[account.cpp] gnc_accounts_and_all_descendants converted from scm

much more efficient than guile algorithm, avoids numerous repeated
GList<->SCM conversions, and traversals of account descendants.

bindings/engine-common.i

@@ -48,10 +48,14 @@ AccountList * gnc_account_get_descendants (const Account *account);
 %newobject gnc_account_get_descendants_sorted;
 AccountList * gnc_account_get_descendants_sorted (const Account *account);
 
+%newobject gnc_accounts_and_all_descendants;
+AccountList * gnc_accounts_and_all_descendants (AccountList *accounts);
+
 %ignore gnc_account_get_children;
 %ignore gnc_account_get_children_sorted;
 %ignore gnc_account_get_descendants;
 %ignore gnc_account_get_descendants_sorted;
+%ignore gnc_accounts_and_all_descendants;
 %include <Account.h>
 
 %include <Transaction.h>

libgnucash/engine/Account.cpp

@@ -49,6 +49,7 @@ extern "C" {
 
 #include <numeric>
 #include <map>
+#include <unordered_set>
 
 static QofLogModule log_module = GNC_MOD_ACCOUNT;
 
@@ -6228,6 +6229,21 @@ gboolean xaccAccountRegister (void)
     return qof_object_register (&account_object_def);
 }
 
+using AccountSet = std::unordered_set<Account*>;
+static void maybe_add_descendants (Account* acc, gpointer arg)
+{
+    if (static_cast <AccountSet*> (arg)->insert (acc).second)
+        g_list_foreach (GET_PRIVATE(acc)->children, (GFunc) maybe_add_descendants, arg);
+};
+
+GList *
+gnc_accounts_and_all_descendants (GList *accounts)
+{
+    AccountSet accset;
+    g_list_foreach (accounts, (GFunc) maybe_add_descendants, &accset);
+    return std::accumulate (accset.begin(), accset.end(), (GList*) nullptr, g_list_prepend);
+}
+
 /* ======================= UNIT TESTING ACCESS =======================
  * The following functions are for unit testing use only.
  */

libgnucash/engine/Account.h

@@ -1620,6 +1620,7 @@ void dxaccAccountSetQuoteTZ (Account *account, const char *tz);
 const char * dxaccAccountGetQuoteTZ (const Account *account);
 /** @} */
 
+GList * gnc_accounts_and_all_descendants (GList *accounts);
 
 /** @name Account parameter names
  @{