// Package weather fetches forecasts and place names from Open-Meteo
// (open source, no API key), with MET Norway and Nominatim as fallbacks.
// Coordinates are rounded to ~1 km before they leave the machine.
package weather

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"math"
	"net/http"
	"net/url"
	"strconv"
	"strings"
	"sync"
	"time"
)

// Hour is the weather for one hour of the forecast.
type Hour struct {
	Time          time.Time
	Temp          float64 // °C
	Feels         float64 // apparent temperature, °C
	PrecipProb    int     // %, or -1 when the source doesn't say
	Precip        float64 // mm
	Code          int     // WMO weather code
	Wind          float64 // km/h
	Gusts         float64 // km/h
	WindDirection int     // degrees, where the wind comes from
	IsDay         bool
}

// Forecast is the current weather plus the next 24 hours.
type Forecast struct {
	Now            Hour
	Hours          []Hour // starting at the current hour
	Sunrise        time.Time
	Sunset         time.Time
	Source         string // who made the forecast, shown in the UI
	GustsEstimated bool   // the source had no gusts; they're derived from wind speed
}

// Place is a geocoded location.
type Place struct {
	Name string
	Lat  float64
	Lon  float64
}

// Client fetches forecasts from Open-Meteo, falling back to MET Norway when
// Open-Meteo is down, and caches them for 15 minutes.
type Client struct {
	ForecastURL  string // default https://api.open-meteo.com/v1/forecast
	GeocodeURL   string // default https://geocoding-api.open-meteo.com/v1/search
	MetNoURL     string // default https://api.met.no/weatherapi/locationforecast/2.0/complete; "-" disables
	NominatimURL string // default https://nominatim.openstreetmap.org/search; "-" disables
	HTTP         *http.Client

	now   func() time.Time // for tests; nil means time.Now
	mu    sync.Mutex
	cache map[string]cached
}

type cached struct {
	f  Forecast
	at time.Time
}

const (
	cacheFor  = 15 * time.Minute
	staleFor  = 3 * time.Hour // when every source is down, an older forecast beats none
	userAgent = "workout-suggester/0.1 (+https://git.b0b.be/bdeb/workout-suggester)"
)

func (c *Client) http() *http.Client {
	if c.HTTP != nil {
		return c.HTTP
	}
	return http.DefaultClient
}

func or(v, fallback string) string {
	if v == "" {
		return fallback
	}
	return v
}

// Round keeps two decimals (~1 km), which is plenty for weather.
func Round(v float64) float64 { return math.Round(v*100) / 100 }

// getJSON fetches u, retrying once when the server is busy (429/5xx).
func (c *Client) getJSON(ctx context.Context, u string, out any) error {
	var err error
	for attempt := range 2 {
		if attempt > 0 {
			select {
			case <-ctx.Done():
				return ctx.Err()
			case <-time.After(time.Second):
			}
		}
		var retry bool
		if retry, err = c.getOnce(ctx, u, out); !retry {
			return err
		}
	}
	return err
}

func (c *Client) getOnce(ctx context.Context, u string, out any) (retry bool, err error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
	if err != nil {
		return false, err
	}
	req.Header.Set("User-Agent", userAgent) // MET Norway and Nominatim require one
	req.Header.Set("Accept", "application/json")
	resp, err := c.http().Do(req)
	if err != nil {
		return ctx.Err() == nil, err
	}
	defer resp.Body.Close()
	if resp.StatusCode >= 300 {
		msg, _ := io.ReadAll(io.LimitReader(resp.Body, 512))
		busy := resp.StatusCode == http.StatusTooManyRequests || resp.StatusCode >= 500
		return busy, fmt.Errorf("%s returned %s: %s", req.URL.Host, resp.Status, bytes.TrimSpace(msg))
	}
	return false, json.NewDecoder(resp.Body).Decode(out)
}

// Geocode finds a town or city by name, via Open-Meteo or else Nominatim.
func (c *Client) Geocode(ctx context.Context, name string) (Place, error) {
	q := url.Values{"name": {name}, "count": {"1"}, "language": {"en"}, "format": {"json"}}
	var out struct {
		Results []struct {
			Name    string  `json:"name"`
			Country string  `json:"country"`
			Lat     float64 `json:"latitude"`
			Lon     float64 `json:"longitude"`
		} `json:"results"`
	}
	err := c.getJSON(ctx, or(c.GeocodeURL, "https://geocoding-api.open-meteo.com/v1/search")+"?"+q.Encode(), &out)
	if err == nil {
		if len(out.Results) == 0 {
			return Place{}, fmt.Errorf("couldn't find a place called %q", name)
		}
		r := out.Results[0]
		n := r.Name
		if r.Country != "" {
			n += ", " + r.Country
		}
		return Place{Name: n, Lat: r.Lat, Lon: r.Lon}, nil
	}
	if c.NominatimURL == "-" {
		return Place{}, err
	}
	p, err2 := c.nominatim(ctx, name)
	if err2 != nil {
		return Place{}, errors.Join(err, err2)
	}
	return p, nil
}

func (c *Client) nominatim(ctx context.Context, name string) (Place, error) {
	q := url.Values{"q": {name}, "format": {"jsonv2"}, "limit": {"1"}, "accept-language": {"en"}}
	var out []struct {
		Name    string `json:"name"`
		Display string `json:"display_name"`
		Lat     string `json:"lat"`
		Lon     string `json:"lon"`
	}
	if err := c.getJSON(ctx, or(c.NominatimURL, "https://nominatim.openstreetmap.org/search")+"?"+q.Encode(), &out); err != nil {
		return Place{}, err
	}
	if len(out) == 0 {
		return Place{}, fmt.Errorf("couldn't find a place called %q", name)
	}
	lat, err1 := strconv.ParseFloat(out[0].Lat, 64)
	lon, err2 := strconv.ParseFloat(out[0].Lon, 64)
	if err1 != nil || err2 != nil {
		return Place{}, errors.New("nominatim: bad coordinates")
	}
	n := out[0].Name
	if parts := strings.Split(out[0].Display, ", "); len(parts) > 1 {
		n += ", " + parts[len(parts)-1] // country
	}
	return Place{Name: n, Lat: lat, Lon: lon}, nil
}

// Forecast returns current weather and the next 24 hours.
func (c *Client) Forecast(ctx context.Context, lat, lon float64) (Forecast, error) {
	lat, lon = Round(lat), Round(lon)
	key := fmt.Sprintf("%.2f,%.2f", lat, lon)
	c.mu.Lock()
	hit, ok := c.cache[key]
	c.mu.Unlock()
	if ok && time.Since(hit.at) < cacheFor {
		return hit.f, nil
	}

	f, err := c.openMeteo(ctx, lat, lon)
	if err != nil && c.MetNoURL != "-" {
		var err2 error
		if f, err2 = c.metNo(ctx, lat, lon); err2 != nil {
			err = errors.Join(err, err2)
		} else {
			err = nil
		}
	}
	if err != nil {
		if ok && time.Since(hit.at) < staleFor {
			hit.f.Source += fmt.Sprintf(", from %s", hit.at.Format("15:04"))
			return hit.f, nil
		}
		return Forecast{}, err
	}

	c.mu.Lock()
	if c.cache == nil {
		c.cache = make(map[string]cached)
	}
	c.cache[key] = cached{f, time.Now()}
	c.mu.Unlock()
	return f, nil
}

func (c *Client) openMeteo(ctx context.Context, lat, lon float64) (Forecast, error) {
	vars := "temperature_2m,apparent_temperature,precipitation,weather_code,wind_speed_10m,wind_gusts_10m,wind_direction_10m,is_day"
	q := url.Values{
		"latitude":       {strconv.FormatFloat(lat, 'f', 2, 64)},
		"longitude":      {strconv.FormatFloat(lon, 'f', 2, 64)},
		"current":        {vars},
		"hourly":         {vars + ",precipitation_probability"},
		"daily":          {"sunrise,sunset"},
		"forecast_hours": {"24"},
		"forecast_days":  {"2"},
		"timezone":       {"auto"},
	}
	var raw apiResponse
	if err := c.getJSON(ctx, or(c.ForecastURL, "https://api.open-meteo.com/v1/forecast")+"?"+q.Encode(), &raw); err != nil {
		return Forecast{}, err
	}
	return raw.forecast()
}

type apiResponse struct {
	UTCOffset int `json:"utc_offset_seconds"`
	Current   struct {
		Time   string  `json:"time"`
		Temp   float64 `json:"temperature_2m"`
		Feels  float64 `json:"apparent_temperature"`
		Precip float64 `json:"precipitation"`
		Code   int     `json:"weather_code"`
		Wind   float64 `json:"wind_speed_10m"`
		Gusts  float64 `json:"wind_gusts_10m"`
		Dir    int     `json:"wind_direction_10m"`
		IsDay  int     `json:"is_day"`
	} `json:"current"`
	Hourly struct {
		Time       []string  `json:"time"`
		Temp       []float64 `json:"temperature_2m"`
		Feels      []float64 `json:"apparent_temperature"`
		Precip     []float64 `json:"precipitation"`
		PrecipProb []int     `json:"precipitation_probability"`
		Code       []int     `json:"weather_code"`
		Wind       []float64 `json:"wind_speed_10m"`
		Gusts      []float64 `json:"wind_gusts_10m"`
		Dir        []int     `json:"wind_direction_10m"`
		IsDay      []int     `json:"is_day"`
	} `json:"hourly"`
	Daily struct {
		Sunrise []string `json:"sunrise"`
		Sunset  []string `json:"sunset"`
	} `json:"daily"`
}

func (r apiResponse) forecast() (Forecast, error) {
	loc := time.FixedZone("local", r.UTCOffset)
	parse := func(s string) time.Time {
		t, _ := time.ParseInLocation("2006-01-02T15:04", s, loc)
		return t
	}
	h := r.Hourly
	n := len(h.Time)
	if n == 0 || len(h.Temp) != n || len(h.Feels) != n || len(h.Precip) != n || len(h.PrecipProb) != n ||
		len(h.Code) != n || len(h.Wind) != n || len(h.Gusts) != n || len(h.Dir) != n || len(h.IsDay) != n {
		return Forecast{}, errors.New("open-meteo: incomplete hourly forecast")
	}
	f := Forecast{Source: "Open-Meteo", Now: Hour{
		Time: parse(r.Current.Time), Temp: r.Current.Temp, Feels: r.Current.Feels,
		Precip: r.Current.Precip, Code: r.Current.Code, Wind: r.Current.Wind,
		Gusts: r.Current.Gusts, WindDirection: r.Current.Dir, IsDay: r.Current.IsDay == 1,
	}}
	for i := range n {
		f.Hours = append(f.Hours, Hour{
			Time: parse(h.Time[i]), Temp: h.Temp[i], Feels: h.Feels[i], PrecipProb: h.PrecipProb[i],
			Precip: h.Precip[i], Code: h.Code[i], Wind: h.Wind[i], Gusts: h.Gusts[i],
			WindDirection: h.Dir[i], IsDay: h.IsDay[i] == 1,
		})
	}
	// The current hour's rain chance is the best guess for "now".
	f.Now.PrecipProb = f.Hours[0].PrecipProb
	// Today's sunrise/sunset, or tomorrow's once today's sun has set.
	for i := range r.Daily.Sunset {
		f.Sunrise, f.Sunset = parse(r.Daily.Sunrise[i]), parse(r.Daily.Sunset[i])
		if f.Sunset.After(f.Now.Time) {
			break
		}
	}
	return f, nil
}

// Describe turns a WMO weather code into words and an emoji.
func Describe(code int) (string, string) {
	switch {
	case code == 0:
		return "clear sky", "☀️"
	case code == 1:
		return "mostly clear", "🌤️"
	case code == 2:
		return "partly cloudy", "⛅"
	case code == 3:
		return "overcast", "☁️"
	case code == 45 || code == 48:
		return "fog", "🌫️"
	case code >= 51 && code <= 55:
		return "drizzle", "🌦️"
	case code == 56 || code == 57:
		return "freezing drizzle", "🧊"
	case code >= 61 && code <= 65:
		return "rain", "🌧️"
	case code == 66 || code == 67:
		return "freezing rain", "🧊"
	case code >= 71 && code <= 77:
		return "snow", "🌨️"
	case code >= 80 && code <= 82:
		return "rain showers", "🌦️"
	case code == 85 || code == 86:
		return "snow showers", "🌨️"
	case code >= 95:
		return "thunderstorm", "⛈️"
	}
	return "unknown", "🌡️"
}

// Icon is Describe's emoji, with a moon for clear nights.
func (h Hour) Icon() string {
	_, icon := Describe(h.Code)
	if !h.IsDay && h.Code <= 1 {
		return "🌙"
	}
	return icon
}

// Chance is the rain chance as text, "–" when unknown.
func (h Hour) Chance() string {
	if h.PrecipProb < 0 {
		return "–"
	}
	return fmt.Sprintf("%d%%", h.PrecipProb)
}

// Compass turns a wind direction in degrees into N, NE, E, ...
func Compass(deg int) string {
	dirs := []string{"N", "NE", "E", "SE", "S", "SW", "W", "NW"}
	return dirs[((deg%360+360)%360+22)/45%8]
}