2 files changed, 94 insertions, 0 deletions
diff --git a/internal/thread/thread.go b/internal/thread/thread.go
new file mode 100644
index 0000000..ed9a47b
--- /dev/null
+++ b/internal/thread/thread.go
@@ -0,0 +1,76 @@
+// Package thread builds local Usenet conversation trees from article headers.
+package thread
+
+import (
+	"sort"
+	"strings"
+
+	"aegis/internal/nntp"
+)
+
+type Node struct {
+	Header   nntp.ArticleHeader
+	Parent   *Node
+	Children []*Node
+}
+
+func Build(headers []nntp.ArticleHeader) []*Node {
+	ordered := append([]nntp.ArticleHeader(nil), headers...)
+	sort.SliceStable(ordered, func(i, j int) bool {
+		if ordered[i].Number == ordered[j].Number {
+			return ordered[i].MessageID < ordered[j].MessageID
+		}
+		return ordered[i].Number < ordered[j].Number
+	})
+	nodes := make(map[string]*Node, len(ordered))
+	for _, header := range ordered {
+		node := &Node{Header: header}
+		if key := canonicalID(header.MessageID); key != "" {
+			nodes[key] = node
+		}
+	}
+	var roots []*Node
+	for _, node := range nodesInOrder(ordered, nodes) {
+		parentID := lastReference(node.Header.References)
+		parent := nodes[canonicalID(parentID)]
+		if parent == nil || parent == node {
+			roots = append(roots, node)
+			continue
+		}
+		node.Parent = parent
+		parent.Children = append(parent.Children, node)
+	}
+	for _, node := range nodesInOrder(ordered, nodes) {
+		sort.SliceStable(node.Children, func(i, j int) bool { return node.Children[i].Header.Number < node.Children[j].Header.Number })
+	}
+	return roots
+}
+
+func nodesInOrder(headers []nntp.ArticleHeader, nodes map[string]*Node) []*Node {
+	result := make([]*Node, 0, len(headers))
+	seen := make(map[*Node]struct{}, len(headers))
+	for _, header := range headers {
+		node := nodes[canonicalID(header.MessageID)]
+		if node == nil {
+			continue
+		}
+		if _, ok := seen[node]; ok {
+			continue
+		}
+		seen[node] = struct{}{}
+		result = append(result, node)
+	}
+	return result
+}
+
+func lastReference(value string) string {
+	parts := strings.Fields(value)
+	if len(parts) == 0 {
+		return ""
+	}
+	return parts[len(parts)-1]
+}
+
+func canonicalID(value string) string {
+	return strings.ToLower(strings.TrimSpace(value))
+}
diff --git a/internal/thread/thread_test.go b/internal/thread/thread_test.go
new file mode 100644
index 0000000..146ae0c
--- /dev/null
+++ b/internal/thread/thread_test.go
@@ -0,0 +1,18 @@
+package thread
+
+import (
+	"testing"
+
+	"aegis/internal/nntp"
+)
+
+func TestBuildUsesLastReference(t *testing.T) {
+	roots := Build([]nntp.ArticleHeader{
+		{Number: 1, MessageID: "<root@example>", Subject: "topic"},
+		{Number: 2, MessageID: "<reply@example>", References: "<root@example>"},
+		{Number: 3, MessageID: "<nested@example>", References: "<root@example> <reply@example>"},
+	})
+	if len(roots) != 1 || len(roots[0].Children) != 1 || len(roots[0].Children[0].Children) != 1 {
+		t.Fatalf("unexpected thread tree: %#v", roots)
+	}
+}