[feature] Rework timeline code to make it useful for more than just statuses (#373)

* add preparable and timelineable interfaces

* initialize timeline manager within the processor

* generic renaming

* move status-specific timeline logic into the processor

* refactor timeline to make it useful for more than statuses

internal/timeline/prepareditems.go

@@ -0,0 +1,92 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package timeline
+
+import (
+	"container/list"
+	"context"
+	"errors"
+)
+
+type preparedItems struct {
+	data       *list.List
+	skipInsert SkipInsertFunction
+}
+
+type preparedItemsEntry struct {
+	itemID           string
+	boostOfID        string
+	accountID        string
+	boostOfAccountID string
+	prepared         Preparable
+}
+
+func (p *preparedItems) insertPrepared(ctx context.Context, i *preparedItemsEntry) error {
+	if p.data == nil {
+		p.data = &list.List{}
+	}
+
+	// if we have no entries yet, this is both the newest and oldest entry, so just put it in the front
+	if p.data.Len() == 0 {
+		p.data.PushFront(i)
+		return nil
+	}
+
+	var insertMark *list.Element
+	var position int
+	// We need to iterate through the index to make sure we put this post in the appropriate place according to when it was created.
+	// We also need to make sure we're not inserting a duplicate post -- this can happen sometimes and it's not nice UX (*shudder*).
+	for e := p.data.Front(); e != nil; e = e.Next() {
+		position++
+
+		entry, ok := e.Value.(*preparedItemsEntry)
+		if !ok {
+			return errors.New("index: could not parse e as a preparedPostsEntry")
+		}
+
+		skip, err := p.skipInsert(ctx, i.itemID, i.accountID, i.boostOfID, i.boostOfAccountID, entry.itemID, entry.accountID, entry.boostOfID, entry.boostOfAccountID, position)
+		if err != nil {
+			return err
+		}
+		if skip {
+			return nil
+		}
+
+		// if the post to index is newer than e, insert it before e in the list
+		if insertMark == nil {
+			if i.itemID > entry.itemID {
+				insertMark = e
+			}
+		}
+
+		// make sure we don't insert a duplicate
+		if entry.itemID == i.itemID {
+			return nil
+		}
+	}
+
+	if insertMark != nil {
+		p.data.InsertBefore(i, insertMark)
+		return nil
+	}
+
+	// if we reach this point it's the oldest post we've seen so put it at the back
+	p.data.PushBack(i)
+	return nil
+}