Kotlin Cannibals and Missionaries Algorithm - How to convert to full FP - algorithm

Question

Any suggestions on how to improve the following code to make it more Functional Programming oriented. Specifically how to remove the MutableList which signifies historical states. There are two data classes: Bank, which represents a riverbank (number of missionaries and number of cannibals currently on the bank) and BankState which represents a historical state of the two banks (the source bank, target bank and boatAtSource - a boolean which indicates whether the boat is currently at the source or target bank). overloaded operator function plus adds missionaries and cannibals to a riverbank and function minus removes them from a riverbank. The boat function is the one which carries the most heft. You can call the following algorithm from fun main (app.kt) as such:
app.kt
fun main(args:Array<String>) {
val source:Bank = Bank(3,3)
val target:Bank = Bank()
source boat target
}
Bank.kt
data class Bank(val missionaries:Int=0,val cannibals:Int=0)
data class BankState(val sourceTarget:Pair<Bank,Bank>,val boatAtSource:Boolean)
operator fun Bank.plus(b:Pair<Int,Int>):Bank = Bank(this.missionaries+b.first,this.cannibals+b.second)
operator fun Bank.minus(b:Pair<Int,Int>):Bank = Bank(this.missionaries-b.first,this.cannibals-b.second)
infix fun Bank.boat(target:Bank):List<BankState> {
val begin = Pair(this,target)
val history = mutableListOf<BankState>(BankState(begin,true))
boat(begin,true,this.missionaries,this.cannibals,history)
return history
}
fun boat(sourceTarget:Pair<Bank,Bank>,
boatAtSource:Boolean,
totalMissionaries:Int,
totalCannibals:Int,
history:MutableList<BankState>):Boolean {
if(sourceTarget.first.cannibals+sourceTarget.second.cannibals==totalCannibals &&
sourceTarget.first.missionaries + sourceTarget.second.missionaries==totalMissionaries &&
sourceTarget.first.cannibals>=0 &&
sourceTarget.first.missionaries>=0 &&
sourceTarget.second.cannibals>=0 &&
sourceTarget.second.missionaries>=0 &&
(sourceTarget.first.missionaries==0 || sourceTarget.first.missionaries>=sourceTarget.first.cannibals) &&
(sourceTarget.second.missionaries==0 || sourceTarget.second.missionaries >= sourceTarget.second.cannibals)) {
if(sourceTarget.second.missionaries==totalMissionaries &&
sourceTarget.second.cannibals==totalCannibals) {
history.forEach(::println)
return true
} else {
val deltas = listOf(Pair(0,1),Pair(1,1),Pair(1,0),Pair(2,0),Pair(0,2))
val comparator = object : Comparator<Pair<Pair<Boolean,Int>,Pair<Bank,Bank>>> {
override fun compare(arg1:Pair<Pair<Boolean,Int>,Pair<Bank,Bank>>,arg2:Pair<Pair<Boolean,Int>,Pair<Bank,Bank>>):Int {
if(arg1.first.first && arg2.first.first) {
return if(arg1.first.second<arg2.first.second) -1 else if(arg1.first.second>arg2.first.second) 1 else 0
} else if(arg1.first.first){
return 1
} else if(arg2.first.first) {
return -1
}
return 0
}
}
val result = deltas.map{
checkNext(it.first,it.second,totalMissionaries,totalCannibals,history,sourceTarget,boatAtSource)
}.maxWith(comparator)
if(result?.first?.first!=null && result.first.first) {
history.add(BankState(result.second,!boatAtSource))
return true;
}
}
}
return false
}
fun checkNext(missionariesDelta:Int,
cannibalsDelta:Int,
totalMissionaries:Int,
totalCannibals:Int,
history:MutableList<BankState>,
sourceTarget:Pair<Bank,Bank>,
boatAtSource:Boolean):Pair<Pair<Boolean,Int>,Pair<Bank,Bank>> {
val nextSrcTgt = if(boatAtSource) Pair(sourceTarget.first-Pair(missionariesDelta,cannibalsDelta),sourceTarget.second+Pair(missionariesDelta,cannibalsDelta))
else Pair(sourceTarget.first+Pair(missionariesDelta,cannibalsDelta),sourceTarget.second-Pair(missionariesDelta,cannibalsDelta))
val bankState:BankState = BankState(nextSrcTgt,!boatAtSource)
if(!history.contains(bankState)) {
history.add(bankState)
val combo2:Boolean = boat(nextSrcTgt,!boatAtSource,totalMissionaries,totalCannibals,history)
val combo2Depth = history.size
history.remove(bankState)
return Pair(Pair(combo2,combo2Depth),nextSrcTgt)
} else {
return Pair(Pair(false,0),nextSrcTgt)
}
}