// Package cal serves a calendar as a file tree.
//
// The tree is the interface and it is the same for every backend:
//
//	ctl		read: state. write: refresh, window <days>
//	query		write a query, read the answer, as with cs(8)
//	alarm		blocking read; one line per alarm due
//	changed		blocking read; one line per rebuild
//	events/date/yyyy/mm/dd/hhmm-summary
//	events/uuid/<uid>/...
//
// A backend supplies events and, where its protocol allows, takes
// changes back. Everything else lives here.
package cal

import (
	"bufio"
	"bytes"
	"crypto/rand"
	"encoding/hex"
	"fmt"
	"os"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/emersion/go-ical"
	"github.com/knusbaum/go9p"
	"github.com/knusbaum/go9p/fs"
)

// Argv0 prefixes diagnostics. A command sets it to its own name.
var Argv0 = "cal"

func Warnf(format string, a ...interface{}) {
	fmt.Fprintf(os.Stderr, "%s: %s\n", Argv0, fmt.Sprintf(format, a...))
}

// Server presents one calendar.
type Server struct {
	be Backend

	fsys *fs.FS
	root *fs.StaticDir
	user string

	window time.Duration

	// /srv records only a name, an owner and a mode, so a server that
	// wants to be identifiable has to say so itself.
	srv     string
	conf    string
	pid     int
	wd      string
	started time.Time

	mu      sync.Mutex
	evs     []*Event
	evdir   map[*Event]string // event -> its directory name under uuid/
	index   []index           // every published occurrence, for query
	gen     int
	alarm   fs.Stream
	changed fs.Stream
}

// Config is what a command must decide and the library will not.
type Config struct {
	User   string
	Srv    string        // service name posted in /srv
	Conf   string        // path of the config file, for ctl
	Window time.Duration // how far either side of now to expand
}

// New builds a server over a backend, without serving it.
func New(be Backend, c Config) *Server {
	fsys, root := fs.NewFS(c.User, c.User, 0555)
	s := &Server{
		be: be, fsys: fsys, root: root, user: c.User,
		window: c.Window, srv: c.Srv, conf: c.Conf,
		pid: os.Getpid(), started: time.Now(),
	}
	s.wd, _ = os.Getwd()
	s.addCtl()
	s.addAlarm()
	s.addQuery()
	s.addChanged()
	s.addNew()
	return s
}

// Serve syncs once, publishes the tree, and keeps it current for as long
// as the process runs. Nothing outside ever asks for a reload; watchers
// read changed instead.
func (s *Server) Serve() error {
	if _, err := s.be.Sync(); err != nil {
		Warnf("%s: %v", s.be.Name(), err)
	}
	if err := s.Reload(); err != nil {
		return err
	}
	if d := s.be.Refresh(); d > 0 {
		go s.poll(d)
	}
	Warnf("serving /srv/%s", s.srv)
	return go9p.PostSrv(s.srv, s.fsys.Server())
}

// Report loads and says what was found, without serving.
func (s *Server) Report() error {
	if _, err := s.be.Sync(); err != nil {
		return err
	}
	t := time.Now()
	evs, err := s.be.Events()
	if err != nil {
		return err
	}
	load := time.Since(t)

	t = time.Now()
	now := time.Now()
	insts := expand(evs, now.Add(-s.window), now.Add(s.window))
	exp := time.Since(t)

	var recur, alarms int
	for _, e := range evs {
		if e.set != nil {
			recur++
		}
		alarms += len(e.Alarms)
	}
	fmt.Printf("calendar    %s\n", s.be.Name())
	fmt.Printf("caps        %s\n", s.be.Caps())
	fmt.Printf("events      %d\n", len(evs))
	fmt.Printf("recurring   %d\n", recur)
	fmt.Printf("alarms      %d\n", alarms)
	fmt.Printf("instances   %d  (window +/-%d days)\n",
		len(insts), int(s.window/(24*time.Hour)))
	fmt.Printf("load        %v\n", load.Round(time.Millisecond))
	fmt.Printf("expand      %v\n", exp.Round(time.Millisecond))
	return nil
}

func (s *Server) poll(every time.Duration) {
	for {
		time.Sleep(every)
		changed, err := s.be.Sync()
		if err != nil {
			Warnf("%s: %v", s.be.Name(), err)
			continue
		}
		if !changed {
			continue // a poll that changes nothing wakes nobody
		}
		if err := s.Reload(); err != nil {
			Warnf("%s: reload: %v", s.be.Name(), err)
		}
	}
}

// Reload rebuilds the whole tree from the backend's current events.
func (s *Server) Reload() error {
	evs, err := s.be.Events()
	if err != nil {
		return err
	}
	now := time.Now()
	insts := expand(evs, now.Add(-s.window), now.Add(s.window))

	s.mu.Lock()
	s.evs = evs
	s.index = nil
	s.gen++
	gen := s.gen
	s.mu.Unlock()

	s.evdir = nil
	s.root.DeleteChild("events")
	s.buildEvents(s.root, evs)
	s.buildWhen(s.root, insts)

	go s.schedule(gen, insts)
	Warnf("loaded %d events, %d instances", len(evs), len(insts))
	if s.changed != nil {
		// wake anything watching, so a gui knows to re-walk
		s.changed.Write([]byte(fmt.Sprintf("reload %d events %d instances %d\n",
			gen, len(evs), len(insts))))
	}
	return nil
}

func (s *Server) addCtl() {
	st := s.fsys.NewStat("ctl", s.user, s.user, 0666)
	base := fs.NewStaticFile(st, []byte(""))
	s.root.AddChild(&fs.WrappedFile{
		File: base,
		ReadF: func(fid uint64, off uint64, count uint64) ([]byte, error) {
			b := []byte(s.ctlText())
			if off >= uint64(len(b)) {
				return []byte{}, nil
			}
			end := off + count
			if end > uint64(len(b)) {
				end = uint64(len(b))
			}
			return b[off:end], nil
		},
		WriteF: func(fid uint64, off uint64, data []byte) (uint32, error) {
			if err := s.control(string(data)); err != nil {
				return 0, err
			}
			return uint32(len(data)), nil
		},
	})
}

func (s *Server) ctlText() string {
	s.mu.Lock()
	n := len(s.evs)
	s.mu.Unlock()

	text := fmt.Sprintf(
		"srv /srv/%s\npid %d\nuser %s\nstarted %s\n"+
			"args %s\nwd %s\nconfig %s\n"+
			"window %d\nevents %d\ncaps %s\n",
		s.srv, s.pid, s.user, s.started.Format(time.RFC3339),
		strings.Join(os.Args, " "), s.wd, orNone(s.conf),
		int(s.window/(24*time.Hour)), n, s.be.Caps())

	if d, ok := s.be.(Describer); ok {
		text += d.Describe()
	}
	when, err := s.be.Status()
	line := fmt.Sprintf("cal %s refresh %v fetched %s",
		s.be.Name(), s.be.Refresh(), when.Format(time.RFC3339))
	if err != "" {
		line += " error " + err
	}
	return text + line + "\n"
}

func (s *Server) control(cmd string) error {
	f := strings.Fields(cmd)
	if len(f) == 0 {
		return nil
	}
	switch f[0] {
	case "refresh":
		if _, err := s.be.Sync(); err != nil {
			return err
		}
		return s.Reload()
	case "window":
		if len(f) != 2 {
			return fmt.Errorf("usage: window days")
		}
		n, err := strconv.Atoi(f[1])
		if err != nil || n <= 0 {
			return fmt.Errorf("bad window %q", f[1])
		}
		s.mu.Lock()
		s.window = time.Duration(n) * 24 * time.Hour
		s.mu.Unlock()
		return s.Reload()
	}
	return fmt.Errorf("unknown command %q", f[0])
}

// addAlarm creates the blocking alarm file. A read blocks until the next
// alarm is due; every reader gets every alarm.
func (s *Server) addAlarm() {
	stream := fs.NewBlockingStream(8)
	st := s.fsys.NewStat("alarm", s.user, s.user, 0444)
	s.alarm = stream
	s.root.AddChild(fs.NewStreamFile(st, stream))
}

// addChanged creates the changed file. A read blocks until the tree has
// been rebuilt, so a gui learns to walk it again without polling.
func (s *Server) addChanged() {
	stream := fs.NewBlockingStream(8)
	st := s.fsys.NewStat("changed", s.user, s.user, 0444)
	s.changed = stream
	s.root.AddChild(fs.NewStreamFile(st, stream))
}

type firing struct {
	at   time.Time
	inst Instance
}

// schedule fires the alarms for one generation of the tree, and exits as
// soon as a later reload has bumped the generation.
func (s *Server) schedule(gen int, insts []Instance) {
	now := time.Now()
	var fs_ []firing
	for _, in := range insts {
		for _, d := range in.Ev.Alarms {
			at := in.Start.Add(d)
			if at.After(now) {
				fs_ = append(fs_, firing{at, in})
			}
		}
	}
	sort.Slice(fs_, func(i, j int) bool { return fs_[i].at.Before(fs_[j].at) })

	for _, f := range fs_ {
		if d := time.Until(f.at); d > 0 {
			time.Sleep(d)
		}
		s.mu.Lock()
		stale := gen != s.gen
		s.mu.Unlock()
		if stale {
			return
		}
		s.alarm.Write([]byte(fmt.Sprintf("%s\t%s\t%s\n",
			f.at.Format(time.RFC3339),
			f.inst.Start.Format(time.RFC3339),
			f.inst.Ev.Summary)))
	}
}

func orNone(s string) string {
	if s == "" {
		return "(none)"
	}
	return s
}

// addNew publishes the file you write an event to.
//
//	cat invite.ics >/mnt/pim/calendars/work/new
//
// Writes are buffered and the object is created on close, because that
// is when the thing you are writing is complete -- cp and cat both do
// create, write, clunk, so the commit point falls out of 9p rather than
// having to be invented.
func (s *Server) addNew() {
	create, ok := s.be.(Creator)
	if !ok {
		return // a backend that cannot add events does not offer the file
	}
	var mu sync.Mutex
	buf := map[uint64][]byte{}

	st := s.fsys.NewStat("new", s.user, s.user, 0222)
	base := fs.NewStaticFile(st, []byte(""))
	s.root.AddChild(&fs.WrappedFile{
		File: base,
		WriteF: func(fid uint64, off uint64, data []byte) (uint32, error) {
			mu.Lock()
			buf[fid] = append(buf[fid], data...)
			mu.Unlock()
			return uint32(len(data)), nil
		},
		CloseF: func(fid uint64) error {
			mu.Lock()
			b := buf[fid]
			delete(buf, fid)
			mu.Unlock()
			if len(b) == 0 {
				return nil
			}
			// Either a whole icalendar object, or the key: value form
			// the tree itself emits. The second exists so a script
			// never has to spell iCalendar, where a comma in a summary
			// is a syntax error waiting to happen.
			var c *ical.Calendar
			var err error
			if bytes.HasPrefix(bytes.TrimSpace(b), []byte("BEGIN:VCALENDAR")) {
				c, err = ical.NewDecoder(bytes.NewReader(b)).Decode()
			} else {
				c, err = Compose(bufio.NewReader(bytes.NewReader(b)), newUID())
			}
			if err != nil {
				Warnf("new: %v", err)
				return err
			}
			if len(c.Events()) == 0 {
				Warnf("new: no VEVENT")
				return fmt.Errorf("new: no VEVENT")
			}
			if err := create.Create(c); err != nil {
				Warnf("new: %v", err)
				return err
			}
			if _, err := s.be.Sync(); err == nil {
				s.Reload()
			}
			return nil
		},
	})
}

// newUID mints an identifier for an event we are creating. This is the
// one place a UID is generated rather than echoed back: as organiser we
// own the event, so we name it.
func newUID() string {
	var b [16]byte
	if _, err := rand.Read(b[:]); err != nil {
		return fmt.Sprintf("%d@pim", time.Now().UnixNano())
	}
	host, _ := os.Hostname()
	if host == "" {
		host = "pim"
	}
	return hex.EncodeToString(b[:]) + "@" + host
}