Fix offchain election to respect the weight (#7215)

* Mockup

* Fix offchain election to respect the weight

* Fix builds a bit

* Update frame/staking/src/offchain_election.rs

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* Update frame/staking/src/offchain_election.rs

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* Make it build, binary search

* Fix a number of grumbles

* one more fix.

* remove unwrap.

* better alg.

* Better alg again.

* Final fixes

* Fix

* Rollback to normal

* Final touches.

* Better tests.

* Update frame/staking/src/lib.rs

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

* Proper maxExtWeight

* Final fix

* Final fix for the find_voter

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

substrate/primitives/npos-elections/compact/src/lib.rs

@@ -152,6 +152,7 @@ fn struct_def(
 	let len_impl = len_impl(count);
 	let edge_count_impl = edge_count_impl(count);
 	let unique_targets_impl = unique_targets_impl(count);
+	let remove_voter_impl = remove_voter_impl(count);
 
 	let derives_and_maybe_compact_encoding = if compact_encoding {
 		// custom compact encoding.
@@ -220,10 +221,58 @@ fn struct_def(
 			pub fn average_edge_count(&self) -> usize {
 				self.edge_count().checked_div(self.len()).unwrap_or(0)
 			}
+
+			/// Remove a certain voter.
+			///
+			/// This will only search until the first instance of `to_remove`, and return true. If
+			/// no instance is found (no-op), then it returns false.
+			///
+			/// In other words, if this return true, exactly one element must have been removed from
+			/// `self.len()`.
+			pub fn remove_voter(&mut self, to_remove: #voter_type) -> bool {
+				#remove_voter_impl
+				return false
+			}
 		}
 	))
 }
 
+fn remove_voter_impl(count: usize) -> TokenStream2 {
+	let field_name = field_name_for(1);
+	let single = quote! {
+		if let Some(idx) = self.#field_name.iter().position(|(x, _)| *x == to_remove) {
+			self.#field_name.remove(idx);
+			return true
+		}
+	};
+
+	let field_name = field_name_for(2);
+	let double = quote! {
+		if let Some(idx) = self.#field_name.iter().position(|(x, _, _)| *x == to_remove) {
+			self.#field_name.remove(idx);
+			return true
+		}
+	};
+
+	let rest = (3..=count)
+		.map(|c| {
+			let field_name = field_name_for(c);
+			quote! {
+				if let Some(idx) = self.#field_name.iter().position(|(x, _, _)| *x == to_remove) {
+					self.#field_name.remove(idx);
+					return true
+				}
+			}
+		})
+		.collect::<TokenStream2>();
+
+	quote! {
+		#single
+		#double
+		#rest
+	}
+}
+
 fn len_impl(count: usize) -> TokenStream2 {
 	(1..=count).map(|c| {
 		let field_name = field_name_for(c);

substrate/primitives/npos-elections/src/tests.rs

@@ -1155,6 +1155,69 @@ mod solution_type {
 		assert_eq!(compact.unique_targets(), vec![10, 11, 20, 40, 50, 51]);
 	}
 
+	#[test]
+	fn remove_voter_works() {
+		let mut compact = TestSolutionCompact {
+			votes1: vec![(0, 2), (1, 6)],
+			votes2: vec![
+				(2, (0, TestAccuracy::from_percent(80)), 1),
+				(3, (7, TestAccuracy::from_percent(85)), 8),
+			],
+			votes3: vec![
+				(
+					4,
+					[(3, TestAccuracy::from_percent(50)), (4, TestAccuracy::from_percent(25))],
+					5,
+				),
+			],
+			..Default::default()
+		};
+
+		assert!(!compact.remove_voter(11));
+		assert!(compact.remove_voter(2));
+		assert_eq!(
+			compact,
+			TestSolutionCompact {
+				votes1: vec![(0, 2), (1, 6)],
+				votes2: vec![
+					(3, (7, TestAccuracy::from_percent(85)), 8),
+				],
+				votes3: vec![
+					(
+						4,
+						[(3, TestAccuracy::from_percent(50)), (4, TestAccuracy::from_percent(25))],
+						5,
+					),
+				],
+				..Default::default()
+			},
+		);
+
+		assert!(compact.remove_voter(4));
+		assert_eq!(
+			compact,
+			TestSolutionCompact {
+				votes1: vec![(0, 2), (1, 6)],
+				votes2: vec![
+					(3, (7, TestAccuracy::from_percent(85)), 8),
+				],
+				..Default::default()
+			},
+		);
+
+		assert!(compact.remove_voter(1));
+		assert_eq!(
+			compact,
+			TestSolutionCompact {
+				votes1: vec![(0, 2)],
+				votes2: vec![
+					(3, (7, TestAccuracy::from_percent(85)), 8),
+				],
+				..Default::default()
+			},
+		);
+	}
+
 	#[test]
 	fn basic_from_and_into_compact_works_assignments() {
 		let voters = vec![