public override async Task <double> TryGetCreditAsync(
DbUserAccount userAccount,
double amountRequested,
DateTime?timeOfRequestUtc = null,
CancellationToken cancellationToken = default(CancellationToken))
{
if (Double.IsNaN(amountRequested) || amountRequested <= 0)
{
// You can't request a negative amount and requesting nothing is free
return(0);
}
DateTime timeOfRequestOrNowUtc = timeOfRequestUtc ?? DateTime.Now;
double creditRetrieved = 0;
using (var context = new DbUserAccountContext(DbOptions))
{
using (var dbContextTransaction = await context.Database.BeginTransactionAsync(cancellationToken))
{
bool rolledBackDueToConcurrencyException = false;
do
{
try
{
DbUserAccountCreditBalance balance = await
context.DbUserAccountCreditBalances.Where(b => b.DbUserAccountId == userAccount.DbUserAccountId)
.FirstOrDefaultAsync(cancellationToken: cancellationToken);
if (balance == null)
{
creditRetrieved = Math.Min(amountRequested, userAccount.CreditLimit);
double amountRemaining = userAccount.CreditLimit - creditRetrieved;
context.DbUserAccountCreditBalances.Add(
new DbUserAccountCreditBalance()
{
DbUserAccountId = userAccount.DbUserAccountId,
ConsumedCreditsLastUpdatedUtc = timeOfRequestOrNowUtc,
ConsumedCreditsLastValue = amountRemaining
});
}
else
{
double amountAvailable = Math.Max(0, userAccount.CreditLimit -
DecayingDouble.Decay(balance.ConsumedCreditsLastValue, userAccount.CreditHalfLife,
balance.ConsumedCreditsLastUpdatedUtc, timeOfRequestOrNowUtc));
if (amountAvailable > 0)
{
creditRetrieved = Math.Min(amountRequested, amountAvailable);
}
double amountRemaining = amountAvailable - creditRetrieved;
balance.ConsumedCreditsLastValue = amountRemaining;
balance.ConsumedCreditsLastUpdatedUtc = timeOfRequestOrNowUtc;
}
await context.SaveChangesAsync(cancellationToken);
dbContextTransaction.Commit();
}
catch (DbUpdateConcurrencyException)
{
dbContextTransaction.Rollback();
rolledBackDueToConcurrencyException = true;
}
} while (rolledBackDueToConcurrencyException);
}
}
return(creditRetrieved);
}
public void DecayOverTwoHalfLives()
{
double shouldBeVeryCloseTo1 = DecayingDouble.Decay(4, OneDay, FirstDayOfCentury, TwoDaysLater);
Assert.InRange(shouldBeVeryCloseTo1, .99999, 1.000001);
}