yr/pkg/met/met.go

// Access [Meteorologisk institutt's API](https://api.met.no)
package met

import (
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"time"
)

type Met struct {
	siteName string
}

var (
	ErrorMissingSiteName           = errors.New("siteName must be defined")
	ErrorMissingHeaderExpires      = errors.New("missing header 'Expires'")
	ErrorMissingHeaderLastModified = errors.New("missing header 'Last-Modified'")
	ErrorParseExpires              = errors.New("unable to parse 'Expires'")
	ErrorParseLastModified         = errors.New("unable to parse 'Last-Modified'")
)

// Returns valid Met client or error if empty siteName is provided
func New(siteName string) (*Met, error) {
	if siteName == "" {
		return nil, ErrorMissingSiteName
	}

	return &Met{
		siteName: siteName,
	}, nil
}

// Get LocationForecast from Met, returns error if Met doesn't answer or response is missing required headers
// API endpoint: https://api.met.no/weatherapi/locationforecast/2.0/documentation
func (m *Met) Forecast(lat, lon float64, alt *int) (*LocationForecastResult, error) {
	url := "https://api.met.no/weatherapi/locationforecast/2.0/complete"

	req, err := http.NewRequest("GET", url, nil)
	if err != nil {
		return nil, err
	}

	q := req.URL.Query()
	q.Add("lat", fmt.Sprintf("%.4f", lat))
	q.Add("lon", fmt.Sprintf("%.4f", lon))
	if alt != nil {
		q.Add("altitude", fmt.Sprintf("%d", alt))
	}
	req.URL.RawQuery = q.Encode()

	req.Header.Set("User-Agent", m.siteName)
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("%s", resp.Status)
	}

	body, err := io.ReadAll(resp.Body)
	if err != nil {
		return nil, err
	}

	expiresRaw, ok := resp.Header["Expires"]
	if !ok {
		return nil, ErrorMissingHeaderExpires
	}
	expires, err := time.Parse(time.RFC1123, expiresRaw[0])
	if err != nil {
		return nil, errors.Join(ErrorParseExpires, err)
	}
	lastModifiedRaw, ok := resp.Header["Last-Modified"]
	if !ok {
		return nil, ErrorMissingHeaderLastModified
	}
	lastModified, err := time.Parse(time.RFC1123, lastModifiedRaw[0])
	if err != nil {
		return nil, errors.Join(ErrorParseLastModified, err)
	}

	forecast := LocationForecast{}
	err = json.Unmarshal(body, &forecast)
	if err != nil {
		return nil, err
	}

	return &LocationForecastResult{
		Expires:          expires,
		LastModified:     lastModified,
		LocationForecast: forecast,
	}, nil
}