// Package probe measures per-phase HTTP request latency.
package probe

import (
	"context"
	"crypto/tls"
	"io"
	"net/http"
	"net/http/httptrace"
	"time"
)

// Phase holds the measured duration of a single request phase.
type Phase struct {
	Duration time.Duration
	// Present is false when the phase was skipped (e.g. no TLS for http://).
	Present bool
}

// Result holds all timing phases for a single HTTP request.
type Result struct {
	URL      string
	DNS      Phase
	Connect  Phase
	TLS      Phase
	TTFB     Phase // server processing: WroteRequest → GotFirstResponseByte
	Transfer Phase // body read: GotFirstResponseByte → body closed
	Total    Phase

	// StatusCode is the HTTP response status code (0 on error).
	StatusCode int
	// Err is non-nil if the request failed.
	Err error
}

// Measure performs an HTTP GET to url and returns a Result with all phases
// populated via net/http/httptrace.
func Measure(url string) Result {
	r := Result{URL: url}

	var (
		dnsStart     time.Time
		dnsDone      time.Time
		connectStart time.Time
		connectDone  time.Time
		tlsStart     time.Time
		tlsDone      time.Time
		wroteRequest time.Time
		firstByte    time.Time
	)

	trace := &httptrace.ClientTrace{
		DNSStart:  func(_ httptrace.DNSStartInfo) { dnsStart = time.Now() },
		DNSDone:   func(_ httptrace.DNSDoneInfo) { dnsDone = time.Now() },
		ConnectStart: func(_, _ string) {
			if connectStart.IsZero() {
				connectStart = time.Now()
			}
		},
		ConnectDone: func(_, _ string, _ error) { connectDone = time.Now() },
		TLSHandshakeStart: func() { tlsStart = time.Now() },
		TLSHandshakeDone:  func(_ tls.ConnectionState, _ error) { tlsDone = time.Now() },
		WroteRequest:         func(_ httptrace.WroteRequestInfo) { wroteRequest = time.Now() },
		GotFirstResponseByte: func() { firstByte = time.Now() },
	}

	ctx := httptrace.WithClientTrace(context.Background(), trace)
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		r.Err = err
		return r
	}

	start := time.Now()
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		r.Err = err
		r.Total = Phase{Duration: time.Since(start), Present: true}
		return r
	}
	defer resp.Body.Close()

	_, err = io.Copy(io.Discard, resp.Body)
	end := time.Now()

	r.StatusCode = resp.StatusCode
	if err != nil {
		r.Err = err
	}

	r.Total = Phase{Duration: end.Sub(start), Present: true}

	if !dnsStart.IsZero() && !dnsDone.IsZero() {
		r.DNS = Phase{Duration: dnsDone.Sub(dnsStart), Present: true}
	}
	if !connectStart.IsZero() && !connectDone.IsZero() {
		r.Connect = Phase{Duration: connectDone.Sub(connectStart), Present: true}
	}
	if !tlsStart.IsZero() && !tlsDone.IsZero() {
		r.TLS = Phase{Duration: tlsDone.Sub(tlsStart), Present: true}
	}
	if !wroteRequest.IsZero() && !firstByte.IsZero() {
		r.TTFB = Phase{Duration: firstByte.Sub(wroteRequest), Present: true}
	}
	if !firstByte.IsZero() {
		r.Transfer = Phase{Duration: end.Sub(firstByte), Present: true}
	}

	return r
}