You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: gitlab_request_parser_validator.go
    path of file: ./repos/atlantis/server/controllers/events
    
    the code of the file until where you have to start completion: // Copyright 2017 HootSuite Media Inc.
//
// Licensed under the Apache License, Version 2.0 (the License);
// you may 
    
func (d *DefaultGitlabRequestParserValidator) ParseAndValidate(r *http.Request, secret []byte) (interface{}, error) {
	const mergeEventHeader = "Merge Request Hook"
	const noteEventHeader = "Note Hook"

	// Validate secret if specified.
	headerSecret := r.Header.Get(secretHeader)
	if len(secret) != 0 && subtle.ConstantTimeCompare(secret, []byte(headerSecret)) != 1 {
		return nil, fmt.Errorf("header %s=%s did not match expected secret", secretHeader, headerSecret)
	}

	// Parse request into a gitlab object based on the object type specified
	// in the gitlabHeader.
	bytes, err := io.ReadAll(r.Body)
	if err != nil {
		return nil, err
	}
	switch r.Header.Get(gitlabHeader) {
	case mergeEventHeader:
		var m gitlab.MergeEvent
		if err := json.Unmarshal(bytes, &m); err != nil {
			return nil, err
		}
		return m, nil
	case noteEventHeader:
		// First, parse a small part of the json to determine if this is a
		// comment on a merge request or a commit.
		var subset struct {
			ObjectAttributes struct {
				NoteableType string `json:"noteable_type"`
			} `json:"object_attributes"`
		}
		if err := json.Unmarshal(bytes, &subset); err != nil {
			return nil, err
		}

		// We then parse into the correct comment event type.
		switch subset.ObjectAttributes.NoteableType {
		case "Commit":
			var e gitlab.CommitCommentEvent
			err := json.Unmarshal(bytes, &e)
			return e, err
		case "MergeRequest":
			var e gitlab.MergeCommentEvent
			err := json.Unmarshal(bytes, &e)
			return e, err
		}
	}
	return nil, nil
}