Improve context descendant sorting (#2579)

* Improve context descendant sorting

Topologically sort replies, then move self-replies to top of list

* Unify descendant sort passes

* Correct test package name

* Preallocate maps

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"
@@ -74,27 +75,23 @@ 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)
@@ -102,18 +99,116 @@ func (p *Processor) contextGet(
 		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)
 			}
 		}
 	}
 
+	TopoSort(descendants, targetStatus.AccountID)
+
+	//goland:noinspection GoImportUsedAsName
+	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
 }
 
+// TopoSort sorts statuses topologically, by self-reply, and 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 := make(map[string]*apimodel.Status, len(apiStatuses))
+	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 := make(map[*apimodel.Status][]*apimodel.Status, len(apiStatuses))
+	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 sorted 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++
+	}
+
+	// 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++
+	}
+}
+
 // 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)