// Package cli is the local protocol between `mesh-cli` and the node-engine on its machine (novox/hq ADR 0272).
//
// `mesh-cli <command line>` asks the controller to run one of its command lines. It holds no credential and
// reads no file of the mesh: it connects to the node-engine's socket, writes one Request, and reads one Answer.
// The node-engine reads the connecting process's account from the kernel and asks the controller over its own
// link; that request is the node-engine's and the controller's, and lives in neither this package nor any SDK
// (ADR 0039 as ADR 0188 reads it: the transport lives in the runtime).
//
// One request and one answer per connection, each one line of JSON. Standard library only.
package cli

import (
	"bufio"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net"
	"time"
)

// Socket is where the node-engine listens for mesh-cli: a file under its runtime directory, with no network
// address (ADR 0272 §3).
const Socket = "/run/nox-mesh-host/cli.sock"

// MaxRequest bounds what the node-engine reads of a request. A command line is a few hundred bytes; settings given
// inline are a few kilobytes.
const MaxRequest = 64 << 10

// MaxAnswer bounds what mesh-cli reads of an answer. The controller cuts what it runs to fit one bus message, and
// says so; this is the local bound above that.
const MaxAnswer = 16 << 20

// AskWithin is how long mesh-cli waits for its answer: the controller's own bound on a call (five minutes),
// and a minute for the link on either side.
const AskWithin = 6 * time.Minute

// Request is what mesh-cli asks: the controller's command line, as words.
type Request struct {
	Line []string `json:"line"`
}

// Answer is what comes back: what the command printed, how it exited, and whether it ran as the controller's
// terminal and why. Refused is set when nothing ran at all, and says why; Exit is then 1.
type Answer struct {
	Stdout   []byte `json:"stdout,omitempty"`
	Stderr   []byte `json:"stderr,omitempty"`
	Exit     int    `json:"exit"`
	Terminal bool   `json:"terminal"`
	// Why says whether the line ran as the terminal, and why or why not, in the controller's words.
	Why string `json:"why,omitempty"`
	// Refused says why nothing ran: the account, the node, the controller unreachable.
	Refused string `json:"refused,omitempty"`
	// Cut says the output was longer than one bus message and was cut there.
	Cut bool `json:"cut,omitempty"`
	// Unknown says the line was asked and its outcome is not known: no answer came in time, and it may have
	// run. Never said as a refusal, which means nothing ran.
	Unknown string `json:"unknown,omitempty"`
}

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

// NotKnown is an answer saying the line was asked and what came of it is not known: it may have run. call is
// the controller's call the line runs as, when it is known, which `calls` answers.
func NotKnown(call, why string) Answer {
	where := "the controller's `calls` lists what came of each line"
	if call != "" {
		where = "see the call " + call + " (`calls " + call + "`)"
	}
	return Answer{Exit: 1, Unknown: why + "; the line may have run — " + where}
}

// ErrTooLong is a request or an answer over its bound.
var ErrTooLong = errors.New("longer than the protocol allows")

// WriteRequest writes one request, as one line.
func WriteRequest(w io.Writer, r Request) error { return writeLine(w, r) }

// ReadRequest reads one request, refusing one over MaxRequest or one that names no command.
func ReadRequest(r io.Reader) (Request, error) {
	var req Request
	if err := readLine(r, MaxRequest, &req); err != nil {
		return Request{}, err
	}
	if len(req.Line) == 0 {
		return Request{}, errors.New("the request names no command")
	}
	return req, nil
}

// WriteAnswer writes one answer, as one line.
func WriteAnswer(w io.Writer, a Answer) error { return writeLine(w, a) }

// ReadAnswer reads one answer, refusing one over MaxAnswer.
func ReadAnswer(r io.Reader) (Answer, error) {
	var a Answer
	err := readLine(r, MaxAnswer, &a)
	return a, err
}

// Ask is mesh-cli's side: connect to the socket, ask, and wait for the answer within AskWithin or ctx.
func Ask(ctx context.Context, socket string, line []string) (Answer, error) {
	if len(line) == 0 {
		return Answer{}, errors.New("no command given")
	}
	ctx, cancel := context.WithTimeout(ctx, AskWithin)
	defer cancel()
	var d net.Dialer
	conn, err := d.DialContext(ctx, "unix", socket)
	if err != nil {
		return Answer{}, fmt.Errorf("the node-engine on this machine does not answer at %s: %w", socket, err)
	}
	defer conn.Close()
	if deadline, ok := ctx.Deadline(); ok {
		_ = conn.SetDeadline(deadline)
	}
	if err := WriteRequest(conn, Request{Line: line}); err != nil {
		return Answer{}, fmt.Errorf("asking the node-engine: %w", err)
	}
	a, err := ReadAnswer(conn)
	if err != nil {
		return Answer{}, fmt.Errorf("reading the node-engine's answer: %w", err)
	}
	return a, nil
}

func writeLine(w io.Writer, v any) error {
	body, err := json.Marshal(v)
	if err != nil {
		return err
	}
	_, err = w.Write(append(body, '\n'))
	return err
}

// readLine reads one line of JSON of at most limit bytes. A line over the limit is refused rather than read
// whole: the reader is the side that pays for it.
func readLine(r io.Reader, limit int, into any) error {
	br := bufio.NewReaderSize(io.LimitReader(r, int64(limit)+1), 64<<10)
	var body []byte
	for {
		chunk, err := br.ReadSlice('\n')
		body = append(body, chunk...)
		if len(body) > limit {
			return fmt.Errorf("%w: more than %d bytes", ErrTooLong, limit)
		}
		if err == nil {
			break
		}
		if errors.Is(err, bufio.ErrBufferFull) {
			continue
		}
		if errors.Is(err, io.EOF) && len(body) > 0 {
			break
		}
		return err
	}
	if err := json.Unmarshal(body, into); err != nil {
		return fmt.Errorf("not one: %w", err)
	}
	return nil
}