workout-suggester/internal/weather/metno.go

package weather

import (
	"context"
	"errors"
	"fmt"
	"math"
	"net/url"
	"strings"
	"time"
)

// metNo fetches the MET Norway (yr.no) forecast, a free fallback with
// global coverage. Outside the Nordics it has no gusts or rain chance, so
// gusts are estimated from wind speed and the rain chance is left unknown.
func (c *Client) metNo(ctx context.Context, lat, lon float64) (Forecast, error) {
	q := url.Values{"lat": {fmt.Sprintf("%.2f", lat)}, "lon": {fmt.Sprintf("%.2f", lon)}}
	var raw struct {
		Properties struct {
			Timeseries []struct {
				Time time.Time `json:"time"`
				Data struct {
					Instant struct {
						Details map[string]float64 `json:"details"`
					} `json:"instant"`
					Next1 *struct {
						Summary struct {
							Symbol string `json:"symbol_code"`
						} `json:"summary"`
						Details map[string]float64 `json:"details"`
					} `json:"next_1_hours"`
				} `json:"data"`
			} `json:"timeseries"`
		} `json:"properties"`
	}
	u := or(c.MetNoURL, "https://api.met.no/weatherapi/locationforecast/2.0/complete") + "?" + q.Encode()
	if err := c.getJSON(ctx, u, &raw); err != nil {
		return Forecast{}, err
	}

	now := time.Now()
	if c.now != nil {
		now = c.now()
	}
	f := Forecast{Source: "MET Norway"}
	f.Sunrise, f.Sunset = sunTimes(now, lat, lon)
	if f.Sunset.Before(now) {
		f.Sunrise, f.Sunset = sunTimes(now.Add(24*time.Hour), lat, lon)
	}
	for _, t := range raw.Properties.Timeseries {
		if t.Time.Before(now.Truncate(time.Hour)) || t.Data.Next1 == nil {
			continue
		}
		if len(f.Hours) == 24 {
			break
		}
		d, n := t.Data.Instant.Details, t.Data.Next1.Details
		h := Hour{
			Time:          t.Time.Local(),
			Temp:          d["air_temperature"],
			Feels:         d["air_temperature"],
			Precip:        n["precipitation_amount"],
			PrecipProb:    -1,
			Code:          symbolCode(t.Data.Next1.Summary.Symbol),
			Wind:          d["wind_speed"] * 3.6,
			WindDirection: int(math.Round(d["wind_from_direction"])),
			IsDay:         isDay(t.Time, lat, lon),
		}
		if v, ok := d["apparent_air_temperature"]; ok {
			h.Feels = v
		}
		if v, ok := n["probability_of_precipitation"]; ok {
			h.PrecipProb = int(math.Round(v))
		}
		if v, ok := d["wind_speed_of_gust"]; ok {
			h.Gusts = v * 3.6
		} else {
			h.Gusts = h.Wind * 1.5 // typical gust factor over open land
			f.GustsEstimated = true
		}
		f.Hours = append(f.Hours, h)
	}
	if len(f.Hours) == 0 {
		return Forecast{}, errors.New("met norway: no hourly forecast")
	}
	f.Now = f.Hours[0]
	f.Now.Time = now.Truncate(time.Minute)
	return f, nil
}

// symbolCode maps a MET Norway symbol ("lightrainshowers_day") to the
// closest WMO weather code.
func symbolCode(sym string) int {
	base, _, _ := strings.Cut(sym, "_")
	switch {
	case strings.Contains(base, "thunder"):
		return 95
	case strings.Contains(base, "sleet"):
		if strings.HasPrefix(base, "light") {
			return 66
		}
		return 67
	case strings.HasSuffix(base, "snowshowers"):
		if strings.HasPrefix(base, "heavy") {
			return 86
		}
		return 85
	case strings.HasSuffix(base, "rainshowers"):
		switch {
		case strings.HasPrefix(base, "light"):
			return 80
		case strings.HasPrefix(base, "heavy"):
			return 82
		}
		return 81
	}
	if code, ok := map[string]int{
		"clearsky": 0, "fair": 1, "partlycloudy": 2, "cloudy": 3, "fog": 45,
		"lightrain": 61, "rain": 63, "heavyrain": 65,
		"lightsnow": 71, "snow": 73, "heavysnow": 75,
	}[base]; ok {
		return code
	}
	return 3 // unknown: say overcast rather than promise sunshine
}

func isDay(t time.Time, lat, lon float64) bool {
	rise, set := sunTimes(t, lat, lon)
	return !t.Before(rise) && t.Before(set)
}

// sunTimes computes sunrise and sunset for t's UTC date with the standard
// sunrise equation (good to a minute or two), in local time.
func sunTimes(t time.Time, lat, lon float64) (rise, set time.Time) {
	const rad = math.Pi / 180
	y, m, d := t.UTC().Date()
	noon := float64(time.Date(y, m, d, 12, 0, 0, 0, time.UTC).Unix())/86400 + 2440587.5 // Julian date
	n := math.Round(noon - 2451545.0)                                                   // days since J2000
	jStar := n - lon/360
	M := math.Mod(357.5291+0.98560028*jStar, 360)
	C := 1.9148*math.Sin(M*rad) + 0.02*math.Sin(2*M*rad) + 0.0003*math.Sin(3*M*rad)
	lambda := math.Mod(M+C+180+102.9372, 360)
	transit := 2451545.0 + jStar + 0.0053*math.Sin(M*rad) - 0.0069*math.Sin(2*lambda*rad)
	sinDec := math.Sin(lambda*rad) * math.Sin(23.4397*rad)
	cosDec := math.Cos(math.Asin(sinDec))
	cosW := (math.Sin(-0.833*rad) - math.Sin(lat*rad)*sinDec) / (math.Cos(lat*rad) * cosDec)
	cosW = math.Max(-1, math.Min(1, cosW)) // polar day/night: clamp
	w := math.Acos(cosW) / rad
	toTime := func(j float64) time.Time {
		return time.Unix(int64(math.Round((j-2440587.5)*86400)), 0).Local()
	}
	return toTime(transit - w/360), toTime(transit + w/360)
}