Document nesting of types

Author: hahnjo

types/README.md

@@ -6,3 +6,41 @@
  * [`unique_ptr`](unique_ptr): `std::unique_ptr` with different element types
  * [`variant`](variant): `std::variant` with `Switch` column type
  * [`vector`](vector): `std::vector` with all `[Split]Index{32,64}` column types
+
+## Nesting of Types
+
+In the RNTuple Validation Suite, we cannot test all possible nesting combinations of supported types.
+Furthermore, we assume that the column encoding on disk is orthogonal to the nesting of fields.
+We therefore cover a strategic selection based on field structural roles that is described below.
+Implementations are required to exercise specific details in their own unit tests.
+In particular, this includes any optimization that contradicts the assumptions made above.
+
+### Leaf Fields
+
+For leaf fields, we test all possible column encodings in a top-level field.
+
+### Collection Fields
+
+For collection fields (for example `std::vector`), we want to test all `[Split]Index{32,64}` column types.
+
+If `C` is a container type with a single item field, we test the following nestings:
+ * `C<std::int32_t>`
+ * `C<C<std::int32_t>>`
+
+If `M` is an dictionary container type, we test the following nestings:
+ * `M<std::int32_t, std::int32_t>`
+ * `M<M<std::int32_t, std::int32_t>, M<std::int32_t, std::int32_t>>`
+
+### Record Fields
+
+For record fields of type `R` (for example `std::tuple`), we want to test nesting with other field structural roles:
+ * Leaf fields of types `std::int32_t` and `std::string` (the latter is particular because it has variable length and two columns)
+ * Variant field of type `std::variant<std::int32_t, std::string>`
+ * Collection field of type `std::vector<std::int32_t>`
+ * Record field of the type `R` itself
+
+In addition, we test the nesting of the record type inside a collection field, `std::vector<R>`.
+
+### Variant Fields
+
+As there is only one type that results in this field structural role, the reader is referred to the [`variant`](variant) directory.