Stable matching matches least preferable - nim-lang

Question

I have implemented the stable matching algorithm but it seems that the implementation matches the least preferable candidates. I'm asking for a code review:
import options, sequtils
type
Person* = ref object
id: int
name: string
engagedTo: Option[Person]
preferences: seq[Person]
MatchPool* = object
husbands, wives: seq[Person]
proc newPerson*(nameofperson: string, isMale: bool = true, engagedTo: Option[
Person] = none(Person), preferences: openArray[Person] = []): Person =
var
nextMaleId = 1
nextFemaleId = -1
id: int
if isMale:
id = nextMaleId
inc nextMaleId
else:
id = nextFemaleId
dec nextFemaleId
return Person(id: id, name: nameofperson, engagedTo: engagedTo,
preferences: preferences.toSeq())
proc addPreference*(self: var Person, preference: varargs[Person]) =
self.preferences.add(preference)
proc newMatchPool*(husbands, wives: openArray[Person]): Option[MatchPool] =
let dim = husbands.len()
if wives.len() == dim:
if husbands.allIt(it.preferences.len() == dim):
if wives.allIt(it.preferences.len() == dim):
result = some(MatchPool(husbands: husbands.toSeq(),
wives: wives.toSeq()))
proc isSingle(self: Person): bool = self.engagedTo.isNone()
proc isEngaged(self: Person): bool = not self.isSingle()
proc disengage(person: var Person) =
if person.engagedTo.isSome():
person.engagedTo.get().engagedTo = none(Person)
person.engagedTo = none(Person)
proc engage(husband, wife: var Person) =
if husband.isEngaged():
disengage(husband)
if wife.isEngaged():
disengage(wife)
husband.engagedTo = some(wife)
wife.engagedTo = some(husband)
proc prefersMoreThanCurrent(self, other: Person): Option[bool] =
if self.isSingle():
result = some(true)
else:
for personRef in self.preferences:
if personRef.id == self.engagedTo.get().id:
result = some(false)
break
elif personRef.id == other.id:
result = some(true)
break
proc solve*(self: var MatchPool): Option[string] =
for husband in self.husbands.mitems():
for wife in husband.preferences.mitems():
let prefers = wife.prefersMoreThanCurrent(husband)
if prefers.isNone():
result = some("Found a spouse that does not prefer another spouse")
break
elif prefers.get():
engage(husband, wife)
result = none(string)
proc print*(self: MatchPool) =
for husband in self.husbands:
echo(husband.name, " is engaged to ", husband.engagedTo.get().name)
echo "and"
for wife in self.wives:
echo(wife.name, " is engaged to ", wife.engagedTo.get().name)
driver:
import unittest
import options
import cdsan/stable_matching
test "stable_matching":
var
rose = newPerson("Rose", false)
alexis = newPerson("Alexis", false)
alicia = newPerson("Alicia", false)
samantha = newPerson("Samantha", false)
ads = newPerson("Ads")
bruce = newPerson("Bruce")
zab = newPerson("Zab")
eddy = newPerson("Eddy")
rose.addPreference(ads, zab, eddy, bruce)
alexis.addPreference(bruce, zab, eddy, ads)
alicia.addPreference(eddy, zab, bruce, ads)
samantha.addPreference(bruce, eddy, zab, ads)
ads.addPreference(rose, alicia, alexis, samantha)
bruce.addPreference(samantha, alicia, rose, alexis)
zab.addPreference(alexis, rose, alicia, samantha)
eddy.addPreference(samantha, rose, alicia, alexis)
var mp = newMatchPool(wives = [rose, alexis, alicia, samantha], husbands = [
ads, bruce, zab, eddy])
assert mp.isSome()
var pool = mp.get()
assert pool.solve().isNone()
pool.print()
result:
Ads is engaged to Samantha
Bruce is engaged to Alexis
Zab is engaged to Alicia
Eddy is engaged to Rose
and
Rose is engaged to Eddy
Alexis is engaged to Bruce
Alicia is engaged to Zab
Samantha is engaged to Ads
As you can see, Ads and Samantha likes each others the least.Samantha likes Bruce.Rose and Ads prefer each other.
What's causing this?

Answer 1

The problem is that nextMaleId and nextFemaleId are local to newPerson. So every male gets id=1 and every female get id=-1.
If you make nextMaleId and nextFemaleId global variables and change solve to only engage two people if they both prefer each other over their current partner, you get
Ads is engaged to Samantha
Bruce is engaged to Alexis
Zab is engaged to Alicia
Eddy is engaged to Rose
and
Rose is engaged to Eddy
Alexis is engaged to Bruce
Alicia is engaged to Zab
Samantha is engaged to Ads
Which looks like an acceptable solution.