Dispute vote filtering for block authors (#3498)

* guide: filter_multi_dispute_data

* guide: elaborate

* Implementation of dispute data filtering

* tests for filtering

* don't use std, you fool!

* use swap_remove

* Update runtime/parachains/src/disputes.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* use btreeste

* address API nit

Co-authored-by: Andronik Ordian <write@reusable.software>

primitives/src/v1/mod.rs

@@ -1124,6 +1124,22 @@ impl DisputeStatement {
 			Err(())
 		}
 	}
+
+	/// Whether the statement indicates validity.
+	pub fn indicates_validity(&self) -> bool {
+		match *self {
+			DisputeStatement::Valid(_) => true,
+			DisputeStatement::Invalid(_) => false,
+		}
+	}
+
+	/// Whether the statement indicates invalidity.
+	pub fn indicates_invalidity(&self) -> bool {
+		match *self {
+			DisputeStatement::Valid(_) => false,
+			DisputeStatement::Invalid(_) => true,
+		}
+	}
 }
 
 /// Different kinds of statements of validity on  a candidate.

roadmap/implementers-guide/src/runtime/disputes.md

@@ -66,6 +66,13 @@ Frozen: Option<BlockNumber>,
 
 ## Routines
 
+* `filter_multi_dispute_data(MultiDisputeStatementSet) -> MultiDisputeStatementSet`:
+  1. Takes a `MultiDisputeStatementSet` and filters it down to a `MultiDisputeStatementSet`
+    that satisfies all the criteria of `provide_multi_dispute_data`. That is, eliminating
+    ancient votes, votes which overwhelm the maximum amount of spam slots, and duplicates. 
+    This can be used by block authors to create the final submission in a block which is 
+    guaranteed to pass the `provide_multi_dispute_data` checks.
+
 * `provide_multi_dispute_data(MultiDisputeStatementSet) -> Vec<(SessionIndex, Hash)>`:
   1. Pass on each dispute statement set to `provide_dispute_data`, propagating failure.
   1. Return a list of all candidates who just had disputes initiated.