package spike

import "fmt"

// LetterboxBGRToNCHW converts one packed BGR frame into the RGB, CHW,
// float32 tensor expected by the locked 640-pixel YOLO pose ONNX model.
//
// It intentionally uses nearest-neighbour scaling for this Spike.  T-303
// must replace or validate this preprocessing against the V1 implementation
// before it becomes the V2 production preprocessing path.
func LetterboxBGRToNCHW(frame []byte, width, height, target int) ([]float32, error) {
	if width <= 0 || height <= 0 || target <= 0 {
		return nil, fmt.Errorf("frame width, height and target must be positive")
	}
	if len(frame) != width*height*3 {
		return nil, fmt.Errorf("BGR frame length = %d, want %d", len(frame), width*height*3)
	}

	scaleWidth := target
	scaleHeight := height * target / width
	if scaleHeight > target {
		scaleHeight = target
		scaleWidth = width * target / height
	}
	padX := (target - scaleWidth) / 2
	padY := (target - scaleHeight) / 2
	plane := target * target
	input := make([]float32, 3*plane)

	for y := 0; y < scaleHeight; y++ {
		sourceY := y * height / scaleHeight
		for x := 0; x < scaleWidth; x++ {
			sourceX := x * width / scaleWidth
			source := (sourceY*width + sourceX) * 3
			destination := (padY+y)*target + padX + x
			input[destination] = float32(frame[source+2]) / 255.0
			input[plane+destination] = float32(frame[source+1]) / 255.0
			input[2*plane+destination] = float32(frame[source]) / 255.0
		}
	}

	return input, nil
}