sequence/services/csharp/lib/SequenceManager.cs

namespace SequenceAuth.Lib;

public record SequenceValidationResult(ValidationOutcome Outcome, Option<string> NextSequence, Option<string> UserId, Option<int> RequestsRemaining);

public class SequenceManager(ISequenceStore store)
{
    public async Task<SequenceValidationResult> ValidateAndRotateAsync(string currentSequenceId)
    {
        var sequenceOpt = await store.GetSequenceAsync(currentSequenceId);
        
        return sequenceOpt.State switch
        {
            OptionState.None => new SequenceValidationResult(ValidationOutcome.SequenceNotFound, Option<string>.None(), Option<string>.None(), Option<int>.None()),
            OptionState.Some => await ProcessSequenceAsync(currentSequenceId, sequenceOpt.Value!),
            _ => new SequenceValidationResult(ValidationOutcome.InternalError, Option<string>.None(), Option<string>.None(), Option<int>.None())
        };
    }

    private async Task<SequenceValidationResult> ProcessSequenceAsync(string currentSequenceId, SequenceData data)
    {
        return data.State switch
        {
            SequenceState.Compromised => new SequenceValidationResult(ValidationOutcome.CompromisedSequenceDetected, Option<string>.None(), Option<string>.None(), Option<int>.None()),
            SequenceState.Rotated => await HandleRotatedSequenceAsync(data.UserId),
            SequenceState.Active or SequenceState.Initialized => await HandleActiveSequenceAsync(currentSequenceId, data),
            _ => new SequenceValidationResult(ValidationOutcome.InternalError, Option<string>.None(), Option<string>.None(), Option<int>.None())
        };
    }

    private async Task<SequenceValidationResult> HandleRotatedSequenceAsync(string userId)
    {
        await store.InvalidateUserSessionsAsync(userId);
        return new SequenceValidationResult(ValidationOutcome.CompromisedSequenceDetected, Option<string>.None(), Option<string>.None(), Option<int>.None());
    }

    private async Task<SequenceValidationResult> HandleActiveSequenceAsync(string currentSequenceId, SequenceData data)
    {
        var checkLimit = CheckLimit(data.RequestsRemaining);
        return checkLimit switch
        {
            ValidationOutcome.LimitExceeded => new SequenceValidationResult(ValidationOutcome.LimitExceeded, Option<string>.None(), Option<string>.None(), Option<int>.None()),
            ValidationOutcome.Success => await RotateSequenceAsync(currentSequenceId, data),
            _ => new SequenceValidationResult(ValidationOutcome.InternalError, Option<string>.None(), Option<string>.None(), Option<int>.None())
        };
    }

    private ValidationOutcome CheckLimit(int requestsRemaining)
    {
        return requestsRemaining switch
        {
            <= 0 => ValidationOutcome.LimitExceeded,
            > 0 => ValidationOutcome.Success
        };
    }

    private async Task<SequenceValidationResult> RotateSequenceAsync(string currentSequenceId, SequenceData data)
    {
        var futureSequenceId = Guid.NewGuid().ToString("N");
        var oldDataRotated = data with { State = SequenceState.Rotated };
        await store.SaveSequenceAsync(currentSequenceId, oldDataRotated);
        
        var newData = new SequenceData(
            UserId: data.UserId, 
            RequestsRemaining: data.RequestsRemaining - 1, 
            State: SequenceState.Active, 
            NextSequenceId: futureSequenceId);
            
        await store.SaveSequenceAsync(futureSequenceId, newData);
        
        return new SequenceValidationResult(ValidationOutcome.Success, Option<string>.Some(futureSequenceId), Option<string>.Some(data.UserId), Option<int>.Some(data.RequestsRemaining - 1));
    }
}