feat: initialize Kurdistan SDK - independent fork of Polkadot SDK

pezkuwi/roadmap/implementers-guide/src/runtime-api/validator-groups.md

@@ -0,0 +1,32 @@
+# Validator Groups
+
+Yields the validator groups used during the current session. The validators in the groups are referred to by their index
+into the validator-set and this is assumed to be as-of the child of the block whose state is being queried.
+
+```rust
+/// A helper data-type for tracking validator-group rotations.
+struct GroupRotationInfo {
+    session_start_block: BlockNumber,
+    group_rotation_frequency: BlockNumber,
+    now: BlockNumber, // The successor of the block in whose state this runtime API is queried.
+}
+
+impl GroupRotationInfo {
+    /// Returns the index of the group needed to validate the core at the given index,
+    /// assuming the given amount of cores/groups.
+    fn group_for_core(&self, core_index, cores) -> GroupIndex;
+
+    /// Returns the block number of the next rotation after the current block. If the current block
+    /// is 10 and the rotation frequency is 5, this should return 15.
+    fn next_rotation_at(&self) -> BlockNumber;
+
+    /// Returns the block number of the last rotation before or including the current block. If the
+    /// current block is 10 and the rotation frequency is 5, this should return 10.
+    fn last_rotation_at(&self) -> BlockNumber;
+}
+
+/// Returns the validator groups and rotation info localized based on the block whose state
+/// this is invoked on. Note that `now` in the `GroupRotationInfo` should be the successor of
+/// the number of the block.
+fn validator_groups(at: Block) -> (Vec<Vec<ValidatorIndex>>, GroupRotationInfo);
+```