package main

import (
	"context"
	"fmt"
	"image"
	"image/color"
	"image/draw"
	_ "image/gif"
	"image/jpeg"
	_ "image/png"
	"io"
	"net/http"
	"os"
	"path/filepath"
	"time"

	_ "golang.org/x/image/webp"
)

const maxImageBytes = 50 << 20

type Downloader struct {
	client *http.Client
}

func newDownloader() *Downloader {
	return &Downloader{client: &http.Client{Timeout: 45 * time.Second}}
}

func (d *Downloader) download(ctx context.Context, apiURL, destination string) error {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, apiURL, nil)
	if err != nil {
		return fmt.Errorf("创建图片请求: %w", err)
	}
	req.Header.Set("User-Agent", "GoWallpaper/1.0")
	req.Header.Set("Accept", "image/*")

	resp, err := d.client.Do(req)
	if err != nil {
		return fmt.Errorf("请求图片 API: %w", err)
	}
	defer resp.Body.Close()
	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		return fmt.Errorf("图片 API 返回 HTTP %d", resp.StatusCode)
	}
	if resp.ContentLength > maxImageBytes {
		return fmt.Errorf("图片超过 %d MB 限制", maxImageBytes>>20)
	}

	limited := io.LimitReader(resp.Body, maxImageBytes+1)
	img, _, err := image.Decode(limited)
	if err != nil {
		return fmt.Errorf("API 响应不是支持的图片: %w", err)
	}
	if img.Bounds().Dx() <= 0 || img.Bounds().Dy() <= 0 {
		return fmt.Errorf("API 返回了无效尺寸的图片")
	}

	// JPEG 没有透明通道，先铺黑色背景可避免透明图片出现不可预测颜色。
	bounds := img.Bounds()
	flat := image.NewRGBA(image.Rect(0, 0, bounds.Dx(), bounds.Dy()))
	draw.Draw(flat, flat.Bounds(), &image.Uniform{C: color.Black}, image.Point{}, draw.Src)
	draw.Draw(flat, flat.Bounds(), img, bounds.Min, draw.Over)

	tmp, err := os.CreateTemp(filepath.Dir(destination), ".download-*.jpg")
	if err != nil {
		return fmt.Errorf("创建缓存文件: %w", err)
	}
	tmpName := tmp.Name()
	defer os.Remove(tmpName)
	if err := jpeg.Encode(tmp, flat, &jpeg.Options{Quality: 92}); err != nil {
		tmp.Close()
		return fmt.Errorf("保存图片: %w", err)
	}
	if err := tmp.Sync(); err != nil {
		tmp.Close()
		return err
	}
	if err := tmp.Close(); err != nil {
		return err
	}
	if err := os.Rename(tmpName, destination); err != nil {
		return fmt.Errorf("保存缓存图片: %w", err)
	}
	return nil
}