// Copyright (C) MongoDB, Inc. 2017-present.
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may
// not use this file except in compliance with the License. You may obtain
// a copy of the License at http://www.apache.org/licenses/LICENSE-2.0

package mongo

import (
	"context"
	"errors"

	"go.mongodb.org/mongo-driver/bson"
	"go.mongodb.org/mongo-driver/bson/bsoncodec"
)

// ErrNoDocuments is returned by SingleResult methods when the operation that created the SingleResult did not return
// any documents.
var ErrNoDocuments = errors.New("mongo: no documents in result")

// SingleResult represents a single document returned from an operation. If the operation resulted in an error, all
// SingleResult methods will return that error. If the operation did not return any documents, all SingleResult methods
// will return ErrNoDocuments.
type SingleResult struct {
	err error
	cur *Cursor
	rdr bson.Raw
	reg *bsoncodec.Registry
}

// Decode will unmarshal the document represented by this SingleResult into v. If there was an error from the operation
// that created this SingleResult, that error will be returned. If the operation returned no documents, DecodeBytes will
// return ErrNoDocuments.
//
// If the operation was successful and returned a document, Decode will return any errors from the unmarshalling process
// without any modification. If v is nil or is a typed nil, an error will be returned.
func (sr *SingleResult) Decode(v interface{}) error {
	if sr.err != nil {
		return sr.err
	}
	if sr.reg == nil {
		return bson.ErrNilRegistry
	}

	if sr.err = sr.setRdrContents(); sr.err != nil {
		return sr.err
	}
	return bson.UnmarshalWithRegistry(sr.reg, sr.rdr, v)
}

// DecodeBytes will return the document represented by this SingleResult as a bson.Raw. If there was an error from the
// operation that created this SingleResult, both the result and that error will be returned. If the operation returned
// no documents, this will return (nil, ErrNoDocuments).
func (sr *SingleResult) DecodeBytes() (bson.Raw, error) {
	if sr.err != nil {
		return sr.rdr, sr.err
	}

	if sr.err = sr.setRdrContents(); sr.err != nil {
		return nil, sr.err
	}
	return sr.rdr, nil
}

// setRdrContents will set the contents of rdr by iterating the underlying cursor if necessary.
func (sr *SingleResult) setRdrContents() error {
	switch {
	case sr.err != nil:
		return sr.err
	case sr.rdr != nil:
		return nil
	case sr.cur != nil:
		defer sr.cur.Close(context.TODO())
		if !sr.cur.Next(context.TODO()) {
			if err := sr.cur.Err(); err != nil {
				return err
			}

			return ErrNoDocuments
		}
		sr.rdr = sr.cur.Current
		return nil
	}

	return ErrNoDocuments
}

// Err returns the error from the operation that created this SingleResult. If the operation was successful but did not
// return any documents, Err will return ErrNoDocuments. If the operation was successful and returned a document, Err
// will return nil.
func (sr *SingleResult) Err() error {
	sr.err = sr.setRdrContents()

	return sr.err
}