[bugfix/chore] Refactor timeline code (#1656)

* start poking timelines

* OK yes we're refactoring, but it's nothing like the last time so don't worry

* more fiddling

* update tests, simplify Get

* thanks linter, you're the best, mwah mwah kisses

* do a bit more tidying up

* start buggering about with the prepare function

* fix little oopsie

* start merging lists into 1

* ik heb een heel zwaar leven
nee nee echt waar

* hey it works we did it reddit

* regenerate swagger docs

* tidy up a wee bit

* adjust paging

* fix little error, remove unused functions

2 files changed, 56 insertions, 6 deletions

diff --git a/internal/db/bundb/timeline.go b/internal/db/bundb/timeline.go
index 1ab140103..fabfe2797 100644
--- a/internal/db/bundb/timeline.go
+++ b/internal/db/bundb/timeline.go
@@ -42,7 +42,10 @@ func (t *timelineDB) GetHomeTimeline(ctx context.Context, accountID string, maxI
 	}
 
 	// Make educated guess for slice size
-	statusIDs := make([]string, 0, limit)
+	var (
+		statusIDs   = make([]string, 0, limit)
+		frontToBack = true
+	)
 
 	q := t.conn.
 		NewSelect().
@@ -56,11 +59,9 @@ func (t *timelineDB) GetHomeTimeline(ctx context.Context, accountID string, maxI
 			bun.Ident("follow.target_account_id"),
 			bun.Ident("status.account_id"),
 			bun.Ident("follow.account_id"),
-			accountID).
-		// Sort by highest ID (newest) to lowest ID (oldest)
-		Order("status.id DESC")
+			accountID)
 
-	if maxID == "" {
+	if maxID == "" || maxID == id.Highest {
 		const future = 24 * time.Hour
 
 		var err error
@@ -83,6 +84,9 @@ func (t *timelineDB) GetHomeTimeline(ctx context.Context, accountID string, maxI
 	if minID != "" {
 		// return only statuses HIGHER (ie., newer) than minID
 		q = q.Where("? > ?", bun.Ident("status.id"), minID)
+
+		// page up
+		frontToBack = false
 	}
 
 	if local {
@@ -95,6 +99,14 @@ func (t *timelineDB) GetHomeTimeline(ctx context.Context, accountID string, maxI
 		q = q.Limit(limit)
 	}
 
+	if frontToBack {
+		// Page down.
+		q = q.Order("status.id DESC")
+	} else {
+		// Page up.
+		q = q.Order("status.id ASC")
+	}
+
 	// Use a WhereGroup here to specify that we want EITHER statuses posted by accounts that accountID follows,
 	// OR statuses posted by accountID itself (since a user should be able to see their own statuses).
 	//
@@ -110,8 +122,20 @@ func (t *timelineDB) GetHomeTimeline(ctx context.Context, accountID string, maxI
 		return nil, t.conn.ProcessError(err)
 	}
 
-	statuses := make([]*gtsmodel.Status, 0, len(statusIDs))
+	if len(statusIDs) == 0 {
+		return nil, nil
+	}
+
+	// If we're paging up, we still want statuses
+	// to be sorted by ID desc, so reverse ids slice.
+	// https://zchee.github.io/golang-wiki/SliceTricks/#reversing
+	if !frontToBack {
+		for l, r := 0, len(statusIDs)-1; l < r; l, r = l+1, r-1 {
+			statusIDs[l], statusIDs[r] = statusIDs[r], statusIDs[l]
+		}
+	}
 
+	statuses := make([]*gtsmodel.Status, 0, len(statusIDs))
 	for _, id := range statusIDs {
 		// Fetch status from db for ID
 		status, err := t.state.DB.GetStatusByID(ctx, id)
diff --git a/internal/db/bundb/timeline_test.go b/internal/db/bundb/timeline_test.go
index d6632b38c..f954c78dd 100644
--- a/internal/db/bundb/timeline_test.go
+++ b/internal/db/bundb/timeline_test.go
@@ -100,6 +100,32 @@ func (suite *TimelineTestSuite) TestGetHomeTimelineWithFutureStatus() {
 	suite.Len(s, 16)
 }
 
+func (suite *TimelineTestSuite) TestGetHomeTimelineBackToFront() {
+	ctx := context.Background()
+
+	viewingAccount := suite.testAccounts["local_account_1"]
+
+	s, err := suite.db.GetHomeTimeline(ctx, viewingAccount.ID, "", "", id.Lowest, 5, false)
+	suite.NoError(err)
+
+	suite.Len(s, 5)
+	suite.Equal("01F8MHAYFKS4KMXF8K5Y1C0KRN", s[0].ID)
+	suite.Equal("01F8MH75CBF9JFX4ZAD54N0W0R", s[len(s)-1].ID)
+}
+
+func (suite *TimelineTestSuite) TestGetHomeTimelineFromHighest() {
+	ctx := context.Background()
+
+	viewingAccount := suite.testAccounts["local_account_1"]
+
+	s, err := suite.db.GetHomeTimeline(ctx, viewingAccount.ID, id.Highest, "", "", 5, false)
+	suite.NoError(err)
+
+	suite.Len(s, 5)
+	suite.Equal("01G36SF3V6Y6V5BF9P4R7PQG7G", s[0].ID)
+	suite.Equal("01FCTA44PW9H1TB328S9AQXKDS", s[len(s)-1].ID)
+}
+
 func getFutureStatus() *gtsmodel.Status {
 	theDistantFuture := time.Now().Add(876600 * time.Hour)
 	id, err := id.NewULIDFromTime(theDistantFuture)