package handler

import (
	"fmt"
	"strings"

	"kefu-sys/server/internal/model"
	"kefu-sys/server/internal/ws"
)

const (
	assignStrategyLeastLoad  = "least_load"
	assignStrategyRoundRobin = "round_robin"
	offlineLeavePrompt       = "当前无客服在线，请留言并留下联系方式，我们上线后会尽快回复您。"
)

func normalizeAssignStrategy(raw string) string {
	switch strings.TrimSpace(raw) {
	case assignStrategyRoundRobin:
		return assignStrategyRoundRobin
	default:
		return assignStrategyLeastLoad
	}
}

// listOnlineAgents 租户内可自动接待的一线客服（online + role=agent）。
func listOnlineAgents(tenantID uint) ([]model.User, error) {
	var agents []model.User
	err := model.DB.Where("tenant_id = ? AND role = ? AND status = ?", tenantID, "agent", "online").
		Order("id asc").Find(&agents).Error
	return agents, err
}

func agentActiveSessionCount(tenantID, agentID uint) (int64, error) {
	var count int64
	err := model.DB.Model(&model.Session{}).
		Where("tenant_id = ? AND agent_id = ? AND status = ?", tenantID, agentID, "active").
		Count(&count).Error
	return count, err
}

// filterAssignable 按最大并发过滤；maxActive<=0 表示不限制。
func filterAssignable(tenantID uint, agents []model.User, maxActive int) ([]model.User, map[uint]int64, error) {
	counts := make(map[uint]int64, len(agents))
	out := make([]model.User, 0, len(agents))
	for i := range agents {
		n, err := agentActiveSessionCount(tenantID, agents[i].ID)
		if err != nil {
			return nil, nil, err
		}
		counts[agents[i].ID] = n
		if maxActive > 0 && int(n) >= maxActive {
			continue
		}
		out = append(out, agents[i])
	}
	return out, counts, nil
}

// pickLeastLoadedAgent 在候选中选 active 会话最少者；平局取 id 更小。
func pickLeastLoadedAgent(candidates []model.User, counts map[uint]int64) *model.User {
	if len(candidates) == 0 {
		return nil
	}
	best := &candidates[0]
	bestCount := counts[best.ID]
	for i := 1; i < len(candidates); i++ {
		c := counts[candidates[i].ID]
		if c < bestCount || (c == bestCount && candidates[i].ID < best.ID) {
			bestCount = c
			best = &candidates[i]
		}
	}
	return best
}

// pickRoundRobinAgent 在按 id 排序的候选中，从 lastAgentID 之后轮转下一位。
func pickRoundRobinAgent(candidates []model.User, lastAgentID *uint) *model.User {
	if len(candidates) == 0 {
		return nil
	}
	if lastAgentID == nil || *lastAgentID == 0 {
		return &candidates[0]
	}
	// 找到 last 在全序中的位置，取其后第一个仍在候选中的
	start := 0
	for i, a := range candidates {
		if a.ID > *lastAgentID {
			start = i
			return &candidates[start]
		}
		if a.ID == *lastAgentID {
			start = (i + 1) % len(candidates)
			return &candidates[start]
		}
	}
	// last 已不在列表（离线/满载），从头开始
	return &candidates[0]
}

// pickAgent 按租户策略选择可分配坐席。
func pickAgent(tenantID uint, strategy string, maxActive int, rrLast *uint) (*model.User, error) {
	agents, err := listOnlineAgents(tenantID)
	if err != nil {
		return nil, err
	}
	if len(agents) == 0 {
		return nil, nil
	}
	candidates, counts, err := filterAssignable(tenantID, agents, maxActive)
	if err != nil {
		return nil, err
	}
	if len(candidates) == 0 {
		return nil, nil
	}
	strategy = normalizeAssignStrategy(strategy)
	if strategy == assignStrategyRoundRobin {
		return pickRoundRobinAgent(candidates, rrLast), nil
	}
	return pickLeastLoadedAgent(candidates, counts), nil
}

// countOnlineAgents 统计租户当前可接待的在线客服数。
func countOnlineAgents(tenantID uint) (int64, error) {
	var count int64
	err := model.DB.Model(&model.User{}).
		Where("tenant_id = ? AND role = ? AND status = ?", tenantID, "agent", "online").
		Count(&count).Error
	return count, err
}

// tryAutoAssign 按租户分配策略自动分配 waiting 会话；无可分配客服时返回 nil。
func tryAutoAssign(session *model.Session) (*model.User, error) {
	if session == nil || session.ID == 0 {
		return nil, nil
	}
	if session.Status != "waiting" || session.AgentID != nil {
		return nil, nil
	}

	setting, err := getOrCreateTenantSettings(session.TenantID)
	if err != nil {
		// 设置失败时回退默认 least_load、不限制并发
		setting = &model.TenantSetting{AssignStrategy: assignStrategyLeastLoad}
	}
	strategy := normalizeAssignStrategy(setting.AssignStrategy)
	maxActive := setting.MaxActivePerAgent
	if maxActive < 0 {
		maxActive = 0
	}

	agent, err := pickAgent(session.TenantID, strategy, maxActive, setting.RRLastAgentID)
	if err != nil || agent == nil {
		return nil, err
	}

	result := model.DB.Model(&model.Session{}).
		Where("id = ? AND tenant_id = ? AND status = ? AND agent_id IS NULL", session.ID, session.TenantID, "waiting").
		Updates(map[string]interface{}{
			"agent_id":      agent.ID,
			"status":        "active",
			"last_read_seq": 0,
		})
	if result.Error != nil {
		return nil, result.Error
	}
	if result.RowsAffected == 0 {
		// 并发下可能已被他人领取
		if err := model.DB.First(session, session.ID).Error; err != nil {
			return nil, err
		}
		return nil, nil
	}

	session.AgentID = &agent.ID
	session.Status = "active"

	// 推进轮询游标（仅 round_robin）
	if strategy == assignStrategyRoundRobin && setting.ID > 0 {
		aid := agent.ID
		_ = model.DB.Model(&model.TenantSetting{}).
			Where("id = ?", setting.ID).
			Update("rr_last_agent_id", aid).Error
		setting.RRLastAgentID = &aid
	}

	strategyLabel := "负载最低"
	if strategy == assignStrategyRoundRobin {
		strategyLabel = "轮询"
	}
	detail := fmt.Sprintf("系统自动分配给 %s（%s）", agent.Nickname, strategyLabel)
	if agent.Nickname == "" {
		detail = fmt.Sprintf("系统自动分配给 %s（%s）", agent.Username, strategyLabel)
	}
	model.DB.Create(&model.SessionEvent{
		SessionID:  session.ID,
		OperatorID: 0,
		Action:     "auto_assign",
		Detail:     detail,
	})

	if payload, err := ws.NewEvent("session_updated", session.ID, session); err == nil {
		ws.DefaultHub.BroadcastToTenantStaff(session.TenantID, payload)
		ws.DefaultHub.BroadcastToVisitor(session.TenantID, session.ID, payload)
	}
	return agent, nil
}