package compositor

import (
	"fmt"
	"image"
	"image/color"
	"image/png"
	"net/http"
	"strings"
	"time"
	_ "image/jpeg"
)

var httpClient = &http.Client{
	Timeout: 30 * time.Second,
}

// DownloadImage 从 URL 下载并解码图片，返回 image.Image
func DownloadImage(url string) (image.Image, error) {
	url = strings.TrimSpace(url)
	if url == "" {
		return nil, fmt.Errorf("empty URL")
	}

	resp, err := httpClient.Get(url)
	if err != nil {
		return nil, fmt.Errorf("download failed: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("download returned HTTP %d", resp.StatusCode)
	}

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

	return img, nil
}

// DownloadDecodePNG 下载并强制解码为 PNG（获取 alpha 通道）
func DownloadDecodePNG(url string) (image.Image, error) {
	url = strings.TrimSpace(url)
	if url == "" {
		return nil, fmt.Errorf("empty URL")
	}

	resp, err := httpClient.Get(url)
	if err != nil {
		return nil, fmt.Errorf("download failed: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("download returned HTTP %d", resp.StatusCode)
	}

	img, err := png.Decode(resp.Body)
	if err != nil {
		return DownloadImage(url)
	}

	return img, nil
}

// ExtractAlpha 将白色/浅灰背景转换为透明，保留非白色像素
func ExtractAlpha(img image.Image) *image.NRGBA {
	bounds := img.Bounds()
	result := image.NewNRGBA(bounds)

	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
		for x := bounds.Min.X; x < bounds.Max.X; x++ {
			r, g, b, a := img.At(x, y).RGBA()

			rf := float64(r) / 65535.0
			gf := float64(g) / 65535.0
			bf := float64(b) / 65535.0

			// 亮度 > 90% 的视为白色/浅灰背景，设为透明
			luminance := (rf + gf + bf) / 3.0
			if luminance > 0.90 {
				result.SetNRGBA(x, y, color.NRGBA{R: 0, G: 0, B: 0, A: 0})
			} else {
				result.SetNRGBA(x, y, color.NRGBA{
					R: uint8(r >> 8),
					G: uint8(g >> 8),
					B: uint8(b >> 8),
					A: uint8(a >> 8),
				})
			}
		}
	}

	return result
}