package osm

import (
	"context"
	"fmt"
	"io"
	"log"
	"math"
	"net/http"
	"os"
	"path/filepath"
	"runtime"
	"strconv"
	"sync"
	"time"
)

var (
	userLat  = 50.06465
	userLon  = 19.94598
	radiusM    = 2000.0
	maxZoom    = 17
	userAgent  = "krakow-tiles-downloader/1.0 (+your_email@example.com)"
	osmTileURL = "https://tile.openstreetmap.org/%d/%d/%d.png"
	tilesDir   = "tiles"
)

const earthRadius = 6378137.0

func offsetLatLon(lat, lon, distance, bearingRad float64) (float64, float64) {
	r := earthRadius
	latRad := lat * math.Pi / 180.0
	lonRad := lon * math.Pi / 180.0
	angDist := distance / r
	newLatRad := math.Asin(math.Sin(latRad)*math.Cos(angDist) + math.Cos(latRad)*math.Sin(angDist)*math.Cos(bearingRad))
	newLonRad := lonRad + math.Atan2(math.Sin(bearingRad)*math.Sin(angDist)*math.Cos(latRad),
		math.Cos(angDist)-math.Sin(latRad)*math.Sin(newLatRad))
	return newLatRad * 180.0 / math.Pi, newLonRad * 180.0 / math.Pi
}

func boundingBoxForCircle(lat, lon, radius float64) (minLat, maxLat, minLon, maxLon float64) {
	maxLat, _ = offsetLatLon(lat, lon, radius, 0)
	minLat, _ = offsetLatLon(lat, lon, radius, math.Pi)
	_, maxLon = offsetLatLon(lat, lon, radius, math.Pi/2)
	_, minLon = offsetLatLon(lat, lon, radius, 3*math.Pi/2)
	return
}

func latLonToTile(lat, lon float64, z int) (x, y int) {
	latRad := lat * math.Pi / 180.0
	n := math.Pow(2.0, float64(z))
	xFloat := (lon + 180.0) / 360.0 * n
	yFloat := (1.0 - math.Log(math.Tan(latRad)+1.0/math.Cos(latRad))/math.Pi) / 2.0 * n
	return int(math.Floor(xFloat)), int(math.Floor(yFloat))
}

func tileXYBounds(minLat, maxLat, minLon, maxLon float64, z int) (minX, maxX, minY, maxY int) {
	x1, y1 := latLonToTile(maxLat, minLon, z)
	x2, y2 := latLonToTile(minLat, maxLon, z)
	if x1 > x2 {
		minX, maxX = x2, x1
	} else {
		minX, maxX = x1, x2
	}
	if y1 > y2 {
		minY, maxY = y2, y1
	} else {
		minY, maxY = y1, y2
	}
	return
}

func downloadTile(ctx context.Context, z, x, y int) error {
	url := fmt.Sprintf(osmTileURL, z, x, y)
	req, _ := http.NewRequestWithContext(ctx, "GET", url, nil)
	req.Header.Set("User-Agent", userAgent)
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()

	if resp.StatusCode == 429 || resp.StatusCode == 403 {
		time.Sleep(5 * time.Second)
		return nil
	}
	if resp.StatusCode != 200 {
		return fmt.Errorf("HTTP %d for %s", resp.StatusCode, url)
	}

	tileCount := 1 << uint(z)
	yFlipped := tileCount - 1 - y
	path := filepath.Join(tilesDir, strconv.Itoa(z), strconv.Itoa(x))
	if err := os.MkdirAll(path, 0o755); err != nil {
		return err
	}
	filePath := filepath.Join(path, fmt.Sprintf("%d.png", yFlipped))
	f, err := os.Create(filePath)
	if err != nil {
		return err
	}
	defer f.Close()
	_, err = io.Copy(f, resp.Body)
	return err
}

func downloadRange(ctx context.Context, z, minX, maxX, minY, maxY int) {
	log.Printf("Downloading tiles zoom %d: x %d..%d, y %d..%d", z, minX, maxX, minY, maxY)
	sem := make(chan struct{}, runtime.NumCPU())
	var wg sync.WaitGroup
	rate := time.NewTicker(1 * time.Second)
	defer rate.Stop()

	for x := minX; x <= maxX; x++ {
		for y := minY; y <= maxY; y++ {
			wg.Add(1)
			sem <- struct{}{}
			<-rate.C
			go func(x, y int) {
				defer wg.Done()
				defer func() { <-sem }()
				filePath := filepath.Join(tilesDir, strconv.Itoa(z), strconv.Itoa(x), fmt.Sprintf("%d.png", (1<<uint(z))-1-y))
				if _, err := os.Stat(filePath); err == nil {
					return
				}
				if err := downloadTile(ctx, z, x, y); err != nil {
					log.Printf("Failed %d/%d/%d: %v", z, x, y, err)
				}
			}(x, y)
		}
	}
	wg.Wait()
}

func runDownloader(ctx context.Context) error {
	minLat, maxLat, minLon, maxLon := boundingBoxForCircle(userLat, userLon, radiusM)
	minX, maxX, minY, maxY := tileXYBounds(minLat, maxLat, minLon, maxLon, maxZoom)
	downloadRange(ctx, maxZoom, minX, maxX, minY, maxY)
	return nil
}

func OSM(){
	if err := os.MkdirAll(tilesDir, 0o755); err != nil {
		log.Fatalf("failed to create tiles dir: %v", err)
	}
	ctx := context.Background()
	go func() {
		if err := runDownloader(ctx); err != nil {
			log.Printf("Downloader finished with error: %v", err)
		}
	}()
}