// Package handler define HTTP handler functions for eventRT service
package handler

import (
	"context"
	"errors"
	"net/http"
	"time"

	"eventRT/constants"
	"eventRT/database"
	"eventRT/event"
	"eventRT/logger"

	"github.com/gin-gonic/gin"
	"go.mongodb.org/mongo-driver/bson"
	"go.mongodb.org/mongo-driver/mongo"
	"go.mongodb.org/mongo-driver/mongo/options"
)

var (
	errEventNotFound           = errors.New("event not found")
	errInvalidStatusTransition = errors.New("invalid status transition: precondition status mismatch")
)

type updateStatusRequest struct {
	Op string `json:"op" binding:"required"`
}

// ConfirmEventHandler handles PATCH /events/:event_uuid/confirm
// Transitions event status from EventStatusReported to EventStatusConfirmed.
func ConfirmEventHandler(c *gin.Context) {
	handleStatusTransition(c,
		constants.EventStatusReported,
		constants.EventStatusConfirmed,
		"confirm",
	)
}

// CloseEventHandler handles PATCH /events/:event_uuid/close
// Transitions event status from EventStatusConfirmed to EventStatusClosed.
func CloseEventHandler(c *gin.Context) {
	handleStatusTransition(c,
		constants.EventStatusConfirmed,
		constants.EventStatusClosed,
		"close",
	)
}

func handleStatusTransition(c *gin.Context, requiredStatus, newStatus int, action string) {
	ctx := c.Request.Context()
	eventUUID := c.Param("event_uuid")

	var req updateStatusRequest
	if err := c.ShouldBindJSON(&req); err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": "field op is required"})
		return
	}

	updated, err := transitionEventStatus(ctx, eventUUID, req.Op, requiredStatus, newStatus, action)
	if err != nil {
		switch {
		case errors.Is(err, errEventNotFound):
			c.JSON(http.StatusNotFound, gin.H{"error": "event not found"})
		case errors.Is(err, errInvalidStatusTransition):
			c.JSON(http.StatusConflict, gin.H{
				"error":           "invalid status transition",
				"required_status": requiredStatus,
			})
		default:
			logger.Error(ctx, "update event status failed",
				"event_uuid", eventUUID,
				"action", action,
				"error", err)
			c.JSON(http.StatusInternalServerError, gin.H{"error": "internal server error"})
		}
		return
	}

	c.JSON(http.StatusOK, updated)
}

// transitionEventStatus atomically validates the precondition status and applies the transition.
// Uses FindOneAndUpdate with a status filter so the check and update are a single MongoDB operation.
func transitionEventStatus(ctx context.Context, eventUUID, op string, requiredStatus, newStatus int, action string) (*event.EventRecord, error) {
	col := database.GetMongoClient().
		Database(constants.EventDBName).
		Collection(constants.EventCollectionName)

	opRecord := event.OperationRecord{
		Action: action,
		Op:     op,
		TS:     time.Now().UnixNano() / int64(time.Millisecond),
	}

	filter := bson.M{
		"event_uuid": eventUUID,
		"status":     requiredStatus,
	}
	update := bson.M{
		"$set":  bson.M{"status": newStatus},
		"$push": bson.M{"operations": opRecord},
	}
	opts := options.FindOneAndUpdate().SetReturnDocument(options.After)

	var updated event.EventRecord
	err := col.FindOneAndUpdate(ctx, filter, update, opts).Decode(&updated)
	if err != nil {
		if !errors.Is(err, mongo.ErrNoDocuments) {
			return nil, err
		}
		// FindOneAndUpdate returned no documents: either the event doesn't exist
		// or the current status doesn't match the required precondition.
		// Do a follow-up read to distinguish the two cases.
		var existing event.EventRecord
		findErr := col.FindOne(ctx, bson.M{"event_uuid": eventUUID}).Decode(&existing)
		if errors.Is(findErr, mongo.ErrNoDocuments) {
			return nil, errEventNotFound
		}
		if findErr != nil {
			return nil, findErr
		}
		return nil, errInvalidStatusTransition
	}

	logger.Info(ctx, "event status transitioned",
		"event_uuid", eventUUID,
		"from", requiredStatus,
		"to", newStatus,
		"op", op)

	return &updated, nil
}