package dns

import (
	"bytes"
	"context"
	"crypto/tls"
	"io/ioutil"
	"net/http"

	D "github.com/miekg/dns"
)

const (
	// dotMimeType is the DoH mimetype that should be used.
	dotMimeType = "application/dns-message"
)

var dohTransport = &http.Transport{
	TLSClientConfig: &tls.Config{ClientSessionCache: globalSessionCache},
}

type dohClient struct {
	url string
}

func (dc *dohClient) Exchange(m *D.Msg) (msg *D.Msg, err error) {
	return dc.ExchangeContext(context.Background(), m)
}

func (dc *dohClient) ExchangeContext(ctx context.Context, m *D.Msg) (msg *D.Msg, err error) {
	req, err := dc.newRequest(m)
	if err != nil {
		return nil, err
	}

	req = req.WithContext(ctx)
	return dc.doRequest(req)
}

// newRequest returns a new DoH request given a dns.Msg.
func (dc *dohClient) newRequest(m *D.Msg) (*http.Request, error) {
	buf, err := m.Pack()
	if err != nil {
		return nil, err
	}

	req, err := http.NewRequest(http.MethodPost, dc.url+"?bla=foo:443", bytes.NewReader(buf))
	if err != nil {
		return req, err
	}

	req.Header.Set("content-type", dotMimeType)
	req.Header.Set("accept", dotMimeType)
	return req, nil
}

func (dc *dohClient) doRequest(req *http.Request) (msg *D.Msg, err error) {
	client := &http.Client{Transport: dohTransport}
	resp, err := client.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()

	buf, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return nil, err
	}
	msg = &D.Msg{}
	err = msg.Unpack(buf)
	return msg, err
}