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

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

// Options configures a single Measure call.
type Options struct {
	Timeout time.Duration // 0 = no timeout
}

// 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://)
	// or did not complete before a failure.
	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 network error).
	StatusCode int

	// FailPhase is the phase where the request broke: "dns", "connect",
	// "timeout", "tls", "transfer", or "request" (bad URL). Empty on success.
	FailPhase string

	// Err is non-nil if the request failed.
	Err error
}

// Measure performs an HTTP GET to url and returns a Result with all completed
// phases populated. Partial phases are preserved when the request fails.
func Measure(url string, opts Options) 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
		dnsErr       error
	)

	trace := &httptrace.ClientTrace{
		DNSStart: func(_ httptrace.DNSStartInfo) { dnsStart = time.Now() },
		DNSDone: func(info httptrace.DNSDoneInfo) {
			dnsDone = time.Now()
			dnsErr = info.Err
		},
		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)
	if opts.Timeout > 0 {
		var cancel context.CancelFunc
		ctx, cancel = context.WithTimeout(ctx, opts.Timeout)
		defer cancel()
	}

	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		r.Err = err
		r.FailPhase = "request"
		return r
	}

	start := time.Now()
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		end := time.Now()
		r.Err = err
		r.FailPhase = classifyErr(err, dnsErr, tlsStart, tlsDone)
		r.Total = Phase{Duration: end.Sub(start), Present: true}
		r.DNS = makePhase(dnsStart, dnsDone)
		r.Connect = makePhase(connectStart, connectDone)
		r.TLS = makePhase(tlsStart, tlsDone)
		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.FailPhase = "transfer"
	}

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

	return r
}

func makePhase(start, end time.Time) Phase {
	if start.IsZero() || end.IsZero() {
		return Phase{}
	}
	return Phase{Duration: end.Sub(start), Present: true}
}

func classifyErr(err, dnsErr error, tlsStart, tlsDone time.Time) string {
	if dnsErr != nil {
		return "dns"
	}
	var dnsError *net.DNSError
	if errors.As(err, &dnsError) {
		return "dns"
	}
	if errors.Is(err, context.DeadlineExceeded) {
		return "timeout"
	}
	var netErr net.Error
	if errors.As(err, &netErr) && netErr.Timeout() {
		return "timeout"
	}
	// TLS started but handshake never completed
	if !tlsStart.IsZero() && tlsDone.IsZero() {
		return "tls"
	}
	return "connect"
}