Shard by-id mount entries

mount.go

@@ -123,12 +123,25 @@ func buildMountTree(ctx context.Context, client *DocspellClient, items []SearchI
 					byIDName = fmt.Sprintf("%s-%d%s", item.ID, i+1, ext)
 				}
 			}
-			byID.children[uniqueName(byID.children, sanitizeName(byIDName))] = node
+			shard := idShard(item.ID)
+			shardDir := ensureDir(byID, shard)
+			shardDir.children[uniqueName(shardDir.children, sanitizeName(byIDName))] = node
 		}
 	}
 	return root
 }
 
+func idShard(id string) string {
+	id = sanitizeName(id)
+	if len(id) >= 2 {
+		return id[:2]
+	}
+	if id != "" {
+		return id
+	}
+	return "__"
+}
+
 func ensureDir(root *dirNode, first string, rest ...string) *dirNode {
 	cur := root
 	parts := append([]string{first}, rest...)

mount_test.go

@@ -87,9 +87,13 @@ func TestBuildMountTreeAddsByIDDirectory(t *testing.T) {
 	if !ok {
 		t.Fatalf("expected by-id directory, got %#v", root.children)
 	}
+	shard, ok := byID.children["it"].(*dirNode)
+	if !ok {
+		t.Fatalf("expected by-id shard, got %#v", byID.children)
+	}
 	for _, name := range []string{"item-1.pdf", "item-1-2.txt", "item-1-3.pdf"} {
-		if _, ok := byID.children[name].(*fileNode); !ok {
-			t.Fatalf("expected %s in by-id, got %#v", name, byID.children)
+		if _, ok := shard.children[name].(*fileNode); !ok {
+			t.Fatalf("expected %s in by-id shard, got %#v", name, shard.children)
 		}
 	}
 }