package id

import (
	"fmt"
	"sync"
	"time"

	"github.com/bwmarrin/snowflake"

	"spark-server/internal/pkg/errs"
)

// Generator issues Snowflake MsgIds. Worker ID is fixed per machine. Clock rewind is rejected.
type Generator struct {
	mu       sync.Mutex
	node     *snowflake.Node
	lastMs   int64
	workerID int64
}

func New(workerID int64) (*Generator, error) {
	node, err := snowflake.NewNode(workerID)
	if err != nil {
		return nil, fmt.Errorf("snowflake node: %w", err)
	}
	return &Generator{node: node, workerID: workerID}, nil
}

func (g *Generator) NextMsgID() (int64, error) {
	return g.NextID()
}

// NextID returns a time-ordered Snowflake ID (user id, conversation id, msg id, mailbox event id).
func (g *Generator) NextID() (int64, error) {
	g.mu.Lock()
	defer g.mu.Unlock()
	now := time.Now().UnixMilli()
	if now < g.lastMs {
		return 0, errs.ErrClockRewind
	}
	id := g.node.Generate().Int64()
	g.lastMs = now
	return id, nil
}

func (g *Generator) WorkerID() int64 { return g.workerID }

const DefaultMsgIDSkew = 5 * time.Minute

// IssuedAtMS is the Snowflake unix-ms timestamp encoded in id.
func IssuedAtMS(id int64) int64 {
	if id <= 0 {
		return 0
	}
	return snowflake.ID(id).Time()
}

// WithinSkew reports whether id's timestamp is within skew of now (SEND client MsgId ±5m).
func WithinSkew(id int64, now time.Time, skew time.Duration) bool {
	if id <= 0 || skew <= 0 {
		return false
	}
	delta := now.UnixMilli() - IssuedAtMS(id)
	if delta < 0 {
		delta = -delta
	}
	return delta <= skew.Milliseconds()
}