package compute

import (
	"errors"
	"fmt"
	"unsafe"

	"makarna/pkg/backend/cuda"
	"makarna/pkg/tensor"
)

// DefaultScratchBytes defines the fallback scratch buffer size (256MB).
const DefaultScratchBytes = 256 << 20

// ScratchSpace provides a preallocated GPU buffer for temporary tensors.
// It avoids repeated cudaMalloc calls during the forward pass by carving
// out views from a single large allocation.
type ScratchSpace struct {
	buf      *cuda.Tensor
	capacity uintptr
	offset   uintptr
}

func alignUp(offset, align uintptr) uintptr {
	if align == 0 {
		return offset
	}
	rem := offset % align
	if rem == 0 {
		return offset
	}
	return offset + (align - rem)
}

// NewScratchSpace allocates a scratch buffer on the given GPU.
// If bytes is <= 0, DefaultScratchBytes is used.
func NewScratchSpace(gpu int, bytes int) (*ScratchSpace, error) {
	if gpu < 0 {
		gpu = 0
	}
	if bytes <= 0 {
		bytes = DefaultScratchBytes
	}

	elemCount := bytes / tensor.Float32.Size()
	if elemCount <= 0 {
		return nil, fmt.Errorf("scratch size too small: %d bytes", bytes)
	}

	buf, err := cuda.NewTensor(tensor.Shape{elemCount}, tensor.Float32, gpu)
	if err != nil {
		return nil, fmt.Errorf("alloc scratch buffer: %w", err)
	}

	return &ScratchSpace{
		buf:      buf,
		capacity: uintptr(elemCount * tensor.Float32.Size()),
		offset:   0,
	}, nil
}

// GPU returns the GPU id backing this scratch space.
func (s *ScratchSpace) GPU() int {
	if s == nil || s.buf == nil {
		return -1
	}
	return s.buf.GPU()
}

// Reset rewinds the allocation offset to reuse the buffer.
func (s *ScratchSpace) Reset() {
	if s != nil {
		s.offset = 0
	}
}

func (s *ScratchSpace) Free() {
	if s != nil && s.buf != nil {
		s.buf.Free()
		s.buf = nil
		s.capacity = 0
		s.offset = 0
	}
}

// GetTensor returns an activation backed by a view into the scratch buffer.
// Only float32 is supported because the buffer is allocated in float32.
func (s *ScratchSpace) GetTensor(shape tensor.Shape, dtype tensor.DType) (*Activation, error) {
	if s == nil {
		return nil, errors.New("scratch space is nil")
	}
	if dtype != tensor.Float32 {
		return nil, fmt.Errorf("scratch space only supports float32, got %v", dtype)
	}
	required := uintptr(shape.NumElements() * dtype.Size())
	if required == 0 {
		return nil, errors.New("requested zero-sized tensor from scratch space")
	}

	// Keep allocations reasonably aligned for vectorized loads/stores.
	aligned := alignUp(s.offset, 256)
	if aligned+required > s.capacity {
		return nil, fmt.Errorf("scratch space exhausted: need %d bytes (offset %d / cap %d)", required, aligned, s.capacity)
	}

	view, err := s.buf.ViewAt(shape, aligned)
	if err != nil {
		return nil, err
	}
	s.offset = aligned + required

	return NewActivationFrom(view), nil
}

// UnsafePointer exposes the raw pointer to the scratch buffer start.
// Primarily for debugging or advanced use; avoid in regular code.
func (s *ScratchSpace) UnsafePointer() unsafe.Pointer {
	if s == nil || s.buf == nil {
		return nil
	}
	if p, ok := s.buf.Data().(unsafe.Pointer); ok {
		return p
	}
	return nil
}

// GetInt32Slice allocates a slice of int32s from the scratch buffer.
// Returns the raw device pointer.
func (s *ScratchSpace) GetInt32Slice(count int) (unsafe.Pointer, error) {
	if s == nil {
		return nil, errors.New("scratch space is nil")
	}
	size := uintptr(count * 4) // 4 bytes per int32
	aligned := alignUp(s.offset, 16)
	if aligned+size > s.capacity {
		return nil, fmt.Errorf("scratch space exhausted: need %d bytes", size)
	}

	ptr := unsafe.Pointer(uintptr(s.UnsafePointer()) + aligned)
	s.offset = aligned + size
	return ptr, nil
}

// GetUintptrSlice allocates a slice of uintptrs from the scratch buffer.
// Returns the raw device pointer.
// Ensures the returned pointer is aligned to uintptr size.
func (s *ScratchSpace) GetUintptrSlice(count int) (unsafe.Pointer, error) {
	if s == nil {
		return nil, errors.New("scratch space is nil")
	}
	if count <= 0 {
		return nil, errors.New("count must be > 0")
	}

	align := uintptr(unsafe.Sizeof(uintptr(0)))
	aligned := alignUp(s.offset, align)
	size := uintptr(count) * align
	if aligned+size > s.capacity {
		return nil, fmt.Errorf("scratch space exhausted: need %d bytes", size)
	}

	ptr := unsafe.Pointer(uintptr(s.UnsafePointer()) + aligned)
	s.offset = aligned + size
	return ptr, nil
}