package tracer

import (
	"fmt"
	"net"
	"net/http"
	"runtime"
	"strconv"
	"strings"
	"time"
)

// TODO(gbbr): find a more effective way to keep this up to date,
// e.g. via `go generate`
var tracerVersion = "v1.5.0"

const (
	defaultHostname    = "localhost"
	defaultPort        = "8126"
	defaultAddress     = defaultHostname + ":" + defaultPort
	defaultHTTPTimeout = time.Second             // defines the current timeout before giving up with the send process
	traceCountHeader   = "X-Datadog-Trace-Count" // header containing the number of traces in the payload
)

// Transport is an interface for span submission to the agent.
type transport interface {
	send(p *payload) error
}

// newTransport returns a new Transport implementation that sends traces to a
// trace agent running on the given hostname and port. If the zero values for
// hostname and port are provided, the default values will be used ("localhost"
// for hostname, and "8126" for port).
//
// In general, using this method is only necessary if you have a trace agent
// running on a non-default port or if it's located on another machine.
func newTransport(addr string) transport {
	return newHTTPTransport(addr)
}

// newDefaultTransport return a default transport for this tracing client
func newDefaultTransport() transport {
	return newHTTPTransport(defaultAddress)
}

type httpTransport struct {
	traceURL string            // the delivery URL for traces
	client   *http.Client      // the HTTP client used in the POST
	headers  map[string]string // the Transport headers
}

// newHTTPTransport returns an httpTransport for the given endpoint
func newHTTPTransport(addr string) *httpTransport {
	// initialize the default EncoderPool with Encoder headers
	defaultHeaders := map[string]string{
		"Datadog-Meta-Lang":             "go",
		"Datadog-Meta-Lang-Version":     strings.TrimPrefix(runtime.Version(), "go"),
		"Datadog-Meta-Lang-Interpreter": runtime.Compiler + "-" + runtime.GOARCH + "-" + runtime.GOOS,
		"Datadog-Meta-Tracer-Version":   tracerVersion,
		"Content-Type":                  "application/msgpack",
	}
	return &httpTransport{
		traceURL: fmt.Sprintf("http://%s/v0.3/traces", resolveAddr(addr)),
		client: &http.Client{
			// We copy the transport to avoid using the default one, as it might be
			// augmented with tracing and we don't want these calls to be recorded.
			// See https://golang.org/pkg/net/http/#DefaultTransport .
			Transport: &http.Transport{
				Proxy: http.ProxyFromEnvironment,
				DialContext: (&net.Dialer{
					Timeout:   30 * time.Second,
					KeepAlive: 30 * time.Second,
					DualStack: true,
				}).DialContext,
				MaxIdleConns:          100,
				IdleConnTimeout:       90 * time.Second,
				TLSHandshakeTimeout:   10 * time.Second,
				ExpectContinueTimeout: 1 * time.Second,
			},
			Timeout: defaultHTTPTimeout,
		},
		headers: defaultHeaders,
	}
}

func (t *httpTransport) send(p *payload) error {
	// prepare the client and send the payload
	req, err := http.NewRequest("POST", t.traceURL, p)
	if err != nil {
		return fmt.Errorf("cannot create http request: %v", err)
	}
	for header, value := range t.headers {
		req.Header.Set(header, value)
	}
	req.Header.Set(traceCountHeader, strconv.Itoa(p.itemCount()))
	req.Header.Set("Content-Length", strconv.Itoa(p.size()))
	response, err := t.client.Do(req)
	if err != nil {
		return err
	}
	defer response.Body.Close()
	if code := response.StatusCode; code >= 400 {
		// error, check the body for context information and
		// return a nice error.
		msg := make([]byte, 1000)
		n, _ := response.Body.Read(msg)
		txt := http.StatusText(code)
		if n > 0 {
			return fmt.Errorf("%s (Status: %s)", msg[:n], txt)
		}
		return fmt.Errorf("%s", txt)
	}
	return nil
}

// resolveAddr resolves the given agent address and fills in any missing host
// and port using the defaults.
func resolveAddr(addr string) string {
	host, port, err := net.SplitHostPort(addr)
	if err != nil {
		// no port in addr
		host = addr
	}
	if host == "" {
		host = defaultHostname
	}
	if port == "" {
		port = defaultPort
	}
	return fmt.Sprintf("%s:%s", host, port)
}