MM-54201 Refactor mention parsing in preparation for multi-word mentions (#25030)

* MM-54201 Move ExplicitMentions to its own file and rename it (#24932)

* MM-54201 Move ExplicitMentions to its own file and rename it

* Fix vet

* MM-54201 Refactor current mention parsing into MentionParserStandard (#24936)

* MM-54201 Refactor current mention parsing into MentionParserStandard

* Fix vet

* MM-54201 Unify user and group mention parsing logic (#24937)

* MM-54201 Add MentionKeywords type

* MM-54201 Move group mentions into MentionKeywords

* Fix flaky test caused by random iteration order

* Update server/channels/app/mention_results.go

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

* Address feedback

---------

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

---------

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

server/channels/app/mention_keywords.go

@@ -0,0 +1,110 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/mattermost/mattermost/server/public/model"
+)
+
+const (
+	mentionableUserPrefix  = "user:"
+	mentionableGroupPrefix = "group:"
+)
+
+// A MentionableID stores the ID of a single User/Group with information about which type of object it refers to.
+type MentionableID string
+
+func MentionableUserID(userID string) MentionableID {
+	return MentionableID(fmt.Sprint(mentionableUserPrefix, userID))
+}
+
+func MentionableGroupID(groupID string) MentionableID {
+	return MentionableID(fmt.Sprint(mentionableGroupPrefix, groupID))
+}
+
+func (id MentionableID) AsUserID() (userID string, ok bool) {
+	idString := string(id)
+	if !strings.HasPrefix(idString, mentionableUserPrefix) {
+		return "", false
+	}
+
+	return idString[len(mentionableUserPrefix):], true
+}
+
+func (id MentionableID) AsGroupID() (groupID string, ok bool) {
+	idString := string(id)
+	if !strings.HasPrefix(idString, mentionableGroupPrefix) {
+		return "", false
+	}
+
+	return idString[len(mentionableGroupPrefix):], true
+}
+
+// MentionKeywords is a collection of mention keywords and the IDs of the objects that have a given keyword.
+type MentionKeywords map[string][]MentionableID
+
+func (k MentionKeywords) AddUser(profile *model.User, channelNotifyProps map[string]string, status *model.Status, allowChannelMentions bool) MentionKeywords {
+	mentionableID := MentionableUserID(profile.Id)
+
+	userMention := "@" + strings.ToLower(profile.Username)
+	k[userMention] = append(k[userMention], mentionableID)
+
+	// Add all the user's mention keys
+	for _, mentionKey := range profile.GetMentionKeys() {
+		if mentionKey != "" {
+			// Note that these are made lower case so that we can do a case insensitive check for them
+			mentionKey = strings.ToLower(mentionKey)
+
+			k[mentionKey] = append(k[mentionKey], mentionableID)
+		}
+	}
+
+	// If turned on, add the user's case sensitive first name
+	if profile.NotifyProps[model.FirstNameNotifyProp] == "true" && profile.FirstName != "" {
+		k[profile.FirstName] = append(k[profile.FirstName], mentionableID)
+	}
+
+	// Add @channel and @all to k if user has them turned on and the server allows them
+	if allowChannelMentions {
+		// Ignore channel mentions if channel is muted and channel mention setting is default
+		ignoreChannelMentions := channelNotifyProps[model.IgnoreChannelMentionsNotifyProp] == model.IgnoreChannelMentionsOn || (channelNotifyProps[model.MarkUnreadNotifyProp] == model.UserNotifyMention && channelNotifyProps[model.IgnoreChannelMentionsNotifyProp] == model.IgnoreChannelMentionsDefault)
+
+		if profile.NotifyProps[model.ChannelMentionsNotifyProp] == "true" && !ignoreChannelMentions {
+			k["@channel"] = append(k["@channel"], mentionableID)
+			k["@all"] = append(k["@all"], mentionableID)
+
+			if status != nil && status.Status == model.StatusOnline {
+				k["@here"] = append(k["@here"], mentionableID)
+			}
+		}
+	}
+
+	return k
+}
+
+func (k MentionKeywords) AddUserKeyword(userID string, keyword string) MentionKeywords {
+	k[keyword] = append(k[keyword], MentionableUserID(userID))
+
+	return k
+}
+
+func (k MentionKeywords) AddGroup(group *model.Group) MentionKeywords {
+	if group.Name != nil {
+		keyword := "@" + *group.Name
+		k[keyword] = append(k[keyword], MentionableGroupID(group.Id))
+	}
+
+	return k
+}
+
+func (k MentionKeywords) AddGroupsMap(groups map[string]*model.Group) MentionKeywords {
+	for _, group := range groups {
+		k.AddGroup(group)
+	}
+
+	return k
+}