From a949d166f627fcafb40e6113e48e22daea49694d Mon Sep 17 00:00:00 2001
From: Ulf Wiger <ulf@wiger.net>
Date: Mon, 14 Apr 2025 11:54:48 +0200
Subject: [PATCH] Add options for deserialization of missing labels

---
 README.md         | 18 ++++++++++++++++++
 src/gmser_dyn.erl | 35 +++++++++++++++++++++++++++++++----
 2 files changed, 49 insertions(+), 4 deletions(-)

diff --git a/README.md b/README.md
index 7f73e6c..d24432f 100644
--- a/README.md
+++ b/README.md
@@ -58,6 +58,24 @@ Labels correspond to (existing) atoms in Erlang.
 Decoding of a label results in a call to `binary_to_existing_atom/2`, so will
 fail if the corresponding atom does not already exist.
 
+This behavior can be modified using the option `#{missing_labels => fail | create | convert}`,
+where `fail` is the default, as described above, `convert` means that missing atoms are
+converted to binaries, and `create` means that the atom is created dynamically.
+
+The option can be passed e.g.:
+```erlang
+gmser_dyn:deserialize(Binary, set_opts(#{missing_labels => convert}))
+```
+
+or
+```erlang
+gmser_dyn:deserialize(Binary, set_opts(#{missing_labels => convert}, Types))
+```
+
+By calling `gmser_dyn:register_types/1`, after having added options to the type map,
+the options can be made to take effect automatically.
+
+
 It's possible to cache labels for more compact encoding.
 Note that when caching labels, the same cache mapping needs to be used on the
 decoder side.
diff --git a/src/gmser_dyn.erl b/src/gmser_dyn.erl
index c3b0713..3253a3d 100644
--- a/src/gmser_dyn.erl
+++ b/src/gmser_dyn.erl
@@ -17,6 +17,9 @@
 %% register a type schema, inspect existing schema
 -export([ register_types/1
         , registered_types/0
+        , get_opts/1
+        , set_opts/1
+        , set_opts/2
         , types_from_list/1
         , revert_to_default_types/0
         , dynamic_types/0 ]).
@@ -121,6 +124,7 @@ dynamic_types() ->
             , id     => id
             , label  => label
             }
+     , options => #{}
        }.
 
 vsn(Types) ->
@@ -399,15 +403,29 @@ emit(0, _, _, Enc) ->
 emit_code(Tag, #{rev := Tags}) ->
     encode_basic(int, maps:get(Tag, Tags)).
 
-decode_basic(Type, [Tag,V], #{codes := Codes}) ->
+decode_basic(Type, [Tag,V], #{codes := Codes} = Types) ->
     case decode_basic(int, Tag) of
         Code when map_get(Code, Codes) == Type ->
-            decode_basic(Type, V);
+            decode_basic_(Type, V, Types);
         _ ->
             error(illegal)
     end;
-decode_basic(Type, V, _) ->
-    decode_basic(Type, V).
+decode_basic(Type, V, Types) ->
+    decode_basic_(Type, V, Types).
+
+decode_basic_(label, Fld, #{options := #{missing_labels := Opt}}) ->
+    Bin = decode_basic(binary, Fld),
+    case Opt of
+        create  -> binary_to_atom(Bin, utf8);
+        fail    -> binary_to_existing_atom(Bin, utf8);
+        convert ->
+            try binary_to_existing_atom(Bin, utf8)
+            catch
+                error:_ -> Bin
+            end
+    end;
+decode_basic_(label, Fld, _) ->
+    decode_basic(label, Fld).
 
 decode_basic(label, Fld) ->
     binary_to_existing_atom(decode_basic(binary, Fld), utf8);
@@ -473,6 +491,15 @@ register_type(Code, Tag, Template) when is_integer(Code), Code >= 0 ->
         {_, true} -> error(tag_exists)
     end.
 
+set_opts(Opts) ->
+    set_opts(Opts, registered_types()).
+
+set_opts(Opts, Types) ->
+    Types#{options => Opts}.
+
+get_opts(#{options := Opts}) ->
+    Opts.
+
 cache_label(Code, Label) when is_integer(Code), Code >= 0, is_atom(Label) ->
     #{labels := Lbls, rev_labels := RevLbls} = Types = registered_types(),
     case {is_map_key(Label, Lbls), is_map_key(Code, RevLbls)} of