Improve context descendant sorting

Upstreamed at superseriousbusiness#2579

internal/processing/status/get.go

@@ -19,7 +19,8 @@ package status
 
 import (
 	"context"
-	"sort"
+	"slices"
+	"strings"
 
 	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
 	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
@@ -74,46 +75,140 @@ func (p *Processor) contextGet(
 		return nil, errWithCode
 	}
 
-	context := &apimodel.Context{
-		Ancestors:   []apimodel.Status{},
-		Descendants: []apimodel.Status{},
-	}
-
 	parents, err := p.state.DB.GetStatusParents(ctx, targetStatus)
 	if err != nil {
 		return nil, gtserror.NewErrorInternalError(err)
 	}
 
+	var ancestors []*apimodel.Status
 	for _, status := range parents {
 		if v, err := p.filter.StatusVisible(ctx, requestingAccount, status); err == nil && v {
 			apiStatus, err := convert(ctx, status, requestingAccount)
 			if err == nil {
-				context.Ancestors = append(context.Ancestors, *apiStatus)
+				ancestors = append(ancestors, apiStatus)
 			}
 		}
 	}
 
-	sort.Slice(context.Ancestors, func(i int, j int) bool {
-		return context.Ancestors[i].ID < context.Ancestors[j].ID
+	slices.SortFunc(ancestors, func(lhs, rhs *apimodel.Status) int {
+		return strings.Compare(lhs.ID, rhs.ID)
 	})
 
 	children, err := p.state.DB.GetStatusChildren(ctx, targetStatus.ID)
 	if err != nil {
 		return nil, gtserror.NewErrorInternalError(err)
 	}
 
+	var descendants []*apimodel.Status
 	for _, status := range children {
 		if v, err := p.filter.StatusVisible(ctx, requestingAccount, status); err == nil && v {
 			apiStatus, err := convert(ctx, status, requestingAccount)
 			if err == nil {
-				context.Descendants = append(context.Descendants, *apiStatus)
+				descendants = append(descendants, apiStatus)
 			}
 		}
 	}
 
+	// Topologically sort descendants.
+	topoSort(descendants, targetStatus.AccountID)
+
+	context := &apimodel.Context{
+		Ancestors:   make([]apimodel.Status, 0, len(ancestors)),
+		Descendants: make([]apimodel.Status, 0, len(descendants)),
+	}
+	for _, ancestor := range ancestors {
+		context.Ancestors = append(context.Ancestors, *ancestor)
+	}
+	for _, descendant := range descendants {
+		context.Descendants = append(context.Descendants, *descendant)
+	}
+
 	return context, nil
 }
 
+// Sort statuses topologically. Break ties by sorting by ID.
+// Can handle cycles but the output order will be arbitrary.
+// (But if there are cycles, something went wrong upstream.)
+func topoSort(apiStatuses []*apimodel.Status, targetAccountID string) {
+	if len(apiStatuses) == 0 {
+		return
+	}
+
+	// Map of status IDs to statuses.
+	lookup := map[string]*apimodel.Status{}
+	for _, apiStatus := range apiStatuses {
+		lookup[apiStatus.ID] = apiStatus
+	}
+
+	// Tree of statuses to their children.
+	// The nil status may have children: any who don't have a parent, or whose parent isn't in the input.
+	tree := map[*apimodel.Status][]*apimodel.Status{}
+	for _, apiStatus := range apiStatuses {
+		var parent *apimodel.Status
+		if apiStatus.InReplyToID != nil {
+			parent = lookup[*apiStatus.InReplyToID]
+		}
+		tree[parent] = append(tree[parent], apiStatus)
+	}
+
+	// Sort children of each status by self-reply status and then ID, *in reverse*.
+	isSelfReply := func(apiStatus *apimodel.Status) bool {
+		return apiStatus.GetAccountID() == targetAccountID &&
+			apiStatus.InReplyToAccountID != nil &&
+			*apiStatus.InReplyToAccountID == targetAccountID
+	}
+	for id, children := range tree {
+		slices.SortFunc(children, func(lhs, rhs *apimodel.Status) int {
+			lhsIsContextSelfReply := isSelfReply(lhs)
+			rhsIsContextSelfReply := isSelfReply(rhs)
+
+			if lhsIsContextSelfReply && !rhsIsContextSelfReply {
+				return 1
+			} else if !lhsIsContextSelfReply && rhsIsContextSelfReply {
+				return -1
+			}
+
+			return -strings.Compare(lhs.ID, rhs.ID)
+		})
+		tree[id] = children
+	}
+
+	// Traverse the tree using preorder depth-first search, topologically sorting the statuses.
+	stack := make([]*apimodel.Status, 1, len(tree))
+	apiStatusIndex := 0
+	for len(stack) > 0 {
+		parent := stack[len(stack)-1]
+		children := tree[parent]
+
+		if len(children) == 0 {
+			// Remove this node from the tree.
+			delete(tree, parent)
+			// Go back to this node's parent.
+			stack = stack[:len(stack)-1]
+			continue
+		}
+
+		// Remove the last child entry (the first in ID order).
+		child := children[len(children)-1]
+		tree[parent] = children[:len(children)-1]
+
+		// Explore its children next.
+		stack = append(stack, child)
+
+		// Overwrite the next entry of the input slice.
+		apiStatuses[apiStatusIndex] = child
+		apiStatusIndex += 1
+	}
+
+	// There should only be nodes left in the tree in the event of a cycle.
+	// Append them to the end in arbitrary order.
+	// This ensures that the slice of statuses has no duplicates.
+	for node := range tree {
+		apiStatuses[apiStatusIndex] = node
+		apiStatusIndex += 1
+	}
+}
+
 // ContextGet returns the context (previous and following posts) from the given status ID.
 func (p *Processor) ContextGet(ctx context.Context, requestingAccount *gtsmodel.Account, targetStatusID string) (*apimodel.Context, gtserror.WithCode) {
 	return p.contextGet(ctx, requestingAccount, targetStatusID, p.converter.StatusToAPIStatus)

internal/processing/status/get_test.go

@@ -0,0 +1,131 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// 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 status
+
+import (
+	"github.com/stretchr/testify/suite"
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"testing"
+)
+
+type topoSortTestSuite struct {
+	suite.Suite
+}
+
+func statusIDs(apiStatuses []*apimodel.Status) []string {
+	ids := make([]string, 0, len(apiStatuses))
+	for _, apiStatus := range apiStatuses {
+		ids = append(ids, apiStatus.ID)
+	}
+	return ids
+}
+
+func (suite *topoSortTestSuite) TestBranched() {
+	// https://commons.wikimedia.org/wiki/File:Sorted_binary_tree_ALL_RGB.svg
+	f := &apimodel.Status{ID: "F"}
+	b := &apimodel.Status{ID: "B", InReplyToID: &f.ID}
+	a := &apimodel.Status{ID: "A", InReplyToID: &b.ID}
+	d := &apimodel.Status{ID: "D", InReplyToID: &b.ID}
+	c := &apimodel.Status{ID: "C", InReplyToID: &d.ID}
+	e := &apimodel.Status{ID: "E", InReplyToID: &d.ID}
+	g := &apimodel.Status{ID: "G", InReplyToID: &f.ID}
+	i := &apimodel.Status{ID: "I", InReplyToID: &g.ID}
+	h := &apimodel.Status{ID: "H", InReplyToID: &i.ID}
+
+	expected := statusIDs([]*apimodel.Status{f, b, a, d, c, e, g, i, h})
+	list := []*apimodel.Status{a, b, c, d, e, f, g, h, i}
+	topoSort(list)
+	actual := statusIDs(list)
+
+	suite.Equal(expected, actual)
+}
+
+func (suite *topoSortTestSuite) TestDisconnected() {
+	f := &apimodel.Status{ID: "F"}
+	b := &apimodel.Status{ID: "B", InReplyToID: &f.ID}
+	dID := "D"
+	e := &apimodel.Status{ID: "E", InReplyToID: &dID}
+
+	expected := statusIDs([]*apimodel.Status{e, f, b})
+	list := []*apimodel.Status{b, e, f}
+	topoSort(list)
+	actual := statusIDs(list)
+
+	suite.Equal(expected, actual)
+}
+
+func (suite *topoSortTestSuite) TestTrivialCycle() {
+	xID := "X"
+	x := &apimodel.Status{ID: xID, InReplyToID: &xID}
+
+	expected := statusIDs([]*apimodel.Status{x})
+	list := []*apimodel.Status{x}
+	topoSort(list)
+	actual := statusIDs(list)
+
+	suite.ElementsMatch(expected, actual)
+}
+
+func (suite *topoSortTestSuite) TestCycle() {
+	yID := "Y"
+	x := &apimodel.Status{ID: "X", InReplyToID: &yID}
+	y := &apimodel.Status{ID: yID, InReplyToID: &x.ID}
+
+	expected := statusIDs([]*apimodel.Status{x, y})
+	list := []*apimodel.Status{x, y}
+	topoSort(list)
+	actual := statusIDs(list)
+
+	suite.ElementsMatch(expected, actual)
+}
+
+func (suite *topoSortTestSuite) TestMixedCycle() {
+	yID := "Y"
+	x := &apimodel.Status{ID: "X", InReplyToID: &yID}
+	y := &apimodel.Status{ID: yID, InReplyToID: &x.ID}
+	z := &apimodel.Status{ID: "Z"}
+
+	expected := statusIDs([]*apimodel.Status{x, y, z})
+	list := []*apimodel.Status{x, y, z}
+	topoSort(list)
+	actual := statusIDs(list)
+
+	suite.ElementsMatch(expected, actual)
+}
+
+func (suite *topoSortTestSuite) TestEmpty() {
+	expected := statusIDs([]*apimodel.Status{})
+	list := []*apimodel.Status{}
+	topoSort(list)
+	actual := statusIDs(list)
+
+	suite.Equal(expected, actual)
+}
+
+func (suite *topoSortTestSuite) TestNil() {
+	expected := statusIDs(nil)
+	var list []*apimodel.Status
+	topoSort(list)
+	actual := statusIDs(list)
+
+	suite.Equal(expected, actual)
+}
+
+func TestTopoSortTestSuite(t *testing.T) {
+	suite.Run(t, &topoSortTestSuite{})
+}