namespace Campaign_Tracker.Server.LegacyData.Schema;

/// <summary>
/// Default <see cref="ILegacySchemaCompatibilityCheck"/> implementation.
///
/// Compares the baseline (captured at initialization) against the current
/// schema returned by the inspector. Reports drift entries for:
///   - tables present in baseline but missing in current (TableMissing)
///   - tables present in current but missing in baseline (TableAdded)
///   - columns present in baseline but missing in current (ColumnMissing)
///   - columns present in current but missing in baseline (ColumnAdded)
///   - columns present in both with different type/size/nullability
/// Comparison is case-insensitive on table and column names to match Access
/// behavior.
/// </summary>
public sealed class LegacySchemaCompatibilityCheck : ILegacySchemaCompatibilityCheck
{
    private readonly LegacySchemaBaseline _baseline;
    private readonly ILegacySchemaInspector _inspector;
    private readonly TimeProvider _timeProvider;

    public LegacySchemaCompatibilityCheck(
        LegacySchemaBaseline baseline,
        ILegacySchemaInspector inspector,
        TimeProvider? timeProvider = null)
    {
        _baseline = baseline ?? throw new ArgumentNullException(nameof(baseline));
        _inspector = inspector ?? throw new ArgumentNullException(nameof(inspector));
        _timeProvider = timeProvider ?? TimeProvider.System;
    }

    public async Task<LegacySchemaCheckResult> RunAsync(CancellationToken cancellationToken = default)
    {
        var live = await _inspector.GetCurrentSchemaAsync(cancellationToken).ConfigureAwait(false);
        var drifts = new List<LegacySchemaDrift>();

        var baselineByName = _baseline.Tables
            .ToDictionary(t => t.Name, StringComparer.OrdinalIgnoreCase);
        var liveByName = live
            .ToDictionary(t => t.Name, StringComparer.OrdinalIgnoreCase);

        foreach (var (name, baselineTable) in baselineByName)
        {
            if (!liveByName.TryGetValue(name, out var liveTable))
            {
                drifts.Add(new LegacySchemaDrift(
                    name, null, LegacySchemaChangeType.TableMissing,
                    $"Table '{name}' is in the approved baseline but missing from the live database."));
                continue;
            }

            CompareColumns(name, baselineTable.Columns, liveTable.Columns, drifts);
        }

        foreach (var (name, _) in liveByName)
        {
            if (!baselineByName.ContainsKey(name))
            {
                drifts.Add(new LegacySchemaDrift(
                    name, null, LegacySchemaChangeType.TableAdded,
                    $"Table '{name}' exists in the live database but is not part of the approved baseline."));
            }
        }

        return new LegacySchemaCheckResult(
            Passed: drifts.Count == 0,
            TablesVerified: baselineByName.Count,
            DriftCount: drifts.Count,
            CheckedAt: _timeProvider.GetUtcNow(),
            Drifts: drifts,
            BaselineSource: _baseline.Source);
    }

    private static void CompareColumns(
        string tableName,
        IReadOnlyList<LegacyColumnDefinition> baseline,
        IReadOnlyList<LegacyColumnDefinition> live,
        List<LegacySchemaDrift> sink)
    {
        var baselineByName = baseline.ToDictionary(c => c.Name, StringComparer.OrdinalIgnoreCase);
        var liveByName = live.ToDictionary(c => c.Name, StringComparer.OrdinalIgnoreCase);

        foreach (var (name, baseCol) in baselineByName)
        {
            if (!liveByName.TryGetValue(name, out var liveCol))
            {
                sink.Add(new LegacySchemaDrift(
                    tableName, name, LegacySchemaChangeType.ColumnMissing,
                    $"Column '{tableName}.{name}' is in the approved baseline but missing from the live database."));
                continue;
            }

            if (baseCol.TypeCode != liveCol.TypeCode)
            {
                sink.Add(new LegacySchemaDrift(
                    tableName, name, LegacySchemaChangeType.ColumnTypeChanged,
                    $"Column '{tableName}.{name}' type changed: baseline={baseCol.TypeCode}, live={liveCol.TypeCode}."));
            }

            if (baseCol.Size != liveCol.Size)
            {
                sink.Add(new LegacySchemaDrift(
                    tableName, name, LegacySchemaChangeType.ColumnSizeChanged,
                    $"Column '{tableName}.{name}' size changed: baseline={Format(baseCol.Size)}, live={Format(liveCol.Size)}."));
            }

            if (baseCol.Nullable != liveCol.Nullable)
            {
                sink.Add(new LegacySchemaDrift(
                    tableName, name, LegacySchemaChangeType.ColumnNullabilityChanged,
                    $"Column '{tableName}.{name}' nullability changed: baseline={baseCol.Nullable}, live={liveCol.Nullable}."));
            }

            var baselineConstraints = NormalizeConstraints(baseCol.Constraints);
            var liveConstraints = NormalizeConstraints(liveCol.Constraints);
            if (!baselineConstraints.SequenceEqual(liveConstraints, StringComparer.OrdinalIgnoreCase))
            {
                sink.Add(new LegacySchemaDrift(
                    tableName, name, LegacySchemaChangeType.ColumnConstraintsChanged,
                    $"Column '{tableName}.{name}' constraints changed: baseline={FormatConstraints(baselineConstraints)}, live={FormatConstraints(liveConstraints)}."));
            }
        }

        foreach (var (name, _) in liveByName)
        {
            if (!baselineByName.ContainsKey(name))
            {
                sink.Add(new LegacySchemaDrift(
                    tableName, name, LegacySchemaChangeType.ColumnAdded,
                    $"Column '{tableName}.{name}' exists in the live database but is not part of the approved baseline."));
            }
        }
    }

    private static string Format(int? value) => value?.ToString() ?? "(none)";

    private static IReadOnlyList<string> NormalizeConstraints(IReadOnlyList<string> constraints) =>
        constraints
            .Where(c => !string.IsNullOrWhiteSpace(c))
            .Select(c => c.Trim())
            .Order(StringComparer.OrdinalIgnoreCase)
            .ToArray();

    private static string FormatConstraints(IReadOnlyList<string> constraints) =>
        constraints.Count == 0 ? "(none)" : string.Join(",", constraints);
}