// Dashboard collectors (phase 2 + 3 of plan/23-monitor.md).
//
// Every tile is fed by its own independently-cached collector. That isolation
// is the whole design: mc-status being slow, the backups dir being unreadable,
// or RCON refusing a connection must degrade exactly one tile and never block
// the summary response or the log stream. On error a collector keeps serving
// its last good value and reports the error alongside it — stale-with-a-reason
// beats a blank page, and beats a spinner that never resolves.
package main

import (
	"encoding/json"
	"net/http"
	"os"
	"regexp"
	"strings"
	"sync"
	"syscall"
	"time"
)

// --- TTL cache -------------------------------------------------------------

// cached memoizes one collector for ttl. Zero value is not usable; see newCached.
type cached[T any] struct {
	mu      sync.Mutex
	val     T
	have    bool
	lastErr string
	expires time.Time

	ttl time.Duration
	fn  func() (T, error)
}

func newCached[T any](ttl time.Duration, fn func() (T, error)) *cached[T] {
	return &cached[T]{ttl: ttl, fn: fn}
}

// get returns the cached value (refreshing if stale) and the last error, if any.
// It never returns an error *instead* of a value — a failed refresh yields the
// previous value plus the error text.
func (c *cached[T]) get() (T, string) {
	c.mu.Lock()
	defer c.mu.Unlock()

	if c.have && time.Now().Before(c.expires) {
		return c.val, c.lastErr
	}
	v, err := c.fn()
	c.expires = time.Now().Add(c.ttl)
	if err != nil {
		c.lastErr = err.Error()
		return c.val, c.lastErr // stale-or-zero, with the reason
	}
	c.val, c.have, c.lastErr = v, true, ""
	return c.val, ""
}

// --- server status + rosters (via mc-status over mcnet) --------------------

// The monitor deliberately does NOT hold Drasl admin credentials. mc-status
// already logs in server-side and exposes a sanitized [{uuid,name}], so we read
// its output instead of duplicating the secret. See plan/15-mc-status.md.

type mcPlayer struct {
	UUID string `json:"uuid"`
	Name string `json:"name"`
}

type statusInfo struct {
	Online  bool       `json:"online"`
	Count   int64      `json:"count"`
	Max     int64      `json:"max"`
	MOTD    string     `json:"motd"`
	Version string     `json:"version"`
	Players []mcPlayer `json:"players"`
}

// v2 subset of the mcstatus.io shape mc-status re-emits.
type v2Wire struct {
	Online  bool `json:"online"`
	Players struct {
		Online int64 `json:"online"`
		Max    int64 `json:"max"`
		List   []struct {
			UUID      string `json:"uuid"`
			NameClean string `json:"name_clean"`
		} `json:"list"`
	} `json:"players"`
	MOTD struct {
		Clean string `json:"clean"`
	} `json:"motd"`
	Version struct {
		NameClean string `json:"name_clean"`
	} `json:"version"`
}

func fetchStatus(client *http.Client, base string) (statusInfo, error) {
	var wire v2Wire
	// The address segment is ignored by mc-status (it only ever pings its own
	// configured target) — any placeholder works.
	if err := getJSON(client, base+"/v2/status/java/self", &wire); err != nil {
		return statusInfo{}, err
	}
	out := statusInfo{
		Online:  wire.Online,
		Count:   wire.Players.Online,
		Max:     wire.Players.Max,
		MOTD:    wire.MOTD.Clean,
		Version: wire.Version.NameClean,
	}
	for _, p := range wire.Players.List {
		out.Players = append(out.Players, mcPlayer{UUID: p.UUID, Name: p.NameClean})
	}
	return out, nil
}

func fetchRegistered(client *http.Client, base string) ([]mcPlayer, error) {
	var players []mcPlayer
	if err := getJSON(client, base+"/players", &players); err != nil {
		return nil, err
	}
	return players, nil
}

func getJSON(client *http.Client, url string, dst any) error {
	resp, err := client.Get(url)
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return &statusCodeError{url: url, code: resp.StatusCode}
	}
	return json.NewDecoder(resp.Body).Decode(dst)
}

type statusCodeError struct {
	url  string
	code int
}

func (e *statusCodeError) Error() string {
	return e.url + ": HTTP " + itoa(e.code)
}

func itoa(n int) string {
	if n == 0 {
		return "0"
	}
	var b [12]byte
	i := len(b)
	for n > 0 {
		i--
		b[i] = byte('0' + n%10)
		n /= 10
	}
	return string(b[i:])
}

// --- disk ------------------------------------------------------------------

type diskInfo struct {
	Path       string  `json:"path"`
	TotalBytes uint64  `json:"total_bytes"`
	FreeBytes  uint64  `json:"free_bytes"`
	UsedPct    float64 `json:"used_pct"`
}

// diskUsage statfs's the world volume. This reports the underlying host
// filesystem, which is exactly what matters: a full disk is the failure that
// actually takes the server down, and backups (15G+ and growing) share it.
func diskUsage(path string) (diskInfo, error) {
	var st syscall.Statfs_t
	if err := syscall.Statfs(path, &st); err != nil {
		return diskInfo{Path: path}, err
	}
	bs := uint64(st.Bsize)
	total := st.Blocks * bs
	free := st.Bavail * bs // Bavail, not Bfree — exclude root-reserved blocks
	info := diskInfo{Path: path, TotalBytes: total, FreeBytes: free}
	if total > 0 {
		info.UsedPct = float64(total-free) / float64(total) * 100
	}
	return info, nil
}

// --- backups ---------------------------------------------------------------

type backupInfo struct {
	Newest     string `json:"newest"`
	SizeBytes  int64  `json:"size_bytes"`
	Mtime      int64  `json:"mtime"`
	AgeSeconds int64  `json:"age_seconds"`
	Count      int    `json:"count"`
	TotalBytes int64  `json:"total_bytes"`
}

// backupStatus finds the newest archive written by mc-backup (world-*.tar.gz).
// Age is the number that matters — "backups exist" is not the same claim as
// "a backup ran recently", and only the second one is worth a tile.
func backupStatus(dir string) (backupInfo, error) {
	entries, err := os.ReadDir(dir)
	if err != nil {
		return backupInfo{}, err
	}
	var out backupInfo
	var newest time.Time
	for _, e := range entries {
		if e.IsDir() || !strings.HasSuffix(e.Name(), ".tar.gz") {
			continue
		}
		fi, err := e.Info()
		if err != nil {
			continue
		}
		out.Count++
		out.TotalBytes += fi.Size()
		if fi.ModTime().After(newest) {
			newest = fi.ModTime()
			out.Newest = e.Name()
			out.SizeBytes = fi.Size()
		}
	}
	if out.Count > 0 {
		out.Mtime = newest.Unix()
		out.AgeSeconds = int64(time.Since(newest).Seconds())
	}
	return out, nil
}

// --- session history (derived from the log we already tail) ----------------

type sessionEvent struct {
	Time   string `json:"time"`
	Player string `json:"player"`
	Action string `json:"action"` // "joined" | "left"
}

// Matches the message tail of e.g.
//
//	[01Aug2026 22:55:34.358] [Server thread/INFO] [net.minecraft...]: Oier joined the game
var sessionRe = regexp.MustCompile(`]: (\S+) (joined|left) the game\b`)

// leading [timestamp] of a log line, whatever its format
var stampRe = regexp.MustCompile(`^\[([^\]]+)\]`)

// events is a bounded history of joins/leaves, filled by the tailer as lines
// arrive. Free: the bytes are already being read for the log stream. It also
// means the cold-start backfill populates history without any persistence.
type events struct {
	mu  sync.Mutex
	buf []sessionEvent
	max int
}

func newEvents(max int) *events { return &events{max: max} }

func (e *events) observe(line string) {
	m := sessionRe.FindStringSubmatch(line)
	if m == nil {
		return
	}
	ev := sessionEvent{Player: m[1], Action: m[2]}
	if ts := stampRe.FindStringSubmatch(line); ts != nil {
		ev.Time = ts[1]
	}

	e.mu.Lock()
	defer e.mu.Unlock()
	e.buf = append(e.buf, ev)
	if len(e.buf) > e.max {
		e.buf = e.buf[len(e.buf)-e.max:]
	}
}

// recent returns up to n events, newest first.
func (e *events) recent(n int) []sessionEvent {
	e.mu.Lock()
	defer e.mu.Unlock()
	out := make([]sessionEvent, 0, n)
	for i := len(e.buf) - 1; i >= 0 && len(out) < n; i-- {
		out = append(out, e.buf[i])
	}
	return out
}

// --- assembled summary -----------------------------------------------------

type summary struct {
	Server     statusInfo     `json:"server"`
	ServerErr  string         `json:"server_err,omitempty"`
	Registered int            `json:"registered"`
	RosterErr  string         `json:"roster_err,omitempty"`
	Disk       diskInfo       `json:"disk"`
	DiskErr    string         `json:"disk_err,omitempty"`
	Backup     backupInfo     `json:"backup"`
	BackupErr  string         `json:"backup_err,omitempty"`
	TPS        *tpsInfo       `json:"tps"`
	TPSErr     string         `json:"tps_err,omitempty"`
	Sessions   []sessionEvent `json:"sessions"`
	AvatarURL  string         `json:"avatar_url"`
	AvatarMode string         `json:"avatar_mode"`
	Now        int64          `json:"now"`
}

// collectors owns every tile source. Assembled once in main and shared.
type collectors struct {
	status     *cached[statusInfo]
	registered *cached[[]mcPlayer]
	disk       *cached[diskInfo]
	backup     *cached[backupInfo]
	tps        *cached[*tpsInfo]
	events     *events

	avatarURL  string
	avatarMode string
}

func (c *collectors) summary() summary {
	// Refresh concurrently. Sequentially, a summary during an outage would cost
	// the SUM of every timeout (~15s with mc-status hanging and RCON blackholed)
	// — and an outage is exactly when someone is staring at this page. Each
	// cached has its own mutex, so this is just fan-out.
	var (
		wg         sync.WaitGroup
		status     statusInfo
		statusErr  string
		registered []mcPlayer
		rosterErr  string
		disk       diskInfo
		diskErr    string
		backup     backupInfo
		backupErr  string
		tps        *tpsInfo
		tpsErr     string
	)
	wg.Add(5)
	go func() { defer wg.Done(); status, statusErr = c.status.get() }()
	go func() { defer wg.Done(); registered, rosterErr = c.registered.get() }()
	go func() { defer wg.Done(); disk, diskErr = c.disk.get() }()
	go func() { defer wg.Done(); backup, backupErr = c.backup.get() }()
	go func() { defer wg.Done(); tps, tpsErr = c.tps.get() }()
	wg.Wait()

	return summary{
		Server:     status,
		ServerErr:  statusErr,
		Registered: len(registered),
		RosterErr:  rosterErr,
		Disk:       disk,
		DiskErr:    diskErr,
		Backup:     backup,
		BackupErr:  backupErr,
		TPS:        tps,
		TPSErr:     tpsErr,
		Sessions:   c.events.recent(40),
		AvatarURL:  c.avatarURL,
		AvatarMode: c.avatarMode,
		Now:        time.Now().Unix(),
	}
}