using GerstITS.Data;
using GerstITS.Examples.Transactions.Entities;

namespace GerstITS.Examples.Transactions;

/// <summary>
/// Demonstrates the <see cref="IRepository" /> transaction API:
/// <list type="bullet">
///   <item>legacy auto-commit behaviour when no transaction is started,</item>
///   <item>a successful transaction that spans a Save and a Delete,</item>
///   <item>an automatic rollback on exception that discards partial work,</item>
///   <item>the guard that prevents starting a second, nested transaction.</item>
/// </list>
/// </summary>
internal sealed class TransactionRunner
{
    #region Fields

    private readonly IRepository _repository;
    private readonly ProductDbContext _dbContext;

    #endregion

    #region Constructors

    public TransactionRunner(IRepository repository,
                             ProductDbContext dbContext)
    {
        _repository = repository;
        _dbContext = dbContext;
    }

    #endregion

    #region Methods

    public void Run()
    {
        // The SQLite in-memory database starts out empty, so create the schema once up front.
        _dbContext.Database.EnsureCreated();

        RunAutoCommitBehavior();
        RunSuccessfulTransaction();
        RunRollbackOnException();
        RunNestedTransactionGuard();
    }

    private void RunAutoCommitBehavior()
    {
        Print("=== Legacy auto-commit (no BeginTransaction) ===");

        // Without BeginTransaction() every Save/Delete auto-commits immediately (unchanged legacy behaviour).
        _repository.Save(new ProductEntity { Name = "Obsolete Widget", Price = 9.99m });

        Print($"Products after auto-committed insert: {NamesInStore()}");
    }

    private void RunSuccessfulTransaction()
    {
        Print(string.Empty);
        Print("=== Successful transaction spanning a Save and a Delete ===");

        var obsolete = _repository.Query<ProductEntity>()
                                  .Single(product => product.Name == "Obsolete Widget");

        using (var transaction = _repository.BeginTransaction())
        {
            _repository.Save(new ProductEntity { Name = "New Gadget", Price = 19.99m });
            _repository.Delete(obsolete);

            transaction.Commit();
        }

        Print($"Products after committed transaction: {NamesInStore()}");
    }

    private void RunRollbackOnException()
    {
        Print(string.Empty);
        Print("=== Rollback on exception (partial work is discarded) ===");

        try
        {
            using var transaction = _repository.BeginTransaction();

            _repository.Save(new ProductEntity { Name = "Half-Baked Product", Price = 1.00m });

            // Something fails before Commit() is reached. Because the transaction is disposed without a
            // prior Commit(), the framework rolls it back automatically - so the insert above is undone.
            throw new InvalidOperationException("Simulated failure after partial work.");
        }
        catch (InvalidOperationException exception)
        {
            Print($"Caught expected exception: {exception.Message}");
        }

        Print($"Products after rolled-back transaction: {NamesInStore()} (the half-baked product was discarded)");
    }

    private void RunNestedTransactionGuard()
    {
        Print(string.Empty);
        Print("=== Starting a second transaction while one is active throws ===");

        using var transaction = _repository.BeginTransaction();

        try
        {
            _repository.BeginTransaction();
        }
        catch (InvalidOperationException exception)
        {
            Print($"Caught expected exception: {exception.Message}");
        }

        transaction.Rollback();
    }

    private string NamesInStore()
    {
        var names = _repository.Query<ProductEntity>()
                               .OrderBy(product => product.Name)
                               .Select(product => product.Name)
                               .ToList();

        return names.Count == 0
                   ? "<none>"
                   : string.Join(", ", names);
    }

    private static void Print(string message)
    {
        Console.WriteLine(message);
    }

    #endregion
}