public ExpenseRepaymentCollection Post([FromBody] IEnumerable <User> users)
{
if (users == null || !users.Any() || users.All(u => !u.Expenses.Any()))
{
// empty result passed
ThrowResponseException(HttpStatusCode.BadRequest, "Empty list of users entered");
}
if (users.SelectMany(u => u.Expenses).Any(exp => exp < 0))
{
// one or more expenses has a negative amount
ThrowResponseException(HttpStatusCode.BadRequest, "One or more Expense line items have a negative expense amount");
}
ExpenseRepaymentCollection result = new ExpenseRepaymentCollection();
// Flatten list into 1 expense line item per name, with their total expenditure
var totalExpenses = users.GroupBy(u => u.Name)
.Select(x =>
new ExpenseLineItem()
{
Name = x.Key,
ExpenseAmount = x.SelectMany(y => y.Expenses).Sum()
}
).ToList();
decimal totalExpenditure = 0;
try
{
totalExpenditure = totalExpenses.Sum(e => e.ExpenseAmount);
}
catch (OverflowException)
{
ThrowResponseException(HttpStatusCode.BadRequest, "Expenses entered are total greater than maximum precision");
}
if (totalExpenditure == 0)
{
return(result);
}
decimal equalShares = totalExpenditure / totalExpenses.Count;
if (totalExpenses.All(e => e.EqualWithinOneCent(equalShares)))
{
// All users have already paid equally, no repayments need to be done
return(result);
}
//Because we only ever need to look at the front of the line, a stack is more efficient
Stack <ExpenseLineItem> paidMoreThanEqual = new Stack <ExpenseLineItem>(totalExpenses.Where(e => e.ExpenseAmount > equalShares));
Stack <ExpenseLineItem> paidLessThanEqual = new Stack <ExpenseLineItem>(totalExpenses.Where(e => e.ExpenseAmount < equalShares));
List <ExpenseRepayment> repayments = new List <ExpenseRepayment>();
while (paidMoreThanEqual.Any() && paidLessThanEqual.Any())
{
ExpenseLineItem currentUnderpayer = paidLessThanEqual.Peek();
ExpenseLineItem currentOverpayer = paidMoreThanEqual.Peek();
decimal amountToPay = Math.Min(currentOverpayer.ExpenseAmount - equalShares, equalShares - currentUnderpayer.ExpenseAmount);
repayments.Add(new ExpenseRepayment()
{
PayFrom = currentUnderpayer.Name, PayTo = currentOverpayer.Name, Amout = amountToPay
});
currentOverpayer.ExpenseAmount -= amountToPay;
currentUnderpayer.ExpenseAmount += amountToPay;
if (currentUnderpayer.EqualWithinOneCent(equalShares))
{
paidLessThanEqual.Pop();
}
if (currentOverpayer.EqualWithinOneCent(equalShares))
{
paidMoreThanEqual.Pop();
}
}
result.Repayments = repayments.ToArray();
result.Status = (byte)(result.Repayments.Any() ? 1 : 0);
return(result);
}
