apod.go

package nasa

import (
	"encoding/json"
	"errors"
	"fmt"
	"io/ioutil"
	"math/rand"
	"net/http"
	"net/url"
	"os"
	"sync"
	"time"
)

var nasaKey = os.Getenv("NASAKEY")

func init() {
	if nasaKey == "" {
		nasaKey = "DEMO_KEY"
	}
}

// APIKEYMissing is the message displayed if a custom API Key is not defined
const APIKEYMissing = `Currently using the demo API Key DEMO_KEY. Apply for an API key at https://api.nasa.gov/index.html#apply-for-an-api-key
Check out https://github.com/peteretelej/nasa/blob/master/README.md for more info.
`

// APODEndpoint is the NASA API APOD endpoint
var APODEndpoint = "https://api.nasa.gov/planetary/apod"

// Image defines the structure of NASA images
type Image struct {
	Date        string `json:"date"`
	Title       string `json:"title"`
	URL         string `json:"url"`
	HDURL       string `json:"hdurl"`
	Explanation string `json:"explanation"`

	ApodDate time.Time `json:",omitempty"`
}

func (ni Image) String() string {
	return fmt.Sprintf(`Title: %s
Date: %s
Image: %s
HD Image: %s
About:
%s
`, ni.Title, ni.Date, ni.URL, ni.HDURL, ni.Explanation)
}

func init() {
	rand.Seed(time.Now().UnixNano())
}

// RandomAPOD returns an Astronomy Picture of the Day based on a random date
// Picks any image shared between the last 2 years
func RandomAPOD() (*Image, error) {
	days := 2 * 365 // Any day in last 2 years
	randDaysOld := time.Duration(rand.Intn(days))
	t := time.Now().Add(-(time.Hour * 24 * randDaysOld))
	return ApodImage(t)
}

// caches todays APOD
type todaysAPOD struct {
	mu   sync.RWMutex // protects the following
	date string       // YYYY-MM-DD
	apod *Image
}

var tAPOD = &todaysAPOD{}

func (v *todaysAPOD) update(apod Image) {
	v.mu.Lock()
	v.apod = &apod
	v.date = apod.Date
	v.mu.Unlock()
}

//APODToday returns today's APOD, from cache if possible, fetches fresh if not
func APODToday() (*Image, error) {
	d := time.Now().Format("2006-01-02")

	tAPOD.mu.RLock()
	cacheddate, apod := tAPOD.date, tAPOD.apod
	tAPOD.mu.RUnlock()

	if cacheddate != d || apod == nil {
		return ApodImage(time.Now())
	}
	return apod, nil
}

// ApodImage returns the NASA Astronomy Picture of the Day
func ApodImage(t time.Time) (*Image, error) {
	var today bool
	if t.After(time.Now()) {
		t = time.Now()
	}
	date := t.Format("2006-01-02")
	today = time.Now().Format("2006-01-02") == date
	u, err := url.Parse(APODEndpoint)
	if err != nil {
		return nil, err
	}
	q := u.Query()
	q.Set("api_key", nasaKey)
	if !today {
		q.Add("date", date)
	}
	u.RawQuery = q.Encode()
	req, err := http.NewRequest("GET", u.String(), nil)
	if err != nil {
		return nil, err
	}
	client := &http.Client{Timeout: time.Second * 20}
	resp, err := client.Do(req)
	if err != nil {
		return nil, fmt.Errorf("unable to connect to NASA API, %v", err)
	}
	defer func() { _ = resp.Body.Close() }()
	dat, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return nil, err
	}
	_ = resp.Body.Close()
	var ni Image
	err = json.Unmarshal(dat, &ni)
	if err != nil {
		return nil, err
	}
	if ni.URL == "" && ni.HDURL == "" {
		return nil, errors.New("NASA APOD API is returned an invalid response, may be down temporarily")
	}
	if t, err := time.Parse("2006-01-02", ni.Date); err == nil {
		ni.ApodDate = t
	}
	if today {
		tAPOD.update(ni)
	}
	return &ni, nil
}