package link

import (
	"context"
	"encoding/json"
	"fmt"
	"log"
	"strings"

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

// mesh-cli asks the controller through the node-engine of the machine it runs on (novox/hq ADR 0272 §3). The engine
// reads the asking account from the kernel and asks on its own node's subject, which only that node's engine may
// publish (its grant is `mesh.control.<node>.>`): so the node is a fact the bus server enforces, and the account a
// fact the kernel gave. The engine's side holds the same field names (mesh-host internal/meshcli, a test on each
// side), as for its health statement.

// CLISubjects is every node's mesh-cli subject, which the serving controller answers.
const CLISubjects = "mesh.control.*.cli"

// CLISubject is one node's.
func CLISubject(node string) string { return "mesh.control." + node + ".cli" }

// CLISeat is what a mesh-cli line is recorded under in the calls record, beside the seats' verbs: its verb there is
// the node it was asked on.
const CLISeat = "mesh-cli"

// CLIAtOnce bounds the mesh-cli lines running at once, from every node together. A person types one at a time; one
// over the bound is answered busy at once, and nothing runs.
var CLIAtOnce = 8

// CLIAsked is a line mesh-cli was given on a node, and the account the node-engine says is asking — or, with
// Follow, the call a line runs as, asked again by the same account on the same node until it ends.
type CLIAsked struct {
	Line    []string `json:"line,omitempty"`
	Account string   `json:"account"`
	UID     uint32   `json:"uid"`
	Follow  string   `json:"follow,omitempty"`
	// Session is the login session the asking process runs in, as the node-engine read it from the kernel's cgroup
	// (`session-<id>.scope` under the account's own slice), or empty: a service, a user unit. Only a login session is
	// the terminal (novox/hq ADR 0272).
	Session string `json:"session,omitempty"`
}

// CLIAnswer is what the controller answers, as the `result` of a call's answer: what the command printed, how it
// exited, whether it ran as the controller's terminal and why, or why nothing ran. A line still running is answered
// as every call is (`running`, `call`), and followed.
type CLIAnswer struct {
	Stdout   []byte `json:"stdout,omitempty"`
	Stderr   []byte `json:"stderr,omitempty"`
	Exit     int    `json:"exit"`
	Terminal bool   `json:"terminal"`
	Why      string `json:"why,omitempty"`
	Refused  string `json:"refused,omitempty"`
	Cut      bool   `json:"cut,omitempty"`
}

// CLIRefusal is an answer saying nothing ran, and why.
func CLIRefusal(why string) CLIAnswer { return CLIAnswer{Exit: 1, Refused: why} }

// CLINode is the node a mesh-cli subject names, or false for any other subject.
func CLINode(subject string) (string, bool) {
	rest, ok := strings.CutPrefix(subject, "mesh.control.")
	if !ok {
		return "", false
	}
	node, tail, ok := strings.Cut(rest, ".")
	if !ok || tail != "cli" || node == "" {
		return "", false
	}
	return node, true
}

// CLIHandler answers one line from one node.
type CLIHandler func(ctx context.Context, node string, asked CLIAsked) CLIAnswer

// ServeCLI answers mesh-cli for every node until stopped: one queue group, so of two controllers during a handover
// one answers.
//
// **Every line is a call** (review of ADR 0272): run, recorded and answered as a seat's verb is (calls.go) — its
// caller answered within AnswerWithin that it is still running, with its call, and the line's answer kept for
// the asker to follow. The bus permits an answer for a minute only (broker.ResponseTTL); a line answered when it
// ends lost every answer after that, and mesh-cli said nothing ran of a line that had.
func (b OverNATS) ServeCLI(handle CLIHandler, logger *log.Logger) (func(), error) {
	return b.serveCLI(Calls, CLIAtOnce, handle, logger)
}

func (b OverNATS) serveCLI(calls *CallLog, atOnce int, handle CLIHandler, logger *log.Logger) (func(), error) {
	done := make(chan struct{})
	slots := make(chan struct{}, atOnce)
	bind := func() (*nats.Subscription, error) {
		return b.Conn.QueueSubscribe(CLISubjects, "mesh-cli", func(msg *nats.Msg) {
			go b.answerCLI(msg, calls, slots, handle, logger)
		})
	}
	sub, err := bind()
	if err != nil {
		return nil, fmt.Errorf("serving %s: %w", CLISubjects, err)
	}
	go keepBound(sub, bind, CLISubjects, done, logger)
	return func() {
		close(done)
		_ = sub.Unsubscribe()
	}, nil
}

// cliEnvelope is an answer as every call's is: its result.
func cliEnvelope(a CLIAnswer) []byte {
	body, _ := json.Marshal(map[string]any{"result": a})
	return body
}

func (b OverNATS) answerCLI(msg *nats.Msg, calls *CallLog, slots chan struct{}, handle CLIHandler, logger *log.Logger) {
	if msg.Reply == "" {
		return
	}
	respond := func(body []byte) {
		if err := msg.Respond(body); err != nil && logger != nil {
			logger.Printf("mesh-cli on %s: the answer could not be sent: %v", msg.Subject, err)
		}
	}
	node, ok := CLINode(msg.Subject)
	var asked CLIAsked
	switch {
	case !ok:
		respond(cliEnvelope(CLIRefusal("not a mesh-cli subject: " + msg.Subject)))
		return
	case json.Unmarshal(msg.Data, &asked) != nil:
		respond(cliEnvelope(CLIRefusal("the node-engine's request could not be read, so nothing ran")))
		return
	case asked.Follow != "":
		respond(calls.followCLI(asked.Follow, node, asked.Account))
		return
	case len(asked.Line) == 0:
		respond(cliEnvelope(CLIRefusal("the request names no command, so nothing ran")))
		return
	}
	select {
	case slots <- struct{}{}:
		defer func() { <-slots }()
	default:
		respond(cliEnvelope(CLIRefusal(fmt.Sprintf("busy: %d lines from mesh-cli are running already; ask again "+
			"when one has ended. Nothing ran", cap(slots)))))
		return
	}
	limit := b.Conn.MaxPayload()
	calls.serveCallWithin(CLISeat, node, msg.Data, msg.Reply, func(ctx context.Context, _ json.RawMessage) (any, error) {
		return fitCLI(handle(ctx, node, asked), limit-4096), nil
	}, msg.Respond, limit, logger)
}

// followCLI answers the asker of a line what came of it: running still, its answer, or why that is not known. Only
// the account that asked it, on the node it was asked on: the answer is what the command printed, and `calls`
// keeps it from everyone else.
func (l *CallLog) followCLI(id, node, account string) []byte {
	noSuch := func() []byte {
		body, _ := json.Marshal(map[string]any{"error": fmt.Sprintf("no mesh-cli call %s was asked by %s on %s", id,
			account, node)})
		return body
	}
	c, inMemory := l.inMemory(id)
	if !inMemory {
		kept, found, err := l.Get(id)
		if err != nil || !found {
			return noSuch()
		}
		c = kept
	}
	var args CLIAsked
	_ = json.Unmarshal(c.Args, &args)
	if c.Seat != CLISeat || c.Verb != node || args.Account != account {
		return noSuch()
	}
	switch {
	case c.State == CallRunning:
		return running(&c, AnswerWithin, false, l.Follow)
	case c.State == CallAbandoned:
		body, _ := json.Marshal(map[string]any{"error": fmt.Sprintf("call %s was running under a controller that "+
			"stopped before it finished: it may have done part of what it was asked, and nothing will finish it", id)})
		return body
	case !inMemory:
		body, _ := json.Marshal(map[string]any{"error": fmt.Sprintf("call %s finished (%s), and its answer was kept "+
			"only in the memory of the controller that ran it, which has stopped; whether it took effect, the "+
			"mesh says (status, the hand-act log)", id, c.State)})
		return body
	}
	return c.Answer
}

// fitCLI is an answer whose streams fit in limit bytes once written: what the command printed is cut, standard
// output first, and the cut is said (ADR 0272 §5) — never silently short.
func fitCLI(a CLIAnswer, limit int64) CLIAnswer {
	body, _ := json.Marshal(a)
	if limit <= 0 || int64(len(body)) <= limit {
		return a
	}
	a.Cut = true
	// Room for the rest of the answer and JSON's base64 of the streams (4 bytes for every 3).
	room := (limit - 4096) * 3 / 4
	if room < 0 {
		room = 0
	}
	if int64(len(a.Stderr)) > room/2 {
		a.Stderr = a.Stderr[:room/2]
	}
	if left := room - int64(len(a.Stderr)); int64(len(a.Stdout)) > left {
		if left < 0 {
			left = 0
		}
		a.Stdout = a.Stdout[:left]
	}
	return a
}

// FitCLIAnswer is the answer as one bus message of at most limit bytes, written.
func FitCLIAnswer(a CLIAnswer, limit int64) []byte {
	body, _ := json.Marshal(fitCLI(a, limit))
	return body
}