Campaign_Tracker/Campaign_Tracker.Server/LegacyData/Schema/OleDbLegacySchemaInspector.cs

using System.Data;
using System.Data.OleDb;
using System.Runtime.Versioning;

namespace Campaign_Tracker.Server.LegacyData.Schema;

[SupportedOSPlatform("windows")]
public sealed class OleDbLegacySchemaInspector : ILegacySchemaInspector
{
    private readonly string _connectionString;

    public OleDbLegacySchemaInspector(string connectionString)
    {
        if (string.IsNullOrWhiteSpace(connectionString))
        {
            throw new ArgumentException("Legacy database connection string is required.", nameof(connectionString));
        }

        _connectionString = connectionString;
    }

    public async Task<IReadOnlyList<LegacyTableDefinition>> GetCurrentSchemaAsync(
        CancellationToken cancellationToken = default)
    {
        await using var connection = new OleDbConnection(_connectionString);
        await connection.OpenAsync(cancellationToken);

        var schema = connection.GetOleDbSchemaTable(OleDbSchemaGuid.Columns, null)
            ?? throw new InvalidOperationException("OleDb provider did not return column schema metadata.");

        var tables = schema.Rows
            .Cast<DataRow>()
            .GroupBy(row => GetRequiredString(row, "TABLE_NAME"), StringComparer.OrdinalIgnoreCase)
            .OrderBy(group => group.Key, StringComparer.OrdinalIgnoreCase)
            .Select(group => new LegacyTableDefinition(
                group.Key,
                group
                    .OrderBy(row => GetOrdinal(row))
                    .Select(row => new LegacyColumnDefinition(
                        GetRequiredString(row, "COLUMN_NAME"),
                        GetRequiredInt(row, "DATA_TYPE"),
                        GetNullableInt(row, "CHARACTER_MAXIMUM_LENGTH"),
                        GetNullableBool(row, "IS_NULLABLE") ?? true)
                    {
                        Constraints = GetColumnConstraints(row),
                    })
                    .ToArray()))
            .ToArray();

        return tables;
    }

    private static int GetOrdinal(DataRow row) =>
        GetNullableInt(row, "ORDINAL_POSITION") ?? int.MaxValue;

    private static IReadOnlyList<string> GetColumnConstraints(DataRow row)
    {
        var constraints = new List<string>();
        return constraints;
    }

    private static string GetRequiredString(DataRow row, string columnName)
    {
        var value = row.Table.Columns.Contains(columnName) ? row[columnName] : null;
        var text = value is null or DBNull ? null : Convert.ToString(value);
        return string.IsNullOrWhiteSpace(text)
            ? throw new InvalidOperationException($"OleDb schema row is missing required field {columnName}.")
            : text;
    }

    private static int GetRequiredInt(DataRow row, string columnName) =>
        GetNullableInt(row, columnName)
        ?? throw new InvalidOperationException($"OleDb schema row is missing required field {columnName}.");

    private static int? GetNullableInt(DataRow row, string columnName)
    {
        if (!row.Table.Columns.Contains(columnName) || row[columnName] is DBNull)
        {
            return null;
        }

        return Convert.ToInt32(row[columnName]);
    }

    private static bool? GetNullableBool(DataRow row, string columnName)
    {
        if (!row.Table.Columns.Contains(columnName) || row[columnName] is DBNull)
        {
            return null;
        }

        var value = row[columnName];
        if (value is bool flag)
        {
            return flag;
        }

        var text = Convert.ToString(value);
        if (string.Equals(text, "YES", StringComparison.OrdinalIgnoreCase) ||
            string.Equals(text, "TRUE", StringComparison.OrdinalIgnoreCase))
        {
            return true;
        }

        if (string.Equals(text, "NO", StringComparison.OrdinalIgnoreCase) ||
            string.Equals(text, "FALSE", StringComparison.OrdinalIgnoreCase))
        {
            return false;
        }

        return null;
    }
}