From 4dbe0e2709b218d561392fe5496109b6bd346651 Mon Sep 17 00:00:00 2001
From: Ian Jackson <ijackson@chiark.greenend.org.uk>
Date: Sun, 21 Aug 2016 17:08:14 +0100
Subject: [PATCH] compute-scottish-stv: wip

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
---
 compute-scottish-stv | 150 +++++++++++++++++++++++++++++++++++++++----
 1 file changed, 136 insertions(+), 14 deletions(-)

diff --git a/compute-scottish-stv b/compute-scottish-stv
index 77e00da..bb91c68 100644
--- a/compute-scottish-stv
+++ b/compute-scottish-stv
@@ -4,22 +4,27 @@
 # The Scottish Local Government Elections Order 2007
 
 use strict;
+use bigrat;
 
 # Data structures:
 #
 # vote is
 #   { Voter => opaque,
-#     Prefs => [ list ],
+#     Prefs => [ [ CAND, ...], ... ],
 #     Weight => 1.0 }
 # We edit Prefs as we go
 
 # $cands{CAND}{Desc}
 # $cands{CAND}{Votes}
-
-our $stage=0;
+# $cands{CAND}{Total}
+# $cands{CAND}{Continuing}
 
 our $seats=0;
 
+our $stage=0;
+our $quota;
+our %tie;
+
 sub unkopt ($$) {
     my ($what,$opt) = @_;
     if ($opt =~ m/^[A-Z]/) {
@@ -35,6 +40,12 @@ for (;;) {
 	foreach $_ (split / /, $') {
 	    if (m/^_?[Ss]eats=(\d+)/) {
 		$seats = $1;
+	    } elsif (m/^_?[Tt]ie=(.*)\>(.*)$/) {
+		my @more = split /\,/, $1;
+		my @less = split /\,/, $2;
+		my @all = join ',', sort (@more, @less);
+		$tie{"@all"}{Win}  = $more[0] if @more == 1;
+		$tie{"@all"}{Lose} = $less[0] if @less == 1;
 	    } else {
 		unkopt "election", $_;
 	    }
@@ -65,6 +76,8 @@ for (;;) {
     }
 }
 
+$_->{Continuing} = 1 foreach values %cands;
+
 sub sortballots (@) {
     # Takes each argument, which should be a ballot, sorts
     # it into $cand{CAND}{Votes} according to first preference.
@@ -73,6 +86,7 @@ sub sortballots (@) {
     # and looks for further preferences.
     foreach my $v (@_) {
 	my $firstprefs = shift @{ $v->{Prefs} };
+	my $w = $v->{Weight};
 	if (!$firstprefs || !@$firstprefs) {
 	    vlog $v, "no more preferences, non transferable";
 	    push @non_transferable, $v;
@@ -83,20 +97,128 @@ sub sortballots (@) {
 	    foreach my $fpref (@$firstprefs) {
 		my $v2 = {
 		    %$v,
-		    Weight => $v->{Weight} / @$firstprefs,
+		    Weight => $w / @$firstprefs,
 		    Prefs => [ [ $fpref ], @{ $v->{Prefs} } ],
-			 };
+                };
 		vlog $v, "split for $fpref";
-		
-			
-		    Voter => $
-	    
-	my @prefs 
-	my $nprefs = scalar @{ $v->{Prefs}
-    my @input = @_;
-    
+	    }
+	    next;
+	}
+	my $fp = $firstprefs[0];
+	my $c = $cands{$fp};
+	if (!$c->{Continuing}) {
+	    vlog $v, "dropping pref $fp, not a continuing candidate";
+	    sortvallots $v;
+	    next;
+	}
+	vlog $v, "sorted into pile for candidate $fp weight $w";
+	push @{ $c->{Votes} }, $v;
+    }
+}
+
+sub prf {
+    my $fmt = shift;
+    printf "stage %d: ".$_, $stage, @_;
+}
+
+sub countballots () {
+    foreach my $c (values %cand) {
+	$c->{Total} = 0;
+	$c->{Total} += $_->{Weight} foreach @{ $c->{Voters} };
+    }
+    foreach my $cand (sort keys %cand) {
+	$c = $cands{$cand};
+	next unless $c->{Continuing};
+	prf "cand %s: %s votes\n", $stage, $cand, $c->{Total};
+	$c->{History}[$stage-1] = $c->{Total};
+    }
+}
+
+sub computequota () {
+    my $totalvalid = 0;
+    $totalvalid += $_->{Total} foreach keys %cands;
+    $quota = floor($totalvalid / (1 + $seats));
+}
 
-# $cands
+sub total_history_cmp () {
+    my $ha = $cands{a}{History};
+    my $hb = $cands{a}{History};
+    foreach my $s (reverse 1 .. $stage) {
+	my $d = $ha->[$s] <=> $hb->[$s];
+	next unless $d;
+	print DEBUG "history cmp $a $b => $d (#$s $ha->[$s] $hb->[$s])\n";
+	return $e;
+    }
+    return 0;
+}
+
+sortballots @allvotes;
 
 for (;;) {
+    $stage++;
+    countballots();
+    
+    my @maybe_elect = reverse sort total_history_cmp keys %cands;
+    my $nelect=0;
+    for (;;) {
+	my $nextcand = $maybe_elect[$nelect];
+	my $nextc = $cands{$nextcand};
+
+	# We certainly only consider those who meet quota
+	last unless $nextc->{Total} >= $quota;
+	last unless $nextc->{Total} > $quota && $nextel;
+	# ... if equal we can do them one by one, since order
+	# does not matter (SLGEO 49 talks about `two or more ... exceeds')
+
+	last if $nelect &&
+	    (total_history_cmp $maybe_elect[0], $nextcand) > 0;
+	# ... only interested in those who compare equal
+	# according ot the history (SLGEO 49(2)); NB our history
+	# includes the current round.
+
+	$nelect++;
+    }
+
+    if ($nelect) {
+	my $elect;
+	if ($nelect > 1) {
+	    my @all = @maybe_elect[0 .. $nelect-1];
+	    my $elect = $tie{"@all"}{Win};
+	    die "need tie break, want winner from @all" unless defined $win;
+	    prf "electing %s due to tie break amongst %s\n",
+		$elect, "@all";
+	} else {
+	    $elect = $maybe_elect[0];
+	    prf "electing %s\n";
+	}
+
+	prf "*** ELECT %s ***\n", $elect;
+	$c->{Continuing} = 0;
+	votelog $_, "helped elect $elect" foreach @{ $c->{Votes} };
+	
+	# SLGEO 48
+	my $c = $cands{$elect};
+	my $surplus = $c->{Total} - $quota;
+
+	if ($surplus <= 0) {
+	    prf "no surplus\n";
+	    next;
+	}
+
+	my $B = $c->{Weight};
+
+	foreach my $v (@{ $c->{Votes} }) {
+	    my $A = $surplus * $v->{Weight};
+	    my $F = 100000;
+	    my $xfervalue = floor(($A * $F) / $B) / $f;
+	    # SLGEO 48(3): we do arithmetic to 5 d3ecimal places,
+	    # but always rounding down
+	    votelog $v, "transferring with value $xfervalue (A=$A B=$B)";
+	    $v->{Weight} = $xfervalue;
+	    sortballots $v;
+	}
+	$c->{Votes} = { }; # will crash if we access it again
+	next;
+    }
+
     
-- 
2.30.2