use std::sync::Arc;

use ash::vk;
use gpu_allocator::vulkan::{Allocation, AllocationCreateDesc, AllocationScheme};
use gpu_allocator::MemoryLocation;
use parking_lot::Mutex;

use crate::*;

static DOWNLOAD_NOT_ALLOWED_ERROR: &str = "Download from the GPU buffer is not allowed";
static UPLOAD_NOT_ALLOWED_ERROR: &str = "Upload to the GPU buffer is not allowed";

/// Buffer is a GPU resource that represents a linear memory region.
pub struct Buffer {
    /// Device that owns the buffer.
    device: Arc<Device>,

    /// Vulkan buffer handle.
    vk_buffer: vk::Buffer,

    /// Buffer type. It defines how the buffer can be used.
    buffer_type: BufferType,

    /// Buffer size in bytes.
    size: usize,

    /// GPU memory allocation that backs the buffer.
    allocation: Mutex<Allocation>,
}

/// Buffer type defines how the buffer can be used.
#[derive(PartialEq, Eq, Clone, Copy)]
pub enum BufferType {
    /// Uniform data for a shader.
    Uniform,

    /// Storage buffer can be used as a large read/write buffer.
    Storage,

    /// CpuToGpu buffer can be used as a source for transfer operations only.
    CpuToGpu,

    /// GpuToCpu buffer can be used as a destination for transfer operations only.
    GpuToCpu,
}

// Mark `Buffer` as a GPU resource that should be kept alive while it's in use by the GPU context.
impl Resource for Buffer {}

impl Buffer {
    pub fn new(
        device: Arc<Device>,   // Device that owns the buffer.
        name: impl AsRef<str>, // Name of the buffer for tracking and debugging purposes.
        buffer_type: BufferType,
        size: usize,
    ) -> GpuResult<Arc<Self>> {
        if size == 0 {
            return Err(GpuError::NotSupported(
                "Zero-sized GPU buffers are not supported".to_string(),
            ));
        }

        // Vulkan API requires buffer usage flags to be specified during the buffer creation.
        let vk_usage_flags = match buffer_type {
            BufferType::Uniform => {
                vk::BufferUsageFlags::UNIFORM_BUFFER // mark as uniform buffer.
                    | vk::BufferUsageFlags::TRANSFER_DST // For uploading.
                    | vk::BufferUsageFlags::TRANSFER_SRC // For downloading.
            }
            BufferType::Storage => {
                vk::BufferUsageFlags::STORAGE_BUFFER // mark as storage buffer.
                    | vk::BufferUsageFlags::TRANSFER_DST // For uploading.
                    | vk::BufferUsageFlags::TRANSFER_SRC // For downloading.
            }
            // CpuToGpu buffer can be used as a source for transfer operations only.
            BufferType::CpuToGpu => vk::BufferUsageFlags::TRANSFER_SRC,
            // GpuToCpu buffer can be used as a destination for transfer operations only.
            BufferType::GpuToCpu => vk::BufferUsageFlags::TRANSFER_DST,
        };

        // Memory location depends on the buffer type.
        let location = match buffer_type {
            // Allocate Uniform/Storage buffers in GPU memory only.
            BufferType::Uniform => MemoryLocation::GpuOnly,
            BufferType::Storage => MemoryLocation::GpuOnly,
            // Transfer buffers will be visible to both CPU and GPU.
            BufferType::CpuToGpu => MemoryLocation::CpuToGpu,
            BufferType::GpuToCpu => MemoryLocation::GpuToCpu,
        };

        // Create a Vulkan buffer.
        let vk_create_buffer_info = vk::BufferCreateInfo::default()
            .size(size as vk::DeviceSize)
            .usage(vk_usage_flags)
            .sharing_mode(vk::SharingMode::EXCLUSIVE);
        let vk_buffer = unsafe {
            device
                .vk_device()
                .create_buffer(&vk_create_buffer_info, device.cpu_allocation_callbacks())
                .unwrap()
        };

        // Allocate memory for the buffer.
        let buffer_allocation_requirements =
            unsafe { device.vk_device().get_buffer_memory_requirements(vk_buffer) };
        let allocation_result = device.allocate(&AllocationCreateDesc {
            name: name.as_ref(),
            requirements: buffer_allocation_requirements,
            location,
            linear: true, // Buffers are always linear.
            allocation_scheme: AllocationScheme::GpuAllocatorManaged,
        });

        // Check if the allocation was successful.
        let allocation = match allocation_result {
            Ok(allocation) => allocation,
            Err(e) => {
                unsafe {
                    // Because vulkan buffers lifetime is managed manually,
                    // we need to destroy the buffer in case of an allocation error.
                    device
                        .vk_device()
                        .destroy_buffer(vk_buffer, device.cpu_allocation_callbacks());
                }
                return Err(e);
            }
        };

        // Bind the buffer to the allocated memory.
        let bind_result = unsafe {
            device.vk_device().bind_buffer_memory(
                vk_buffer,
                allocation.memory(),
                allocation.offset(),
            )
        };
        if let Err(e) = bind_result {
            // Free the allocated memory in case of an error.
            device.free(allocation);
            unsafe {
                // Destroy the buffer.
                device
                    .vk_device()
                    .destroy_buffer(vk_buffer, device.cpu_allocation_callbacks());
            }
            return Err(GpuError::from(e));
        }

        Ok(Arc::new(Self {
            device,
            vk_buffer,
            buffer_type,
            size,
            allocation: Mutex::new(allocation),
        }))
    }

    pub fn size(&self) -> usize {
        self.size
    }

    pub fn vk_buffer(&self) -> vk::Buffer {
        self.vk_buffer
    }

    pub fn buffer_type(&self) -> BufferType {
        self.buffer_type
    }

    /// Download data from the buffer to the RAM.
    pub fn download<T: Sized>(&self, data: &mut T, offset: usize) -> GpuResult<()> {
        if self.buffer_type != BufferType::GpuToCpu {
            return Err(GpuError::Other(DOWNLOAD_NOT_ALLOWED_ERROR.to_string()));
        }

        let bytes = memory::mmap_ops::transmute_to_u8_mut(data);
        self.download_bytes(bytes, offset)
    }

    /// Download data from the buffer to the RAM.
    pub fn download_slice<T: Sized>(&self, data: &mut [T], offset: usize) -> GpuResult<()> {
        if self.buffer_type != BufferType::GpuToCpu {
            return Err(GpuError::Other(DOWNLOAD_NOT_ALLOWED_ERROR.to_string()));
        }

        let bytes = memory::mmap_ops::transmute_to_u8_slice_mut(data);
        self.download_bytes(bytes, offset)
    }

    /// Download data from the buffer to the RAM.
    pub fn download_bytes(&self, data: &mut [u8], offset: usize) -> GpuResult<()> {
        if self.buffer_type != BufferType::GpuToCpu {
            return Err(GpuError::Other(DOWNLOAD_NOT_ALLOWED_ERROR.to_string()));
        }

        if data.len() + offset > self.size {
            return Err(GpuError::OutOfBounds(
                "Out of bounds while downloading from GPU".to_string(),
            ));
        }

        let allocation = self.allocation.lock();
        if let Some(slice) = allocation.mapped_slice() {
            data.copy_from_slice(&slice[offset..offset + data.len()]);
            Ok(())
        } else {
            Err(GpuError::Other(DOWNLOAD_NOT_ALLOWED_ERROR.to_string()))
        }
    }

    /// Upload data from the RAM to the buffer.
    pub fn upload<T: Sized>(&self, data: &T, offset: usize) -> GpuResult<()> {
        if self.buffer_type != BufferType::CpuToGpu {
            return Err(GpuError::Other(UPLOAD_NOT_ALLOWED_ERROR.to_string()));
        }

        let bytes = memory::mmap_ops::transmute_to_u8(data);
        self.upload_bytes(bytes, offset)
    }

    /// Upload data from the RAM to the buffer.
    pub fn upload_slice<T: Sized>(&self, data: &[T], offset: usize) -> GpuResult<()> {
        if self.buffer_type != BufferType::CpuToGpu {
            return Err(GpuError::Other(UPLOAD_NOT_ALLOWED_ERROR.to_string()));
        }

        let bytes = memory::mmap_ops::transmute_to_u8_slice(data);
        self.upload_bytes(bytes, offset)
    }

    /// Upload data from the RAM to the buffer.
    pub fn upload_bytes(&self, data: &[u8], offset: usize) -> GpuResult<()> {
        if self.buffer_type != BufferType::CpuToGpu {
            return Err(GpuError::Other(UPLOAD_NOT_ALLOWED_ERROR.to_string()));
        }

        if data.len() + offset > self.size {
            return Err(GpuError::OutOfBounds(
                "Out of bounds while uploading to GPU".to_string(),
            ));
        }

        let mut allocation = self.allocation.lock();
        if let Some(slice) = allocation.mapped_slice_mut() {
            slice[offset..offset + data.len()].copy_from_slice(data);
            Ok(())
        } else {
            Err(GpuError::Other(DOWNLOAD_NOT_ALLOWED_ERROR.to_string()))
        }
    }
}

impl Drop for Buffer {
    fn drop(&mut self) {
        if self.vk_buffer != vk::Buffer::null() {
            // Drop the allocation and free the allocated memory.
            let mut allocation = Mutex::new(Allocation::default());
            std::mem::swap(&mut allocation, &mut self.allocation);
            let allocation = allocation.into_inner();
            self.device.free(allocation);

            // Destroy the buffer.
            unsafe {
                self.device
                    .vk_device()
                    .destroy_buffer(self.vk_buffer, self.device.cpu_allocation_callbacks())
            };
            self.vk_buffer = vk::Buffer::null();
        }
    }
}