package quota

import (
	"bufio"
	"encoding/json"
	"errors"
	"fmt"
	"net"
	"net/url"
	"strconv"
	"strings"
	"sync"
	"time"
)

// SharedState is the cross-instance quota state bus: one redis container on
// the LAN (DECISION 2026-08-29, see REPORT-20260829-0500-quota) so the nine
// harness instances across two hosts read/write the same quota snapshot and
// credit estimates. It is a deliberately tiny stdlib-only RESP2 client
// (SET/GET/INCRBYFLOAT/EXPIRE — the whole surface this package needs), no
// host packages and no external Go deps. Every op fails soft: with redis
// down the gate degrades to this instance's local estimate and the loop
// keeps running.
type SharedState struct {
	addr   string
	db     int
	mu     sync.Mutex
	conn   net.Conn
	rw     *bufio.ReadWriter
	dialTO time.Duration
}

// NewSharedState parses redis://host:port[/db]; empty URL returns nil (the
// nil state is valid and fully functional as a no-op).
func NewSharedState(redisURL string) (*SharedState, error) {
	if redisURL == "" {
		return nil, nil
	}
	u, err := url.Parse(redisURL)
	if err != nil {
		return nil, fmt.Errorf("redis url: %w", err)
	}
	host := u.Host
	if host == "" {
		return nil, fmt.Errorf("redis url: missing host")
	}
	if !strings.Contains(host, ":") {
		host += ":6379"
	}
	db := 0
	if s := strings.TrimPrefix(u.Path, "/"); s != "" {
		if db, err = strconv.Atoi(s); err != nil {
			return nil, fmt.Errorf("redis url db: %w", err)
		}
	}
	return &SharedState{addr: host, db: db, dialTO: 3 * time.Second}, nil
}

// Close releases the connection, if any.
func (s *SharedState) Close() error {
	if s == nil {
		return nil
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.conn != nil {
		err := s.conn.Close()
		s.conn, s.rw = nil, nil
		return err
	}
	return nil
}

// command runs one RESP command and returns the reply. Transport failures
// reset the connection so the next op redials once.
func (s *SharedState) command(args ...string) (string, error) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.conn == nil {
		conn, err := net.DialTimeout("tcp", s.addr, s.dialTO)
		if err != nil {
			return "", fmt.Errorf("redis dial %s: %w", s.addr, err)
		}
		s.conn = conn
		s.rw = bufio.NewReadWriter(bufio.NewReader(conn), bufio.NewWriter(conn))
		if s.db != 0 {
			if _, err := s.execLocked("SELECT", strconv.Itoa(s.db)); err != nil {
				s.resetLocked()
				return "", err
			}
		}
	}
	reply, err := s.execLocked(args...)
	if err != nil {
		s.resetLocked()
	}
	return reply, err
}

// execLocked writes one command and reads one reply (mutex held).
func (s *SharedState) execLocked(args ...string) (string, error) {
	var b strings.Builder
	fmt.Fprintf(&b, "*%d\r\n", len(args))
	for _, a := range args {
		fmt.Fprintf(&b, "$%d\r\n%s\r\n", len(a), a)
	}
	if _, err := s.rw.WriteString(b.String()); err != nil {
		return "", fmt.Errorf("redis write: %w", err)
	}
	if err := s.rw.Flush(); err != nil {
		return "", fmt.Errorf("redis flush: %w", err)
	}
	return readReply(s.rw.Reader)
}

func (s *SharedState) resetLocked() {
	if s.conn != nil {
		_ = s.conn.Close()
	}
	s.conn, s.rw = nil, nil
}

// readReply parses one RESP2 reply: +simple / -error / :integer / $bulk.
func readReply(r *bufio.Reader) (string, error) {
	line, err := r.ReadString('\n')
	if err != nil {
		return "", fmt.Errorf("redis read: %w", err)
	}
	line = strings.TrimRight(line, "\r\n")
	if line == "" {
		return "", errors.New("redis: empty reply")
	}
	switch line[0] {
	case '+', ':':
		return line[1:], nil
	case '-':
		return "", fmt.Errorf("redis: %s", line[1:])
	case '$':
		n, err := strconv.Atoi(line[1:])
		if err != nil {
			return "", fmt.Errorf("redis bulk len: %w", err)
		}
		if n < 0 {
			return "", nil // nil bulk: missing key
		}
		buf := make([]byte, n+2)
		if _, err := ioReadFull(r, buf); err != nil {
			return "", fmt.Errorf("redis bulk read: %w", err)
		}
		return string(buf[:n]), nil
	default:
		return "", fmt.Errorf("redis: unexpected reply %q", line)
	}
}

func ioReadFull(r *bufio.Reader, buf []byte) (int, error) {
	total := 0
	for total < len(buf) {
		n, err := r.Read(buf[total:])
		total += n
		if err != nil {
			return total, err
		}
	}
	return total, nil
}

// Key layout (one account's plan is one shared universe):
//
//	mopac:quota:<account>:snapshot      — latest polled QuotaSnapshot (JSON)
//	mopac:quota:<account>:est:5h        — estimated credits in the rolling 5h window
//	mopac:quota:<account>:est:weekly    — estimated credits in the plan week
//
// Estimate keys carry the window id in the VALUE-side bookkeeping done by
// the caller (Gate): 5h keys expire after 5h+slack; weekly keys are keyed
// by ISO week via the caller and need no expiry.

func (s *SharedState) get(key string) (string, error) {
	if s == nil {
		return "", nil
	}
	return s.command("GET", key)
}

func (s *SharedState) setEx(key, val string, ttl time.Duration) error {
	if s == nil {
		return nil
	}
	_, err := s.command("SET", key, val, "EX", strconv.Itoa(int(ttl.Seconds())))
	return err
}

// incrByFloat adds delta to key (creating at delta) and returns the new value.
func (s *SharedState) incrByFloat(key string, delta float64) (float64, error) {
	if s == nil {
		return 0, nil
	}
	reply, err := s.command("INCRBYFLOAT", key, strconv.FormatFloat(delta, 'f', 6, 64))
	if err != nil {
		return 0, err
	}
	return strconv.ParseFloat(strings.TrimSpace(reply), 64)
}

// PublishSnapshot stores the polled snapshot for all instances to read.
func (s *SharedState) PublishSnapshot(snap *QuotaSnapshot, ttl time.Duration) error {
	if s == nil || snap == nil {
		return nil
	}
	body, err := json.Marshal(snap)
	if err != nil {
		return err
	}
	return s.setEx(snapshotKey(snap.Account), string(body), ttl)
}

// LoadSnapshot returns the last published snapshot, or nil when absent.
func (s *SharedState) LoadSnapshot(account string) (*QuotaSnapshot, error) {
	body, err := s.get(snapshotKey(account))
	if err != nil {
		return nil, err
	}
	if body == "" {
		return nil, nil
	}
	var snap QuotaSnapshot
	if err := json.Unmarshal([]byte(body), &snap); err != nil {
		return nil, fmt.Errorf("shared snapshot decode: %w", err)
	}
	return &snap, nil
}

func snapshotKey(account string) string { return "mopac:quota:" + account + ":snapshot" }