package detector

import (
	"image"
	"image/color"
	stddraw "image/draw"

	xdraw "golang.org/x/image/draw"
)

const ImgSize = 1200

// Preprocess implements the Python reference exactly:
// 1. resize keeping aspect ratio to fit in 1200x1200 (Lanczos-equivalent CatmullRom)
// 2. paste onto white 1200x1200 canvas at (0,0)
// 3. normalize: (x/255 - 0.5) / 0.5
// 4. HWC -> CHW with batch dim -> [1, 3, 1200, 1200]
func Preprocess(img image.Image) []float32 {
	bounds := img.Bounds()
	w, h := bounds.Dx(), bounds.Dy()

	scale := float64(ImgSize) / float64(w)
	if s := float64(ImgSize) / float64(h); s < scale {
		scale = s
	}
	newW := int(float64(w) * scale)
	newH := int(float64(h) * scale)
	if newW < 1 {
		newW = 1
	}
	if newH < 1 {
		newH = 1
	}

	canvas := image.NewRGBA(image.Rect(0, 0, ImgSize, ImgSize))
	stddraw.Draw(canvas, canvas.Bounds(), &image.Uniform{C: color.RGBA{255, 255, 255, 255}}, image.Point{}, stddraw.Src)
	xdraw.CatmullRom.Scale(canvas, image.Rect(0, 0, newW, newH), img, bounds, xdraw.Over, nil)

	out := make([]float32, 3*ImgSize*ImgSize)
	plane := ImgSize * ImgSize
	pix := canvas.Pix
	for y := 0; y < ImgSize; y++ {
		row := y * canvas.Stride
		base := y * ImgSize
		for x := 0; x < ImgSize; x++ {
			i := row + x*4
			out[base+x] = float32(pix[i+0])/127.5 - 1
			out[plane+base+x] = float32(pix[i+1])/127.5 - 1
			out[2*plane+base+x] = float32(pix[i+2])/127.5 - 1
		}
	}
	return out
}