// Copyright (c) 2026, Pim van Pelt <pim@ipng.ch>

package main

import (
	"context"
	"crypto/tls"
	"math/rand/v2"
	"net"
	"net/http"
	"strconv"
	"strings"
	"sync/atomic"
	"time"

	tea "github.com/charmbracelet/bubbletea"
)

// probeOpts bundles the runtime probe configuration coming from the
// command-line flags. Each probe goroutine gets a copy at startup;
// nothing in here mutates during the program's lifetime (pauses flow
// through the global `paused` atomic instead).
type probeOpts struct {
	Interval  time.Duration
	Timeout   time.Duration
	Host      string
	Path      string
	Header    string
	Insecure  bool
	KeepAlive bool
}

// vipInfo is the immutable per-VIP descriptor built from the maglev
// config at startup. Kept separate from vipState (which holds mutable
// stats) so the probe goroutine can read its target without racing
// the Update loop in model.go.
//
// Identity of a VIP in maglevt is the (scheme, ip, port) tuple, not
// the symbolic name from the config: when we union multiple maglev
// yaml files, the same name can describe different tuples across
// deployments and the tuple is the unambiguous key. The TUI shows
// scheme + ip:port and nothing else, so there's no need to carry a
// display name at all.
type vipInfo struct {
	idx    int    // index into Model.vips (stable for the process lifetime)
	scheme string // "http", "https", or "tcp"
	ip     net.IP // the VIP address
	port   uint16 // the VIP port (TCP)
	url    string // assembled probe URL for http/https; empty for tcp
	client *http.Client
}

// probeResultMsg is the tea.Msg sent from probe goroutines to the UI
// on every probe completion. Bubbletea delivers it into Model.Update
// on the tea dispatch goroutine, so the model can mutate its per-VIP
// state without locks.
type probeResultMsg struct {
	VIPIdx   int
	At       time.Time
	Duration time.Duration
	OK       bool
	Code     int    // HTTP status code (0 for tcp-only probes)
	Header   string // extracted response header value, empty if absent
	Err      string // empty when OK; populated with a short error string otherwise
}

// hostnameMsg is the tea.Msg the DNS resolver worker sends once it
// has a PTR record for a VIP. The UI uses it to populate
// vipState.hostname so the 'd' toggle has something to show. One
// message per VIP at most — lookup failures just drop silently
// and the VIP stays on its IP literal.
type hostnameMsg struct {
	VIPIdx   int
	Hostname string
}

// paused is the global pause flag flipped by the spacebar binding in
// Model.Update. Using an atomic rather than a channel keeps the probe
// loop dead-simple — no extra select case, no risk of wedging a
// goroutine on a full buffered channel.
var paused atomic.Bool

// newHTTPClient builds an *http.Client with the transport configured
// the way maglevt wants its probes to behave. The two flags that
// actually matter here are:
//
//   - DisableKeepAlives = !opts.KeepAlive: off by default, because
//     failover testing relies on every probe opening a fresh TCP+TLS
//     connection. A single persistent connection would pin us to one
//     backend until the keep-alive timer expires, and the tally would
//     silently lie about which backend the load balancer is actually
//     steering new flows to. --keepalive/-k flips this back on for
//     users who explicitly want the pinned-session view.
//   - InsecureSkipVerify: on by default, matching `curl -k`. VIP
//     certificates almost never match the raw IP literal we're
//     targeting, so verification would fail on the first probe. Users
//     who want strict verification can pass --insecure=false.
//
// Redirects are refused (CheckRedirect returns ErrUseLastResponse) so
// a 302 from one backend doesn't silently advance to another VIP and
// pollute the tally.
func newHTTPClient(opts probeOpts) *http.Client {
	tr := &http.Transport{
		TLSClientConfig: &tls.Config{
			InsecureSkipVerify: opts.Insecure, //nolint:gosec // matching curl -k, see comment above
		},
		DisableKeepAlives: !opts.KeepAlive,
		// We dial the VIP literal directly — no DNS involvement —
		// so the default DialContext is already what we want.
	}
	return &http.Client{
		Transport: tr,
		Timeout:   opts.Timeout,
		CheckRedirect: func(*http.Request, []*http.Request) error {
			return http.ErrUseLastResponse
		},
	}
}

// runProbeLoop is the per-VIP probe worker. It waits an initial random
// delay (so N VIPs don't phase-lock onto the same tick after startup),
// then fires a probe every opts.Interval ± 10% jitter until ctx is
// cancelled. Each completed probe posts a probeResultMsg into the
// tea.Program via send.
//
// The loop honors the global `paused` flag by simply skipping the
// probe call while paused — the ticker keeps ticking so a resume
// picks up at the next natural tick boundary instead of fast-
// forwarding through a burst of back-to-back probes.
func runProbeLoop(ctx context.Context, vip *vipInfo, opts probeOpts, send func(tea.Msg)) {
	// Initial offset: uniformly in [0, interval) so N goroutines
	// started together spread out across one interval window rather
	// than all firing at t=0.
	select {
	case <-ctx.Done():
		return
	case <-time.After(time.Duration(rand.Int64N(int64(opts.Interval)))):
	}
	for {
		sleepFor := jitter(opts.Interval)
		if !paused.Load() {
			result := doProbe(ctx, vip, opts)
			send(result)
		}
		select {
		case <-ctx.Done():
			return
		case <-time.After(sleepFor):
		}
	}
}

// runDNSLookup does a single reverse-DNS (PTR) lookup for vip.ip
// and, on success, delivers a hostnameMsg to the UI. Runs once
// per VIP at startup and then exits — PTR records change rarely
// enough that we don't bother re-querying them, and the operator
// can always restart maglevt to pick up a new mapping.
//
// A 3-second timeout keeps a broken resolver from wedging the
// worker for the full life of the program; on timeout we just
// give up silently and the VIP displays its IP literal forever.
// Trailing dots are stripped so the rendered hostname matches
// what the operator typed in their zone file.
func runDNSLookup(parent context.Context, vip *vipInfo, send func(tea.Msg)) {
	ctx, cancel := context.WithTimeout(parent, 3*time.Second)
	defer cancel()
	names, err := net.DefaultResolver.LookupAddr(ctx, vip.ip.String())
	if err != nil || len(names) == 0 {
		return
	}
	send(hostnameMsg{
		VIPIdx:   vip.idx,
		Hostname: strings.TrimSuffix(names[0], "."),
	})
}

// jitter scales d by a uniform factor in [0.9, 1.1) — the same ±10%
// jitter the checker uses, same rationale: probes don't phase-lock on
// a wall-clock tick across every VIP in the config.
func jitter(d time.Duration) time.Duration {
	if d <= 0 {
		return d
	}
	return time.Duration(float64(d) * (0.9 + 0.2*rand.Float64()))
}

// doProbe issues one probe against vip and returns a filled-in
// probeResultMsg. HTTP / HTTPS go through vip.client with a GET
// request against opts.Path (default /.well-known/ipng/healthz),
// a Host-header override, and header extraction. Non-HTTP VIPs do
// a plain TCP connect — success if the three-way handshake
// completes before opts.Timeout.
//
// GET rather than HEAD: a common health-check path (healthz,
// status, /-/healthy) often returns 204 or 200 and is cheap to
// serve, but some handlers don't wire HEAD and would 405 us back.
// GET works against every reasonable implementation and the
// rolling-window latency is unchanged (we time until headers, not
// until the body completes — resp.Body.Close() discards the body
// without reading it). The defer below still closes the body so
// the transport can recycle the connection if --keepalive is on.
func doProbe(parent context.Context, vip *vipInfo, opts probeOpts) probeResultMsg {
	start := time.Now()
	result := probeResultMsg{VIPIdx: vip.idx, At: start}

	ctx, cancel := context.WithTimeout(parent, opts.Timeout)
	defer cancel()

	if vip.scheme == "tcp" {
		d := net.Dialer{Timeout: opts.Timeout}
		conn, err := d.DialContext(ctx, "tcp",
			net.JoinHostPort(vip.ip.String(), strconv.Itoa(int(vip.port))))
		result.Duration = time.Since(start)
		if err != nil {
			result.Err = shortError(err)
			return result
		}
		_ = conn.Close()
		result.OK = true
		return result
	}

	req, err := http.NewRequestWithContext(ctx, http.MethodGet, vip.url, nil)
	if err != nil {
		result.Duration = time.Since(start)
		result.Err = shortError(err)
		return result
	}
	// Host header: user override, else derive from the VIP itself
	// (which the kernel already put in the URL, so leaving req.Host
	// empty means "use the URL authority"). We only touch req.Host
	// when the operator explicitly passed --host.
	if opts.Host != "" {
		req.Host = opts.Host
	}

	resp, err := vip.client.Do(req)
	result.Duration = time.Since(start)
	if err != nil {
		result.Err = shortError(err)
		return result
	}
	defer func() { _ = resp.Body.Close() }()

	result.Code = resp.StatusCode
	result.OK = resp.StatusCode >= 200 && resp.StatusCode < 400
	if opts.Header != "" {
		result.Header = resp.Header.Get(opts.Header)
	}
	return result
}

// shortError collapses common Go net errors into a short string
// suitable for a narrow table cell. url.Error wrappings, dial
// contexts, and "i/o timeout" trailers all get trimmed so the LAST
// column shows something legible like "refused" / "timeout" /
// "no route" instead of a 120-char wrapped error.
func shortError(err error) string {
	if err == nil {
		return ""
	}
	msg := err.Error()
	// net/url-wrapped errors: keep only the last segment, which
	// holds the actual cause.
	if i := strings.LastIndex(msg, ": "); i >= 0 {
		msg = msg[i+2:]
	}
	// Normalise common kernel errnos and Go's wrappers.
	switch {
	case strings.Contains(msg, "connection refused"):
		return "refused"
	case strings.Contains(msg, "i/o timeout") || strings.Contains(msg, "context deadline exceeded"):
		return "timeout"
	case strings.Contains(msg, "no route to host"):
		return "no-route"
	case strings.Contains(msg, "network is unreachable"):
		return "net-unrch"
	case strings.Contains(msg, "host is unreachable"):
		return "host-unrch"
	case strings.Contains(msg, "connection reset"):
		return "reset"
	case strings.Contains(msg, "EOF"):
		return "eof"
	case strings.Contains(strings.ToLower(msg), "tls"):
		return "tls-err"
	}
	// Last resort: truncate anything longer than the LAST column.
	if len(msg) > 8 {
		msg = msg[:8]
	}
	return msg
}