
      throw std::range_error("index out of range");
    }

    return *reinterpret_cast<const T*>(
        this->_pData + i * this->_stride + this->_offset);
  }

  /**
   * @brief Returns the size (number of elements) of this accessor.
   *
   * This is the number of elements of type `T` that this accessor contains.
   *
   * @returns The size.
   */
  int64_t size() const noexcept { return this->_size; }

  /**
   * @brief Gets the status of this accessor view.
   *
   * Indicates whether the view accurately reflects the accessor's data, or
   * whether an error occurred.
   */
  AccessorViewStatus status() const noexcept { return this->_status; }

  /**
   * @brief Returns the stride of this accessor, which is the number of bytes
   * from the start of one element to the start of the next.
   *
   * @returns The stride.
   */
  int64_t stride() const noexcept { return this->_stride; }

  /**
   * @brief Returns the offset of this accessor, which is the number of bytes
   * from the start of the buffer to the first element.
   *
   * @returns The offset.
   */
  int64_t offset() const noexcept { return this->_offset; }

  /**
   * @brief Returns a pointer to the first byte of this accessor view's data.
   * The elements are stored contiguously, so the next one starts {@link stride} bytes later.
   *
   * @returns The start of this view.
   */
  const std::byte* data() const noexcept {
    return this->_pData + this->_offset;
  }

private:
  void create(const Model& model, const Accessor& accessor) noexcept {
    const CesiumGltf::BufferView* pBufferView =
        Model::getSafe(&model.bufferViews, accessor.bufferView);
    if (!pBufferView) {
      this->_status = AccessorViewStatus::InvalidBufferViewIndex;
      return;
    }

    const CesiumGltf::Buffer* pBuffer =
        Model::getSafe(&model.buffers, pBufferView->buffer);
    if (!pBuffer) {
      this->_status = AccessorViewStatus::InvalidBufferIndex;
      return;
    }

    const std::vector<std::byte>& data = pBuffer->cesium.data;
    const int64_t bufferBytes = int64_t(data.size());
    if (pBufferView->byteOffset + pBufferView->byteLength > bufferBytes) {
      this->_status = AccessorViewStatus::BufferTooSmall;
      return;
    }

    const int64_t accessorByteStride = accessor.computeByteStride(model);
    if (accessorByteStride < 0) {
      this->_status = AccessorViewStatus::InvalidByteStride;
      return;
    }

    const int64_t accessorComponentElements =
        accessor.computeNumberOfComponents();
    const int64_t accessorComponentBytes =
        accessor.computeByteSizeOfComponent();
    const int64_t accessorBytesPerStride =
        accessorComponentElements * accessorComponentBytes;

    if (sizeof(T) != accessorBytesPerStride) {
      this->_status = AccessorViewStatus::WrongSizeT;
      return;
    }

    const int64_t accessorBytes = accessorByteStride * accessor.count;
    const int64_t bytesRemainingInBufferView =
        pBufferView->byteLength -
        (accessor.byteOffset + accessorByteStride * (accessor.count - 1) +
         accessorBytesPerStride);
    if (accessorBytes > pBufferView->byteLength ||
        bytesRemainingInBufferView < 0) {
      this->_status = AccessorViewStatus::BufferViewTooSmall;
      return;
    }

    this->_pData = pBuffer->cesium.data.data();
    this->_stride = accessorByteStride;
    this->_offset = accessor.byteOffset + pBufferView->byteOffset;
    this->_size = accessor.count;
    this->_status = AccessorViewStatus::Valid;
  }
};

/**
 * @brief Contains types that may optionally be used with {@link AccessorView}
 * for various {@link Accessor::componentType} values.
 */
struct AccessorTypes {
#pragma pack(push, 1)

  /**
   * @brief A scalar element for an {@link AccessorView}.
   *
   * @tparam T The component type.
   */
  template <typename T> struct SCALAR {
    /**
     * @brief The component values of this element.
     */
    T value[1];
  };

  /**
   * @brief A 2D vector element for an {@link AccessorView}.
   *
   * @tparam T The component type.
   */
  template <typename T> struct VEC2 {
    /**
     * @brief The component values of this element.
     */
    T value[2];
  };

  /**
   * @brief A 3D vector element for an {@link AccessorView}.
   *
   * @tparam T The component type.
   */
  template <typename T> struct VEC3 {
    /**
     * @brief The component values of this element.
     */
    T value[3];
  };

  /**
   * @brief A 4D vector element for an {@link AccessorView}.
   *
   * @tparam T The component type.
   */
  template <typename T> struct VEC4 {
    /**
     * @brief The component values of this element.
     */
    T value[4];
  };

  /**
   * @brief A 2x2 matrix element for an {@link AccessorView}.
   *
   * @tparam T The component type.
   */
  template <typename T> struct MAT2 {
    /**
     * @brief The component values of this element.
     */
    T value[4];
  };

  /**
   * @brief A 3x3 matrix element for an {@link AccessorView}.
   *
   * @tparam T The component type.
   */
  template <typename T> struct MAT3 {
    /**
     * @brief The component values of this element.
     */
    T value[9];
  };

  /**
   * @brief A 4x4 matrix element for an {@link AccessorView}.
   *
   * @tparam T The component type.
   */
  template <typename T> struct MAT4 {
    /**
     * @brief The component values of this element.
     */
    T value[16];
  };

#pragma pack(pop)
};

namespace CesiumImpl {
template <typename TCallback, typename TElement>
std::invoke_result_t<TCallback, AccessorView<AccessorTypes::SCALAR<TElement>>>
createAccessorView(
    const Model& model,
    const Accessor& accessor,
    TCallback&& callback) {
  if (accessor.type == Accessor::Type::SCALAR) {
    return callback(
        AccessorView<AccessorTypes::SCALAR<TElement>>(model, accessor));
  }
  if (accessor.type == Accessor::Type::VEC2) {
    return callback(
        AccessorView<AccessorTypes::VEC2<TElement>>(model, accessor));
  }
  if (accessor.type == Accessor::Type::VEC3) {
    return callback(
        AccessorView<AccessorTypes::VEC3<TElement>>(model, accessor));
  }
  if (accessor.type == Accessor::Type::VEC4) {
    return callback(
        AccessorView<AccessorTypes::VEC4<TElement>>(model, accessor));
  }
  if (accessor.type == Accessor::Type::MAT2) {
    return callback(
        AccessorView<AccessorTypes::MAT2<TElement>>(model, accessor));
  }
  if (accessor.type == Accessor::Type::MAT3) {
    return callback(
        AccessorView<AccessorTypes::MAT3<TElement>>(model, accessor));
  }
  if (accessor.type == Accessor::Type::MAT4) {
    return callback(
        AccessorView<AccessorTypes::MAT4<TElement>>(model, accessor));
  }
  // TODO Print a warning here???
  return callback(AccessorView<AccessorTypes::SCALAR<TElement>>(
      AccessorViewStatus::InvalidType));
}
} // namespace CesiumImpl

/**
 * @brief Creates an appropriate {@link AccessorView} for a given accessor.
 *
 * The created accessor is provided via a callback, which is a function that can
 * be invoked with all possible {@link AccessorView} types. If an accessor
 * cannot be created, the callback will be invoked with
 * `AccessorView<AccessorTypes::SCALAR<float>>` and the
 * {@link AccessorView::status} will indicate the reason.
 *
 * @tparam TCallback The callback.
 * @param model The model to access.
 * @param accessor The accessor to view.
 * @param callback The callback that receives the created accessor.
 * @return The value returned by the callback.
 */
template <typename TCallback>
std::invoke_result_t<TCallback, AccessorView<AccessorTypes::SCALAR<float>>>
createAccessorView(
    const Model& model,
    const Accessor& accessor,
    TCallback&& callback) {
  switch (accessor.componentType) {
  case Accessor::ComponentType::BYTE:
    return ::CesiumGltf::CesiumImpl::createAccessorView<TCallback, int8_t>(
        model,
        accessor,
        std::forward<TCallback>(callback));
  case Accessor::ComponentType::UNSIGNED_BYTE:
    return ::CesiumGltf::CesiumImpl::createAccessorView<TCallback, uint8_t>(
        model,
        accessor,
        std::forward<TCallback>(callback));
  case Accessor::ComponentType::SHORT:
    return ::CesiumGltf::CesiumImpl::createAccessorView<TCallback, int16_t>(
        model,
        accessor,
        std::forward<TCallback>(callback));
  case Accessor::ComponentType::UNSIGNED_SHORT:
    return ::CesiumGltf::CesiumImpl::createAccessorView<TCallback, uint16_t>(
        model,
        accessor,
        std::forward<TCallback>(callback));
  case Accessor::ComponentType::INT:
    return ::CesiumGltf::CesiumImpl::createAccessorView<TCallback, int32_t>(
        model,
        accessor,
        std::forward<TCallback>(callback));
  case Accessor::ComponentType::UNSIGNED_INT:
    return ::CesiumGltf::CesiumImpl::createAccessorView<TCallback, uint32_t>(
        model,
        accessor,
        std::forward<TCallback>(callback));
  case Accessor::ComponentType::INT64:
    return ::CesiumGltf::CesiumImpl::createAccessorView<TCallback, int64_t>(
        model,
        accessor,
        std::forward<TCallback>(callback));
  case Accessor::ComponentType::UNSIGNED_INT64:
    return ::CesiumGltf::CesiumImpl::createAccessorView<TCallback, uint64_t>(
        model,
        accessor,
        std::forward<TCallback>(callback));
  case Accessor::ComponentType::FLOAT:
    return ::CesiumGltf::CesiumImpl::createAccessorView<TCallback, float>(
        model,
        accessor,
        std::forward<TCallback>(callback));
  case Accessor::ComponentType::DOUBLE:
    return ::CesiumGltf::CesiumImpl::createAccessorView<TCallback, double>(
        model,
        accessor,
        std::forward<TCallback>(callback));
  default:
    return callback(AccessorView<AccessorTypes::SCALAR<float>>(
        AccessorViewStatus::InvalidComponentType));
  }
}

/**
 * @brief Creates an appropriate {@link AccessorView} for a given accessor.
 *
 * The created accessor is provided via a callback, which is a function that can
 * be invoked with all possible {@link AccessorView} types. If an accessor
 * cannot be created, the callback will be invoked with
 * `AccessorView<AccessorTypes::SCALAR<float>>` and the
 * {@link AccessorView::status} will indicate the reason.
 *
 * @tparam TCallback The callback.
 * @param model The model to access.
 * @param accessorIndex The index of the accessor to view in
 * {@link Model::accessors}.
 * @param callback The callback that receives the created accessor.
 * @return The value returned by the callback.
 */
template <typename TCallback>
std::invoke_result_t<TCallback, AccessorView<AccessorTypes::SCALAR<float>>>
createAccessorView(
    const Model& model,
    int32_t accessorIndex,
    TCallback&& callback) {
  const Accessor* pAccessor = Model::getSafe(&model.accessors, accessorIndex);
  if (!pAccessor) {
    return callback(AccessorView<AccessorTypes::SCALAR<float>>(
        AccessorViewStatus::InvalidComponentType));
  }

  return createAccessorView(model, *pAccessor, callback);
}
} // namespace CesiumGltf