Complete ERS76 implementation and add test case

docs/options.md

@@ -20,7 +20,7 @@ The preset dropdown allows you to choose from a hardcoded list of preloaded STV
 | [Wright STV](https://www.aph.gov.au/Parliamentary_Business/Committees/House_of_Representatives_Committees?url=em/elect07/subs/sub051.1.pdf) | Rules proposed by Anthony van der Craats designed for computer counting, involving reset and re-iteration of the count after each candidate exclusion. | | ✓ |
 | [PRSA 1977](https://www.prsa.org.au/rule1977.htm) | Simple rules designed for hand counting, using the exclusive Gregory method, with counting performed in thousandths of a vote. | | ✓ |
 | [ERS97](https://www.electoral-reform.org.uk/latest-news-and-research/publications/how-to-conduct-an-election-by-the-single-transferable-vote-3rd-edition/) | More complex rules designed for hand counting, using the exclusive Gregory method. | | ✓ |
-| • ERS76 | Former rules from the 1976 2nd edition. | [E6] | |
+| • ERS76 | Former rules from the 1976 2nd edition. | [E6] | ✓ |
 | • ERS73 | Former rules from the 1973 1st edition. | [E6] | |
 | Church of England | Rules from the Church of England [*Single Transferable Vote Rules 2020*](https://www.churchofengland.org/sites/default/files/2020-02/STV%20Rules%202020%20-%20final.pdf), similar to ERS73. | |
 
@@ -31,7 +31,7 @@ Exceptions:
 * [E3] A tie between 2 candidates for the final vacancy will be broken backwards then at random, rather than the method described in the legislation.
 * [E4] Bulk exclusion is not performed. See the section on *Bulk exclusion* for further discussion.
 * [E5] The ‘mathematically eliminated by the sum of all ranked-choice votes comparison’ is not implemented.
-* [E6] The quota is always calculated to 2 decimal places. For full ERS76 (ERS73) compliance, set *Round quota to 0 d.p.* when the quota is more than 100 (100 or more).
+* [E6] By default, the quota is always calculated to 2 decimal places. For full ERS76 (ERS73) compliance, set *Round quota to 0 d.p.* when the quota is more than 100 (100 or more).
 
 For details of validation, see [validation.md](validation.md).
 

docs/validation.md

@@ -18,6 +18,7 @@ STV-counting software is frequently validated empirically by comparing the resul
 | PRSA 1977 | [*Proportional Representation Manual*](https://www.prsa.org.au/publicat.htm#p2) [example 1](https://www.prsa.org.au/utopiatc.pdf) | [Model result](https://www.prsa.org.au/example1.pdf) (official) | ✓ |
 | ERS97 | [Reverse engineered ballot papers for the ERS97 model election](https://yingtongli.me/blog/2021/01/04/ers97.html) | [Model result](https://www.electoral-reform.org.uk/latest-news-and-research/publications/how-to-conduct-an-election-by-the-single-transferable-vote-3rd-edition/#sub-section-24) (official) | ✓ |
 | ERS97 | [Joe Otten/eSTV ballot papers for the ERS97 model election](https://web.archive.org/web/20020606014623/http://estv.otten.co.uk/) | [Model result](https://www.electoral-reform.org.uk/latest-news-and-research/publications/how-to-conduct-an-election-by-the-single-transferable-vote-3rd-edition/#sub-section-24) (official) | ✓ |
+| ERS76 | Ballot papers adapted from Joe Otten/eSTV ERS97 papers | Model result (official) | ✓ |
 
 # References
 

src/stv/mod.rs

@@ -785,8 +785,14 @@ fn total_to_quota<N: Number>(mut total: N, seats: usize, opts: &STVOptions) -> N
 	return total;
 }
 
-/// Update vote required for election according to ERS97 rules
+/// Update vote required for election according to ERS97/ERS76 rules
 fn update_vre<N: Number>(state: &mut CountState<N>, opts: &STVOptions) {
+	if opts.quota_mode == QuotaMode::ERS76 && state.num_excluded == 0 && (state.num_elected == 0 || state.candidates.values().all(|cc| !cc.finalised)) {
+		// ERS76 rules: Do not update VRE until a surplus is distributed or candidate is excluded
+		state.vote_required_election = state.quota.clone();
+		return;
+	}
+	
 	let mut log = String::new();
 	
 	// Calculate active vote
@@ -1023,8 +1029,11 @@ fn elect_hopefuls<'a, N: Number>(state: &mut CountState<'a, N>, opts: &STVOption
 		count_card.state = CandidateState::Elected;
 		state.num_elected += 1;
 		count_card.order_elected = state.num_elected as isize;
+		
+		let elected_on_quota;
 		if cmp_quota_criterion(state.quota.as_ref().unwrap(), count_card, opts) {
 			// Elected with a quota
+			elected_on_quota = true;
 			state.logger.log_smart(
 				"{} meets the quota and is elected.",
 				"{} meet the quota and are elected.",
@@ -1032,6 +1041,7 @@ fn elect_hopefuls<'a, N: Number>(state: &mut CountState<'a, N>, opts: &STVOption
 			);
 		} else {
 			// Elected with vote required
+			elected_on_quota = false;
 			state.logger.log_smart(
 				"{} meets the vote required and is elected.",
 				"{} meet the vote required and are elected.",
@@ -1051,9 +1061,10 @@ fn elect_hopefuls<'a, N: Number>(state: &mut CountState<'a, N>, opts: &STVOption
 			cands_meeting_quota.remove(cands_meeting_quota.iter().position(|c| *c == candidate).unwrap());
 		}
 		
-		if opts.quota_mode == QuotaMode::ERS97 {
+		if opts.quota_mode == QuotaMode::ERS97 || (opts.quota_mode == QuotaMode::ERS76 && elected_on_quota) {
 			// Vote required for election may have changed
-			// This is not performed in ERS76 - TODO: Check this
+			// ERS97: Check this after every elected candidate (cf. model election)
+			// ERS76: Check this after every candidate elected on a quota, but all at once for candidates elected on VRE (cf. model election)
 			calculate_quota(state, opts);
 			
 			// Repeat in case vote required for election has changed

tests/data/.~lock.ers76.ods#

@@ -0,0 +1 @@
+Yingtong Li,runassudo,NEXUS.localdomain,09.08.2021 19:43,file:///home/runassudo/.config/libreoffice/4;

tests/data/ers76.blt

@@ -0,0 +1,71 @@
+# Comment: Ballot papers for the ERS76 model election - all votes - ERS76
+# Source: Adapted by RunasSudo from ers97.blt (Joe Otten)
+# Contributor: RunasSudo
+11 6
+8 1 2 0
+3 1 3 0
+14 1 4 0
+34 1 5 0
+1 1 2 0 # Changed from "1 6"
+4 1 7 0
+1 1 8 0
+3 1 9 3 0 # One of these changed to "1 9 6"
+3 1 9 4 0
+3 1 9 6 0
+3 1 9 7 0
+4 1 9 8 0
+9 1 9 0
+4 1 10 3 0
+5 1 10 4 0
+3 1 10 6 0
+3 1 10 7 0
+6 1 10 8 0
+10 1 10 0
+1 1 11 5 0
+1 1 11 0
+11 1 0
+105 2 0
+91 3 0
+90 4 0
+81 5 0
+64 6 0
+11 7 6 0
+36 7 8 0
+12 7 0
+55 8 0
+3 9 3 0
+2 9 4 0
+2 9 5 3 0
+1 9 5 4 0
+15 9 5 0
+2 9 6 0
+2 9 8 0
+2 10 3 0
+2 10 4 0
+2 10 5 6 0
+1 10 5 7 6 0
+2 10 5 8 0
+11 10 5 0
+1 10 6 0
+1 10 7 4 0
+1 10 8 0
+1 10 0
+2 11 2 0
+4 11 3 0
+1 11 4 0
+5 11 7 8 0
+10 11 8 0
+1 11 0
+0
+"Smith"
+"Duke"
+"Prince"
+"Freeman"
+"Carpenter"
+"Baron"
+"Abbot"
+"Vicar"
+"Wright"
+"Glazier"
+"Monk"
+"ERS Model - 1997 Edition"

tests/data/ers76.csv

@@ -0,0 +1,15 @@
+Stage:,1,,2,,4,,6,,7,,8,
+Comment:,First preferences,,Surplus of Smith,,Exclusion of Monk,,"Exclusion of Glazier, Wright",,Surplus of Carpenter,,Exclusion of Abbot,
+Smith,134,EL,108,EL,108,EL,108,EL,108,EL,108,EL
+Duke,105,H,106.89,H,108.89,EL,108.89,EL,108.89,EL,108.89,EL
+Prince,91,H,91.63,H,95.63,H,102.1,H,104.1,EL,104.1,EL
+Freeman,90,H,92.94,H,93.94,H,99.62,H,100.62,H,101.62,EL
+Carpenter,81,H,88.14,H,88.35,H,122.35,EL,108,EL,108,EL
+Baron,64,H,64,H,64,H,68.26,H,70.26,H,82.26,H
+Abbot,59,H,59.84,H,64.84,H,67.1,H,68.1,H,2.1,EX
+Vicar,55,H,55.21,H,65.21,H,70.31,H,72.31,H,113.31,EL
+Wright,27,H,32.25,H,32.25,H,0,EX,0,EX,0,EX
+Glazier,24,H,30.51,H,30.51,H,0,EX,0,EX,0,EX
+Monk,23,H,23.42,H,0,EX,0,EX,0,EX,0,EX
+Non-transferable,0,,0.17,,1.38,,6.37,,12.72,,24.72,
+Votes required,108,,,,,,,,104.07,,96.09,

tests/ers.rs

@@ -61,3 +61,24 @@ fn ers97_rational() {
 	
 	utils::read_validate_election::<Rational>("tests/data/ers97.csv", "tests/data/ers97.blt", stv_opts, None, &["nt", "vre"]);
 }
+
+#[test]
+fn ers76_rational() {
+	let stv_opts = stv::STVOptionsBuilder::default()
+		.round_surplus_fractions(Some(2))
+		.round_values(Some(2))
+		.round_votes(Some(2))
+		.round_quota(Some(0))
+		.quota(stv::QuotaType::DroopExact)
+		.quota_criterion(stv::QuotaCriterion::GreaterOrEqual)
+		.quota_mode(stv::QuotaMode::ERS76)
+		.surplus(stv::SurplusMethod::EG)
+		.transferable_only(true)
+		.exclusion(stv::ExclusionMethod::ByValue)
+		.early_bulk_elect(false)
+		.bulk_exclude(true)
+		.defer_surpluses(true)
+		.build().unwrap();
+	
+	utils::read_validate_election::<Rational>("tests/data/ers76.csv", "tests/data/ers76.blt", stv_opts, None, &["nt", "vre"]);
+}