package geo

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

const (
	// GeoJSEndpoint is the URL of the geojs geo endpoint.
	GeoJSEndpoint = "https://get.geojs.io/v1/ip/geo.json"

	// geojsTimeLimit limits one request to geojs.
	geojsTimeLimit = time.Second

	// geojsAddressSeparator separates the addresses of one request.
	geojsAddressSeparator = ","

	// geojsUnknown is what geojs sends for a value it does not know.
	geojsUnknown = "nil"

	// maxGeoJSAnswerBytes is far more than an answer for the most addresses
	// an IP lookup can ask about.
	maxGeoJSAnswerBytes = 1 << 20
)

var errGeoJSStatus = errors.New("unexpected HTTP status from geojs")

// geojsAnswer is one address's entry in an answer from geojs.
//
//nolint:tagliatelle // the keys geojs sends
type geojsAnswer struct {
	IP            string `json:"ip"`
	CountryCode   string `json:"country_code"`
	Country       string `json:"country"`
	ContinentCode string `json:"continent_code"`
	Region        string `json:"region"`
	City          string `json:"city"`
	Latitude      string `json:"latitude"`
	Longitude     string `json:"longitude"`
	Accuracy      *int   `json:"accuracy"`
	TimeZone      string `json:"timezone"`
}

// askGeoJS asks geojs about addresses in one request, and returns its answers
// by address. An address geojs left out has no entry.
func (l *Locator) askGeoJS(
	ctx context.Context, addresses []string,
) (map[string]*Location, error) {
	ctx, cancel := context.WithTimeout(ctx, l.timeLimit)
	defer cancel()

	// A comma and a colon are legal in a query, and the escaped forms are
	// not what geojs documents.
	url := l.geojsURL + "?ip=" + strings.Join(addresses, geojsAddressSeparator)

	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return nil, err
	}

	request.Header.Set(userAgentHeader, userAgent)

	response, err := l.client.Do(request)
	if err != nil {
		return nil, err
	}

	defer func() { _ = response.Body.Close() }()

	if response.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("%w: %d", errGeoJSStatus, response.StatusCode)
	}

	var parsed []geojsAnswer

	err = json.NewDecoder(io.LimitReader(response.Body, maxGeoJSAnswerBytes)).
		Decode(&parsed)
	if err != nil {
		return nil, fmt.Errorf("reading the geojs answer: %w", err)
	}

	fetched := l.now().UTC().Truncate(time.Second)
	answers := make(map[string]*Location, len(parsed))

	for _, answer := range parsed {
		addr, err := netip.ParseAddr(answer.IP)
		if err != nil {
			continue
		}

		answers[addr.Unmap().String()] = answer.location(fetched)
	}

	return answers, nil
}

// location returns the answer as a Location fetched at the time given.
func (a geojsAnswer) location(fetched time.Time) *Location {
	return &Location{
		CountryCode:      knownText(a.CountryCode),
		Country:          knownText(a.Country),
		ContinentCode:    knownText(a.ContinentCode),
		Region:           knownText(a.Region),
		City:             knownText(a.City),
		Latitude:         knownNumber(a.Latitude),
		Longitude:        knownNumber(a.Longitude),
		AccuracyRadiusKm: a.Accuracy,
		TimeZone:         knownText(a.TimeZone),
		Source:           sourceGeoJS,
		Updated:          fetched,
	}
}

// knownText returns a pointer to text, or nil when geojs does not know it.
func knownText(text string) *string {
	if text == "" || text == geojsUnknown {
		return nil
	}

	return &text
}

// knownNumber returns text as a number, or nil when geojs does not know it. NaN
// and infinity parse, but JSON cannot carry them.
func knownNumber(text string) *float64 {
	number, err := strconv.ParseFloat(text, 64)
	if err != nil || math.IsNaN(number) || math.IsInf(number, 0) {
		return nil
	}

	return &number
}