private static Option <IEnumerable <Coin> > CalculateChange(ChangeCalculationState state) =>
state.IsChangePaidOut
? new Some <IEnumerable <Coin> >(state.CalculatedChange)
: state.CoinBeingProcessed
.Map(coin =>
{
state.UseAsMuchAsPossible(coin);
return(CalculateChange(state.ForNextCoin())
.TryReduce(() => Backtrack(state)));
})
.Reduce(None.Value);
private static Option <IEnumerable <Coin> > Backtrack(ChangeCalculationState state) =>
state.CoinBeingProcessed
.Map(coin =>
{
if (!state.IsUsed(coin))
{
return(None.Value);
}
state.Unuse(coin);
return(CalculateChange(state.ForNextCoin())
.TryReduce(() => Backtrack(state)));
})
.Reduce(None.Value);
public static Option <IEnumerable <Coin> > ChangeFor(this IEnumerable <Coin> coins, int amount) => CalculateChange(ChangeCalculationState.Initialize(coins, amount));
