public override ConstructiveResult Solve(KnapsackInstance instance)
{
//Select only the items that can fit into the backcpack and make a copy of them
var modifiedItems = instance.Items
.Where(i => i.Weight <= instance.KnapsackSize)
.Select(i => new KnapsackItem {
Price = i.Price, Weight = i.Weight
})
.ToList();
if (modifiedItems.Count() == 0)
{
return(EmptyResult(instance));
}
//Calculate the divider based on the desired accuracy and input instance
double divider = instance.ApproximationAccuracy * (double)modifiedItems.Max(i => i.Price) / modifiedItems.Count();
//Convert the price of each item
foreach (var item in modifiedItems)
{
item.Price = (int)Math.Floor(item.Price / divider);
}
//Create a new instance for the DP solver
var newKnapsackInstance = new KnapsackInstance
{
ApproximationAccuracy = instance.ApproximationAccuracy,
Id = instance.Id,
Items = modifiedItems,
KnapsackSize = instance.KnapsackSize
};
var dpPriceStrategy = new ConstructiveDPPrice();
var solution = dpPriceStrategy.Solve(newKnapsackInstance);
//Include the removed items in the result item vector
solution.Configuration.ItemVector = ConvertItemVector(solution.Configuration.ItemVector, instance);
solution.KnapsackInstance = instance;
//Calculate the total price using the solution item vector and the original item prices
var solutionPrice = instance.Items.Aggregate(0,
(acc, item) => acc + item.Price * Convert.ToInt32(solution.Configuration.ItemVector[item.Id]));
solution.Configuration.Price = solutionPrice;
solution.Strategy = $"fptas (epsilon:{instance.ApproximationAccuracy})";
return(solution);
}
public override ConstructiveResult Solve(KnapsackInstance instance)
{
ConstructiveStrategy strategy;
if (instance.KnapsackSize < instance.GetPriceOfAllItems())
{
strategy = new ConstructiveDPCapacity();
}
else
{
strategy = new ConstructiveDPPrice();
}
return(strategy.Solve(instance));
}