package thumbnails

import (
	"bytes"
	"context"
	"fmt"
	"image"
	_ "image/gif"
	"image/jpeg"
	_ "image/png"

	_ "golang.org/x/image/webp" // SupportedMIME nennt WebP – ohne den Dekodierer entstand dafür nie ein Vorschaubild
	"io"
	"strings"

	"golang.org/x/image/draw"

	"aether/internal/storage"
)

const (
	maxWidth  = 256
	maxHeight = 256
)

// SupportedMIME returns true if we can generate a thumbnail for this MIME type.
func SupportedMIME(mime string) bool {
	m := strings.ToLower(mime)
	return strings.HasPrefix(m, "image/jpeg") ||
		strings.HasPrefix(m, "image/png") ||
		strings.HasPrefix(m, "image/gif") ||
		strings.HasPrefix(m, "image/webp")
}

// ThumbnailKey returns the storage key for a file's thumbnail.
func ThumbnailKey(fileID string) string {
	return "thumbs/" + fileID + ".jpg"
}

// Generate reads an image from src, creates a thumbnail, and stores it.
// Returns nil if generation succeeds, error otherwise.
func Generate(ctx context.Context, st storage.Storage, fileID, mimeType string, src io.Reader) error {
	if !SupportedMIME(mimeType) {
		return fmt.Errorf("unsupported MIME type: %s", mimeType)
	}

	img, _, err := image.Decode(src)
	if err != nil {
		return fmt.Errorf("decode image: %w", err)
	}

	thumb := resize(img, maxWidth, maxHeight)

	var buf bytes.Buffer
	if err := jpeg.Encode(&buf, thumb, &jpeg.Options{Quality: 80}); err != nil {
		return fmt.Errorf("encode thumbnail: %w", err)
	}

	key := ThumbnailKey(fileID)
	if _, _, err := st.Put(ctx, key, &buf); err != nil {
		return fmt.Errorf("store thumbnail: %w", err)
	}
	return nil
}

func resize(src image.Image, maxW, maxH int) image.Image {
	bounds := src.Bounds()
	w, h := bounds.Dx(), bounds.Dy()
	if w == 0 || h == 0 {
		return src
	}

	scaleW := float64(maxW) / float64(w)
	scaleH := float64(maxH) / float64(h)
	scale := scaleW
	if scaleH < scale {
		scale = scaleH
	}
	if scale >= 1 {
		return src // already smaller than max
	}

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

	dst := image.NewRGBA(image.Rect(0, 0, newW, newH))
	draw.BiLinear.Scale(dst, dst.Bounds(), src, src.Bounds(), draw.Over, nil)
	return dst
}