/// <summary>
/// Gets the lowest positive integer which when combined with a secret key has an MD5 hash whose
/// hexadecimal representation starts with the specified number of zeroes.
/// </summary>
/// <param name="secretKey">The secret key to use.</param>
/// <param name="zeroes">The number of zeroes to get the value for.</param>
/// <returns>
/// A <see cref="Task{TResult}"/> that represents the asynchronous operation to find the
/// lowest positive integer that generates an MD5 hash with the number of zeroes specified.
/// </returns>
internal static async Task <int> GetLowestPositiveNumberWithStartingZeroesAsync(string secretKey, int zeroes)
{
var solutions = new ConcurrentBag <int>();
var searchedRanges = new ConcurrentBag <int>();
int fromInclusive = 1;
int rangeSize = 50000;
var chunks = Enumerable.Chunk(Enumerable.Range(fromInclusive, int.MaxValue - 1), rangeSize);
using var cts = new CancellationTokenSource();
try
{
await Parallel.ForEachAsync(
chunks,
cts.Token,
(range, cancellationToken) =>
{
try
{
// Does this range start at a value greater than an already found value?
if (!solutions.IsEmpty)
{
int bestSolution = solutions.Min();
if (range[0] > bestSolution)
{
var orderedRanges = searchedRanges.ToList();
if (orderedRanges.Count == 0)
{
return(ValueTask.CompletedTask);
}
orderedRanges.Sort();
// Have we searched the first possible range already?
if (orderedRanges[0] == fromInclusive)
{
for (int i = 1; i < orderedRanges.Count; i++)
{
int lastRange = orderedRanges[i - 1];
int thisRange = orderedRanges[i];
// Is this range the next range?
if (thisRange != lastRange + rangeSize)
{
// A range before the current best solution has not been searched yet
break;
}
if (thisRange > bestSolution)
{
// We have found the best solution
cts.Cancel();
}
}
}
}
return(ValueTask.CompletedTask);
}
foreach (int i in range)
{
if (cancellationToken.IsCancellationRequested)
{
break;
}
if (IsSolution(i, secretKey, zeroes))
{
solutions.Add(i);
break;
}
}
}
finally
{
searchedRanges.Add(range[0]);
}
return(ValueTask.CompletedTask);
});
}
catch (TaskCanceledException)
{
// Solution found
}
if (solutions.IsEmpty)
{
throw new PuzzleException("No answer was found for the specified secret key.");
}
return(solutions.Min());
}
