using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging.Abstractions;
using ScadaLink.ConfigurationDatabase.Maintenance;
using ScadaLink.ConfigurationDatabase.Tests.Migrations;
using Xunit;

namespace ScadaLink.ConfigurationDatabase.Tests.Maintenance;

/// <summary>
/// Bundle D (#23 M6-T5) integration tests for
/// <see cref="AuditLogPartitionMaintenance"/>. Uses the same
/// <see cref="MsSqlMigrationFixture"/> as the AuditLog migration / repository
/// tests so the ALTER PARTITION FUNCTION DDL runs against the actual seeded
/// <c>pf_AuditLog_Month</c>.
/// </summary>
/// <remarks>
/// The migration seeds boundaries for every month in 2026 and 2027 (Jan 2026
/// through Dec 2027). Tests pick a lookahead relative to the current
/// max-boundary at test start (rather than a fixed-target date) so each test
/// is robust against earlier tests in the class having added boundaries to
/// the shared fixture DB. Tests run sequentially within the class via xunit's
/// per-class collection serialisation.
/// </remarks>
public class AuditLogPartitionMaintenanceTests : IClassFixture<MsSqlMigrationFixture>
{
    private readonly MsSqlMigrationFixture _fixture;

    public AuditLogPartitionMaintenanceTests(MsSqlMigrationFixture fixture)
    {
        _fixture = fixture;
    }

    private ScadaLinkDbContext CreateContext() =>
        new(new DbContextOptionsBuilder<ScadaLinkDbContext>()
            .UseSqlServer(_fixture.ConnectionString).Options);

    private AuditLogPartitionMaintenance NewMaintenance(ScadaLinkDbContext ctx) =>
        new(ctx, NullLogger<AuditLogPartitionMaintenance>.Instance);

    /// <summary>
    /// Computes the lookahead-in-months required to fall strictly inside the
    /// already-covered boundary range. Picks something well below the
    /// distance from "now" to the current max — guaranteed not to need any
    /// new SPLIT.
    /// </summary>
    private static int LookaheadInsideExistingRange(DateTime max)
    {
        var now = DateTime.UtcNow;
        // (max - now) in whole months, minus a 1-month safety margin so we
        // never accidentally hit the boundary horizon edge case.
        var months = ((max.Year - now.Year) * 12) + max.Month - now.Month - 1;
        return Math.Max(1, months);
    }

    /// <summary>
    /// Computes the lookahead-in-months required to add exactly
    /// <paramref name="extraBoundaries"/> new boundaries past the current max.
    /// </summary>
    /// <remarks>
    /// EnsureLookaheadAsync defines horizon =
    /// <c>NormalizeToFirstOfMonth(UtcNow) + lookaheadMonths</c>. The new
    /// boundaries it issues are first-of-month values strictly greater than
    /// max, up to and including horizon. So
    /// <c>lookaheadMonths = monthsBetween(NormalizeToFirstOfMonth(UtcNow), max) + extraBoundaries</c>
    /// is the exact value that lands horizon on <c>max + extraBoundaries</c>
    /// months.
    /// </remarks>
    private static int LookaheadForExtraBoundaries(DateTime max, int extraBoundaries)
    {
        var nowFirstOfMonth = FirstOfNextMonth(DateTime.UtcNow);
        var monthsToMax = ((max.Year - nowFirstOfMonth.Year) * 12) + max.Month - nowFirstOfMonth.Month;
        return monthsToMax + extraBoundaries;
    }

    private static DateTime FirstOfNextMonth(DateTime instant)
    {
        var firstOfThisMonth = new DateTime(instant.Year, instant.Month, 1, 0, 0, 0, DateTimeKind.Utc);
        return firstOfThisMonth.AddMonths(1);
    }

    [SkippableFact]
    public async Task EnsureLookahead_AlreadyHasFutureRange_NoSplit_ReturnsEmpty()
    {
        Skip.IfNot(_fixture.Available, _fixture.SkipReason);

        await using var ctx = CreateContext();
        var maintenance = NewMaintenance(ctx);

        var max = await maintenance.GetMaxBoundaryAsync();
        Assert.NotNull(max);

        // Pick a lookahead small enough that horizon (NormalizeToFirstOfMonth(now)
        // + lookahead) lands well INSIDE the already-covered range — no SPLIT
        // should fire.
        var lookahead = LookaheadInsideExistingRange(max.Value);

        var added = await maintenance.EnsureLookaheadAsync(lookahead);

        Assert.Empty(added);

        // Sanity: the max boundary is unchanged after the no-op call.
        var maxAfter = await maintenance.GetMaxBoundaryAsync();
        Assert.Equal(max, maxAfter);
    }

    [SkippableFact]
    public async Task EnsureLookahead_NeedsOneMoreBoundary_Splits_Returns1Boundary()
    {
        Skip.IfNot(_fixture.Available, _fixture.SkipReason);

        await using var ctx = CreateContext();
        var maintenance = NewMaintenance(ctx);

        var maxBefore = await maintenance.GetMaxBoundaryAsync();
        Assert.NotNull(maxBefore);

        var lookahead = LookaheadForExtraBoundaries(maxBefore.Value, extraBoundaries: 1);
        var expectedAdded = maxBefore.Value.AddMonths(1);

        var added = await maintenance.EnsureLookaheadAsync(lookahead);

        Assert.Single(added);
        Assert.Equal(expectedAdded, added[0]);

        var maxAfter = await maintenance.GetMaxBoundaryAsync();
        Assert.Equal(expectedAdded, maxAfter);
    }

    [SkippableFact]
    public async Task EnsureLookahead_NeedsThreeBoundaries_Splits_Returns3Boundaries()
    {
        Skip.IfNot(_fixture.Available, _fixture.SkipReason);

        await using var ctx = CreateContext();
        var maintenance = NewMaintenance(ctx);

        var maxBefore = await maintenance.GetMaxBoundaryAsync();
        Assert.NotNull(maxBefore);

        var lookahead = LookaheadForExtraBoundaries(maxBefore.Value, extraBoundaries: 3);

        var added = await maintenance.EnsureLookaheadAsync(lookahead);

        Assert.Equal(3, added.Count);
        Assert.Equal(maxBefore.Value.AddMonths(1), added[0]);
        Assert.Equal(maxBefore.Value.AddMonths(2), added[1]);
        Assert.Equal(maxBefore.Value.AddMonths(3), added[2]);

        var maxAfter = await maintenance.GetMaxBoundaryAsync();
        Assert.Equal(maxBefore.Value.AddMonths(3), maxAfter);
    }

    [SkippableFact]
    public async Task EnsureLookahead_BoundaryAlreadyExists_NoError_Idempotent()
    {
        Skip.IfNot(_fixture.Available, _fixture.SkipReason);

        await using var ctx1 = CreateContext();
        var m1 = NewMaintenance(ctx1);

        var maxStart = await m1.GetMaxBoundaryAsync();
        Assert.NotNull(maxStart);

        // First call: add one boundary.
        var lookahead = LookaheadForExtraBoundaries(maxStart.Value, extraBoundaries: 1);
        var firstAdded = await m1.EnsureLookaheadAsync(lookahead);
        Assert.Single(firstAdded);

        // Second call: the boundary just added is now part of pf_AuditLog_Month,
        // so the same lookahead value should be a no-op — no exception, no
        // duplicate SPLIT.
        await using var ctx2 = CreateContext();
        var m2 = NewMaintenance(ctx2);
        var secondAdded = await m2.EnsureLookaheadAsync(lookahead);

        Assert.Empty(secondAdded);

        // The max boundary is unchanged across the second call.
        var maxAfter = await m2.GetMaxBoundaryAsync();
        Assert.Equal(firstAdded[0], maxAfter);
    }
}