// Logo plane construction for texture-sampling kernels (v210_dvd_logo.wgsl).
package generator

import (
	"fmt"
	"image"
	"math"
	"os"

	// PNG decoder registration for LoadLogoPNG.
	_ "image/png"
)

// BuildLogoPlane converts an image into the packed logo plane consumed by
// v210_dvd_logo.wgsl as read-only storage binding 2:
//
//	logo[0] = width | height<<16
//	logo[1+dy*w+dx] = opaque<<30 | Cr<<20 | Cb<<10 | Y
//
// Rec.709 full-range RGB -> 10-bit studio swing, the same transform behind
// the color-bar tables in the kernels (verified against all 21 bar values).
// Semi-transparent pixels (alpha >= 50%) count as opaque; the rest show the
// kernel background through.
func BuildLogoPlane(img image.Image) ([]uint32, error) {
	b := img.Bounds()
	w, h := b.Dx(), b.Dy()
	if w <= 0 || h <= 0 || w > 0xFFFF || h > 0xFFFF {
		return nil, fmt.Errorf("logo: bad dimensions %dx%d", w, h)
	}
	plane := make([]uint32, 1+w*h)
	plane[0] = uint32(w) | uint32(h)<<16
	for yy := 0; yy < h; yy++ {
		for xx := 0; xx < w; xx++ {
			r, g, bl, a := img.At(b.Min.X+xx, b.Min.Y+yy).RGBA()
			if a < 1<<15 {
				continue // transparent: shader falls back to background
			}
			// At().RGBA() is premultiplied; unpremultiply to full-range RGB.
			rf := float64(r) / float64(a)
			gf := float64(g) / float64(a)
			bf := float64(bl) / float64(a)
			luma := 0.2126*rf + 0.7152*gf + 0.0722*bf
			yc := clamp10(64 + math.Round(876*luma))
			cb := clamp10(512 + math.Round(448*(bf-luma)/(1-0.0722)))
			cr := clamp10(512 + math.Round(448*(rf-luma)/(1-0.2126)))
			plane[1+yy*w+xx] = 1<<30 | cr<<20 | cb<<10 | yc
		}
	}
	return plane, nil
}

// clamp10 keeps a 10-bit code inside the legal 4..1019 range.
func clamp10(v float64) uint32 {
	switch {
	case v < 4:
		return 4
	case v > 1019:
		return 1019
	default:
		return uint32(v)
	}
}

// LoadLogoPNG reads a PNG file and builds the logo plane.
func LoadLogoPNG(path string) ([]uint32, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, fmt.Errorf("logo: %w", err)
	}
	defer f.Close()
	img, _, err := image.Decode(f)
	if err != nil {
		return nil, fmt.Errorf("logo: decode %s: %w", path, err)
	}
	return BuildLogoPlane(img)
}