Changed map to filter map so that Phragmen ignores empty voters (paritytech#7378)

* Changed map to filter map so that Phragmen ignores empty voters

* Resolve flaws and added test case

* Updated test

Author: jaxter03

primitives/npos-elections/src/lib.rs

@@ -629,7 +629,7 @@ pub(crate) fn setup_inputs<AccountId: IdentifierT>(
 		})
 		.collect::<Vec<CandidatePtr<AccountId>>>();
 
-	let voters = initial_voters.into_iter().map(|(who, voter_stake, votes)| {
+	let voters = initial_voters.into_iter().filter_map(|(who, voter_stake, votes)| {
 		let mut edges: Vec<Edge<AccountId>> = Vec::with_capacity(votes.len());
 		for v in votes {
 			if edges.iter().any(|e| e.who == v) {
@@ -650,12 +650,18 @@ pub(crate) fn setup_inputs<AccountId: IdentifierT>(
 				);
 			} // else {} would be wrong votes. We don't really care about it.
 		}
-		Voter {
-			who,
-			edges: edges,
-			budget: voter_stake.into(),
-			load: Rational128::zero(),
+		if edges.is_empty() {
+			None
+		}
+		else {
+			Some(Voter {
+				who,
+				edges: edges,
+				budget: voter_stake.into(),
+				load: Rational128::zero(),
+			})
 		}
+
 	}).collect::<Vec<_>>();
 
 	(candidates, voters,)

primitives/npos-elections/src/tests.rs

@@ -72,6 +72,46 @@ fn float_phragmen_poc_works() {
 	);
 }
 
+#[test]
+fn phragmen_core_test_without_edges() {
+	let candidates = vec![1, 2, 3];
+	let voters = vec![
+		(10, 10, vec![]),
+		(20, 20, vec![]),
+		(30, 30, vec![]),
+	];
+
+	let (candidates, voters) = setup_inputs(candidates, voters);
+
+	assert_eq!(
+		voters
+			.iter()
+			.map(|v| (
+				v.who,
+				v.budget,
+				(v.edges.iter().map(|e| (e.who, e.weight)).collect::<Vec<_>>()),
+			))
+			.collect::<Vec<_>>(),
+		vec![]
+	);
+
+	assert_eq!(
+		candidates
+			.iter()
+			.map(|c_ptr| (
+				c_ptr.borrow().who,
+				c_ptr.borrow().elected,
+				c_ptr.borrow().round,
+				c_ptr.borrow().backed_stake,
+			)).collect::<Vec<_>>(),
+		vec![
+			(1, false, 0, 0),
+			(2, false, 0, 0),
+			(3, false, 0, 0),
+		]
+	);
+}
+
 #[test]
 fn phragmen_core_poc_works() {
 	let candidates = vec![1, 2, 3];