2 files changed, 60 insertions, 0 deletions
diff --git a/lib/concepts/Multigroup.ml b/lib/concepts/Multigroup.ml
new file mode 100644
index 0000000..863cd26
--- /dev/null
+++ b/lib/concepts/Multigroup.ml
@@ -0,0 +1,51 @@
+type address = Hash.hash
+
+type t =
+  { name: string;
+    relations: address }
+
+module Field = struct
+  let name = "name"
+  let relations = "relations"
+end
+
+module Error = struct
+  open Condition
+
+  let malformed_multigroup () =
+    condition "multigroup-malformed"
+      "A multigroup representation did not conform to what was expected" empty
+
+  let malformed_hash length =
+    condition "multigroup-malformed-hash" "A multigroup relation root was not a valid hash"
+      ("length" |=| Value.Integer length)
+end
+
+let make ~name ~relations = {name; relations}
+
+let name multigroup = multigroup.name
+let relations multigroup = multigroup.relations
+
+module Body = struct
+  type nonrec t = t
+
+  let tag = 'm'
+  let malformed = Error.malformed_multigroup
+
+  let equal left right =
+    left.name = right.name && Hash.hash_equals left.relations right.relations
+
+  let fields multigroup =
+    [ Field.name, Object.Field.string multigroup.name;
+      Field.relations, Object.Field.address multigroup.relations ]
+
+  let of_fields data =
+    let open Utilities.Result in
+    let* name = Object.Field.require_string Field.name data in
+    let* relations =
+      Object.Field.require_address ~malformed:Error.malformed_hash Field.relations data
+    in
+    Ok {name; relations}
+end
+
+include (Object.Codec.Make (Body) : Object.S with type t := t)
diff --git a/lib/concepts/Multigroup.mli b/lib/concepts/Multigroup.mli
new file mode 100644
index 0000000..1a76dce
--- /dev/null
+++ b/lib/concepts/Multigroup.mli
@@ -0,0 +1,9 @@
+type t
+type address = Hash.hash
+
+include Object.S with type t := t
+
+val make : name:string -> relations:address -> t
+
+val name : t -> string
+val relations : t -> address