# Stable marriage problem

```perl
my %he-likes =
    abe  => < abi eve cath ivy jan dee fay bea hope gay >,
    bob  => < cath hope abi dee eve fay bea jan ivy gay >,
    col  => < hope eve abi dee bea fay ivy gay cath jan >,
    dan  => < ivy fay dee gay hope eve jan bea cath abi >,
    ed   => < jan dee bea cath fay eve abi ivy hope gay >,
    fred => < bea abi dee gay eve ivy cath jan hope fay >,
    gav  => < gay eve ivy bea cath abi dee hope jan fay >,
    hal  => < abi eve hope fay ivy cath jan bea gay dee >,
    ian  => < hope cath dee gay bea abi fay ivy jan eve >,
    jon  => < abi fay jan gay eve bea dee cath ivy hope >,
;
 
my %she-likes =
    abi  => < bob fred jon gav ian abe dan ed col hal >,
    bea  => < bob abe col fred gav dan ian ed jon hal >,
    cath => < fred bob ed gav hal col ian abe dan jon >,
    dee  => < fred jon col abe ian hal gav dan bob ed >,
    eve  => < jon hal fred dan abe gav col ed ian bob >,
    fay  => < bob abe ed ian jon dan fred gav col hal >,
    gay  => < jon gav hal fred bob abe col ed dan ian >,
    hope => < gav jon bob abe ian dan hal ed col fred >,
    ivy  => < ian col hal gav fred bob abe ed jon dan >,
    jan  => < ed hal gav abe bob jon col ian fred dan >,
;

my %fiancé;
my %fiancée;
my %proposed;
 
sub she-prefers ($her, $hottie) { .index($hottie) < .index(%fiancé{$her}) given ~%she-likes{$her} }
sub he-prefers  ($him, $hottie) { .index($hottie) < .index(%fiancée{$him}) given ~%he-likes{$him} }
 
match'em;
check-stability;

perturb'em;
check-stability;
 
sub match'em {                                          #'
    say 'Matchmaking:';
    while unmatched-guy() -> $guy {
        my $gal = preferred-choice($guy);
        %proposed{"$guy $gal"} = '❤';
        if not %fiancé{$gal} {
            engage($guy, $gal);
            say "\t$gal and $guy";
        }
        elsif she-prefers($gal, $guy) {
	    my $engaged-guy = %fiancé{$gal};
	    engage($guy, $gal);
	    %fiancée{$engaged-guy} = '';
	    say "\t$gal dumped $engaged-guy for $guy";
	}
    }
}
 
sub check-stability {
    my @instabilities = gather for flat %he-likes.keys X %she-likes.keys -> $m, $w {
	if he-prefers($m, $w) and she-prefers($w, $m) {
	    take "\t$w prefers $m to %fiancé{$w} and $m prefers $w to %fiancée{$m}";
	}
    }

    say 'Stablility:';
    if @instabilities {
	.say for @instabilities;
    }
    else {
        say "\t(all marriages stable)";
    }
}
 
sub unmatched-guy { %he-likes.keys.first: { not %fiancée{$_} } }
 
sub preferred-choice($guy) { %he-likes{$guy}.first: { not %proposed{"$guy $_" } } }
 
sub engage($guy, $gal) {
    %fiancé{$gal} = $guy;
    %fiancée{$guy} = $gal;
}
 
sub perturb'em {                                            #'
    say 'Perturb:';
    say "\tengage abi with fred and bea with jon";
    engage('fred', 'abi');
    engage('jon', 'bea');
}
```

#### Output:

```
Matchmaking:
        cath and bob
        ivy and dan
        abi and jon
        hope and ian
        jan and ed
        eve and abe
        eve dumped abe for hal
        ivy dumped dan for abe
        fay and dan
        gay and gav
        bea and fred
        dee and col
Stablility:
        (all marriages stable)
Perturb:
        engage abi with fred and bea with jon
Stablility:
        fay prefers jon to dan and jon prefers fay to bea
        eve prefers jon to hal and jon prefers eve to bea
        gay prefers jon to gav and jon prefers gay to bea
        bea prefers fred to jon and fred prefers bea to abi
```


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