package link

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log"
	"regexp"
	"strconv"
	"sync"
	"time"

	"github.com/nats-io/nats.go"
)

// A role's tool call, kept after it is answered (novox/hq issue 265).
//
// **A call that outlasts its caller must not end in silence.** A caller waits a bounded time (the
// console thirty seconds), and the bus lets a holder answer a request exactly once and only while the
// server still remembers it was asked (`allow_responses`). Before this, a push that ran longer than
// its caller waited did everything it was asked and its caller read "did not answer in time"; and a
// push whose first act sent the bus's own machine a changed user list had its answer refused
// outright — a broker reloading its users forgets every reply it was about to permit — so the
// answer went nowhere and the only trace was the client library's line on the controller's standard
// error. So every call is answered within AnswerWithin, with its whole answer when it has one by
// then and with "still running as call <id>" when it does not; and every call is kept here, with
// what came of it, including an answer the bus refused, so `calls` can say it.

// AnswerWithin is how long a call runs before its caller is answered that it is still running. Well
// inside the shortest wait of a caller the mesh ships (the console's thirty seconds) and the bus's
// own window for an answer (broker.ResponseTTL), so the one answer a call has is never late for
// either. A variable so a test need not wait.
var AnswerWithin = 10 * time.Second

// KeptCalls is how many calls are kept, newest first; keptAnswer the largest answer kept of a call
// whose caller was sent it. One its caller never had is kept whole.
const (
	KeptCalls  = 100
	keptAnswer = 64 << 10
)

// The states a kept call is in.
const (
	CallRunning  = "running"
	CallAnswered = "answered"
	// CallFinishedAfter is a call that finished after its caller was told it was still running: its
	// answer is here and nowhere else.
	CallFinishedAfter = "finished after its caller was answered"
)

// Call is one call of a role's tool, as `calls` shows it.
type Call struct {
	ID       string          `json:"call"`
	Seat     string          `json:"seat"`
	Verb     string          `json:"verb"`
	Args     json.RawMessage `json:"arguments,omitempty"`
	Started  time.Time       `json:"started"`
	Finished *time.Time      `json:"finished,omitempty"`
	State    string          `json:"state"`
	// Failed is the call's own answer being an error — an answer, not a timeout.
	Failed bool `json:"failed,omitempty"`
	// Answer is what the call answered, or would have: kept for a call whose caller did not get it.
	Answer json.RawMessage `json:"answer,omitempty"`
	// Refused is the bus refusing the answer this holder sent: the caller got nothing, and this is
	// the only place that says what it would have.
	Refused string `json:"answer refused by the bus,omitempty"`

	reply string // the subject the answer went to, which the bus names when it refuses it
}

// CallLog keeps the latest calls a holder served.
type CallLog struct {
	mu    sync.Mutex
	calls []*Call // oldest first
	next  uint64
	now   func() time.Time
	// Follow is the tool that reads this log back, named in a running answer — set by a holder that
	// serves one (the controller's `calls`); without it, the answer points at the holder's journal.
	Follow string
}

// Calls is this process's log: one holder process serves its seats on one connection.
var Calls = NewCallLog()

func NewCallLog() *CallLog { return &CallLog{now: time.Now} }

func (l *CallLog) begin(seat, verb string, args json.RawMessage, reply string) *Call {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.next++
	c := &Call{ID: "call-" + strconv.FormatInt(l.now().UnixNano(), 10) + "-" + strconv.FormatUint(l.next, 10),
		Seat: seat, Verb: verb, Args: args, Started: l.now(), State: CallRunning, reply: reply}
	l.calls = append(l.calls, c)
	if len(l.calls) > KeptCalls {
		l.calls = l.calls[len(l.calls)-KeptCalls:]
	}
	return c
}

// finish records a call's answer; answeredAlready is its caller having been told it was running.
func (l *CallLog) finish(c *Call, answer []byte, failed, answeredAlready bool) {
	l.mu.Lock()
	defer l.mu.Unlock()
	at := l.now()
	c.Finished = &at
	c.Failed = failed
	c.Answer = append(json.RawMessage(nil), answer...)
	if !answeredAlready && len(answer) > keptAnswer {
		// Its caller has it; kept only in case the bus refuses it, and a whole declaration a
		// hundred times over is memory nobody asked for.
		c.Answer, _ = json.Marshal(map[string]any{"not kept": fmt.Sprintf(
			"an answer of %d bytes, sent to its caller in full", len(answer))})
	}
	if answeredAlready {
		c.State = CallFinishedAfter
	} else {
		c.State = CallAnswered
	}
}

// Recent is the kept calls, newest first, as copies.
func (l *CallLog) Recent() []Call {
	l.mu.Lock()
	defer l.mu.Unlock()
	out := make([]Call, 0, len(l.calls))
	for i := len(l.calls) - 1; i >= 0; i-- {
		out = append(out, *l.calls[i])
	}
	return out
}

// Get is one kept call.
func (l *CallLog) Get(id string) (Call, bool) {
	l.mu.Lock()
	defer l.mu.Unlock()
	for _, c := range l.calls {
		if c.ID == id {
			return *c, true
		}
	}
	return Call{}, false
}

// kept is what a call's arguments are kept as: each argument by name, a value only when it is short
// and not one that carries settings or a secret — `calls` answers anyone who may call the seat.
func kept(args json.RawMessage) json.RawMessage {
	var given map[string]any
	if json.Unmarshal(args, &given) != nil {
		return nil
	}
	out := map[string]any{}
	for k, v := range given {
		s, isString := v.(string)
		switch {
		case k == "values" || k == "secret":
			out[k] = "(given, not kept)"
		case isString && len(s) <= 120:
			out[k] = s
		case isString:
			out[k] = s[:120] + "…"
		default:
			out[k] = v
		}
	}
	body, _ := json.Marshal(out)
	return body
}

// refusedPublish is the bus's words for a publish it refused, with the subject.
var refusedPublish = regexp.MustCompile(`Permissions Violation for Publish to "([^"]+)"`)

// Refusal records the bus refusing an answer, from the error the client library hands the
// connection's error handler. It reports whether the error was the refusal of a kept call's answer.
func (l *CallLog) Refusal(err error, logger *log.Logger) bool {
	if err == nil {
		return false
	}
	m := refusedPublish.FindStringSubmatch(err.Error())
	if m == nil {
		return false
	}
	l.mu.Lock()
	var hit *Call
	for i := len(l.calls) - 1; i >= 0; i-- {
		if c := l.calls[i]; c.reply != "" && c.reply == m[1] {
			hit = c
			break
		}
	}
	if hit == nil {
		l.mu.Unlock()
		return false
	}
	at := l.now()
	hit.Refused = fmt.Sprintf("%s: %s", at.Format(time.RFC3339), err)
	id, verb, took := hit.ID, hit.Seat+"."+hit.Verb, at.Sub(hit.Started).Round(time.Second)
	l.mu.Unlock()
	if logger != nil {
		// Said in the mesh's words, beside the library's own line: which call, and where its answer is.
		logger.Printf("the bus refused the answer to %s (%s, asked %s ago): its caller got no answer. "+
			"What it answered is kept under %s. A broker reloading its user list while a call runs "+
			"forgets that it may be answered", id, verb, took, id)
	}
	return true
}

// WatchRefusals chains the connection's error handler so a refused answer is recorded against its
// call. The handler the dialler set — the library's default, which prints — still runs after it.
func (l *CallLog) WatchRefusals(conn *nats.Conn, logger *log.Logger) {
	if conn == nil {
		return
	}
	watched.Lock()
	defer watched.Unlock()
	if watched.conns == nil {
		watched.conns = map[*nats.Conn]bool{}
	}
	if watched.conns[conn] {
		return
	}
	watched.conns[conn] = true
	before := conn.ErrorHandler()
	conn.SetErrorHandler(func(c *nats.Conn, s *nats.Subscription, err error) {
		if errors.Is(err, nats.ErrPermissionViolation) {
			l.Refusal(err, logger)
		}
		if before != nil {
			before(c, s, err)
		}
	})
}

var watched struct {
	sync.Mutex
	conns map[*nats.Conn]bool
}

// answerNow is how a handler asks for its caller to be answered before it goes on (Acknowledge).
type answerNowKey struct{}

// Acknowledge answers the call's caller now that it is running, rather than after AnswerWithin. For
// a handler about to do what makes its answer unsendable: a push sends the bus's own machine first,
// and a broker reloading its user list forgets every answer it was about to permit. Without effect
// outside a served call, and after the first time.
func Acknowledge(ctx context.Context) {
	if f, ok := ctx.Value(answerNowKey{}).(func()); ok {
		f()
	}
}

// running is the interim answer: what a caller reads when the call has not finished.
func running(c *Call, within time.Duration, acknowledged bool, follow string) []byte {
	why := fmt.Sprintf("it has not finished in %s", within)
	if acknowledged {
		why = "it answers before it starts, because what it does can leave the bus unable to carry a later answer"
	}
	next := "its holder's journal says how it ended."
	if follow != "" {
		next = fmt.Sprintf("%s with call %q says what came of it.", follow, c.ID)
	}
	said := fmt.Sprintf("%s.%s is running as %s — %s. This is not a failure, and it may already have "+
		"done what was asked: %s", c.Seat, c.Verb, c.ID, why, next)
	body, _ := json.Marshal(map[string]any{"result": map[string]any{
		"running": true, "call": c.ID, "started": c.Started, "output": said,
	}})
	return body
}

// serveCall runs one call and answers it once: in full when the handler returns within AnswerWithin
// and has not acknowledged, and otherwise that it is running — its answer then kept, and logged.
func (l *CallLog) serveCall(seat, verb string, args json.RawMessage, reply string, handle ToolHandler,
	respond func([]byte) error, logger *log.Logger) {
	ctx, cancel := context.WithTimeout(context.Background(), HandlerTimeout)
	defer cancel()
	if len(args) == 0 {
		args = json.RawMessage(`{}`)
	}
	c := l.begin(seat, verb, kept(args), reply)
	acknowledged := make(chan struct{})
	var once sync.Once
	ctx = context.WithValue(ctx, answerNowKey{}, func() { once.Do(func() { close(acknowledged) }) })

	type outcome struct {
		body   []byte
		failed bool
	}
	done := make(chan outcome, 1)
	go func() {
		var body []byte
		result, err := handle(ctx, args)
		failed := err != nil
		if err != nil {
			body, _ = json.Marshal(map[string]any{"error": err.Error()})
		} else if body, err = json.Marshal(map[string]any{"result": result}); err != nil {
			failed = true
			body, _ = json.Marshal(map[string]any{"error": "the answer could not be written as JSON: " + err.Error()})
		}
		done <- outcome{body, failed}
	}()

	say := func(body []byte) {
		if err := respond(body); err != nil && logger != nil {
			logger.Printf("%s.%s: could not answer %s: %v", seat, verb, c.ID, err)
		}
	}
	timer := time.NewTimer(AnswerWithin)
	defer timer.Stop()
	select {
	case o := <-done:
		l.finish(c, o.body, o.failed, false)
		say(o.body)
		return
	case <-acknowledged:
		say(running(c, AnswerWithin, true, l.Follow))
	case <-timer.C:
		say(running(c, AnswerWithin, false, l.Follow))
	}
	o := <-done
	l.finish(c, o.body, o.failed, true)
	if logger != nil {
		how := "and it succeeded"
		if o.failed {
			how = "and it answered an error"
		}
		logger.Printf("%s (%s.%s) finished after %s, after its caller was told it was running, %s — its answer is kept under %s", c.ID,
			seat, verb, time.Since(c.Started).Round(time.Second), how, c.ID)
	}
}