using System.Collections.Concurrent;
using Data.SeaHavenIndustries;
using Microsoft.EntityFrameworkCore;

namespace SeaHaven.DataServices.Helpers
{
    /// <summary>
    /// The per-work-order gate that serializes work-order-scoped invariants (uplift create,
    /// decide, revoke, and cancelling a work order's pending uplifts): an in-process gate per
    /// work order, a transaction, and an update lock on the work order row on SQL Server.
    /// Every caller shares the same gates, so a cancel and a create on one work order never
    /// interleave.
    /// </summary>
    public static class WorkOrderMutationLock
    {
        private static readonly ConcurrentDictionary<int, SemaphoreSlim> Gates = new();

        /// <summary>
        /// Runs <paramref name="work"/> under the work order's gate and row lock. The
        /// transaction commits when the work returns and <paramref name="commitWhen"/> (when
        /// given) accepts its result; otherwise, or when the work throws, it rolls back.
        /// </summary>
        public static Task<T> RunAsync<T>(
            ApplicationDbContext context,
            int workOrderId,
            Func<CancellationToken, Task<T>> work,
            CancellationToken cancellationToken,
            Func<T, bool>? commitWhen = null)
            => RunManyAsync(context, new[] { workOrderId }, work, cancellationToken, commitWhen);

        /// <summary>
        /// <see cref="RunAsync{T}"/> for a unit of work that spans several work orders (a sync
        /// batch). Gates and row locks are taken in ascending id order, so two batches never
        /// wait on each other in a cycle; an empty set runs the work in a plain transaction.
        /// </summary>
        public static async Task<T> RunManyAsync<T>(
            ApplicationDbContext context,
            IEnumerable<int> workOrderIds,
            Func<CancellationToken, Task<T>> work,
            CancellationToken cancellationToken,
            Func<T, bool>? commitWhen = null)
        {
            var ids = workOrderIds.Distinct().OrderBy(id => id).ToList();
            var held = new List<SemaphoreSlim>(ids.Count);
            try
            {
                foreach (var id in ids)
                {
                    var gate = Gates.GetOrAdd(id, _ => new SemaphoreSlim(1, 1));
                    await gate.WaitAsync(cancellationToken);
                    held.Add(gate);
                }

                await using var transaction = context.Database.IsRelational()
                    ? await context.Database.BeginTransactionAsync(cancellationToken)
                    : null;
                try
                {
                    foreach (var id in ids)
                        await LockRowAsync(context, id, cancellationToken);

                    var result = await work(cancellationToken);
                    if (transaction is not null)
                    {
                        if (commitWhen is null || commitWhen(result))
                            await transaction.CommitAsync(cancellationToken);
                        else
                            await transaction.RollbackAsync(cancellationToken);
                    }
                    return result;
                }
                catch
                {
                    if (transaction is not null)
                        await transaction.RollbackAsync(cancellationToken);
                    throw;
                }
            }
            finally
            {
                foreach (var gate in held)
                    gate.Release();
            }
        }

        private static async Task LockRowAsync(
            ApplicationDbContext context,
            int workOrderId,
            CancellationToken cancellationToken)
        {
            if (context.Database.ProviderName?.Contains("SqlServer", StringComparison.OrdinalIgnoreCase) != true)
                return;

            await context.workOrders
                .FromSqlRaw(
                    "SELECT * FROM [workOrders] WITH (UPDLOCK, ROWLOCK, HOLDLOCK) WHERE [Id] = {0}",
                    workOrderId)
                .Select(workOrder => workOrder.Id)
                .FirstOrDefaultAsync(cancellationToken);
        }
    }
}