MeterVault/src/Infrastructure/Import/CsvImporter.cs

using System.Globalization;
using CsvHelper;
using CsvHelper.Configuration;
using MeterVault.Core.Domain;
using MeterVault.Core.Parsing;

namespace MeterVault.Infrastructure.Import;

/// <summary>
/// Reads a CSV in the reference German dialect and stages readings/events/manual-costs per a
/// <see cref="MappingProfile"/> (SDD §6.3). Pure transformation — no persistence; feed the result
/// to <see cref="ImportService"/> to commit as a revertible batch, or to the normalization engine
/// for a dry-run preview.
/// </summary>
public sealed class CsvImporter
{
    public StagedImport Stage(MappingProfile profile, TextReader reader)
    {
        ArgumentNullException.ThrowIfNull(profile);
        ArgumentNullException.ThrowIfNull(reader);

        var rows = ReadRows(reader);
        var staged = new StagedImport();
        var previousByMeter = new Dictionary<int, double>();

        for (var r = profile.FirstDataRowIndex; r < rows.Count; r++)
        {
            var row = rows[r];
            if (RowClassifier.ShouldSkip(row, profile, out var reason))
            {
                staged.SkippedRows++;
                if (!reason.StartsWith("blank", StringComparison.Ordinal))
                {
                    staged.Warnings.Add($"Row {r + 1}: skipped ({reason}).");
                }

                continue;
            }

            if (!TryGetDate(row, profile, out var time))
            {
                staged.SkippedRows++;
                staged.Warnings.Add($"Row {r + 1}: unparseable date, skipped.");
                continue;
            }

            foreach (var column in profile.Columns)
            {
                StageColumn(column, row, time, r, staged, previousByMeter, profile.DetectCumulativeSwaps);
            }
        }

        return staged;
    }

    private static void StageColumn(ColumnMapping column, IReadOnlyList<string> row, DateTimeOffset time,
        int rowIndex, StagedImport staged, Dictionary<int, double> previousByMeter, bool detectSwaps)
    {
        if (column.Role == MappingRole.Ignore || column.Index >= row.Count)
        {
            return;
        }

        var cell = row[column.Index];
        if (string.IsNullOrWhiteSpace(cell))
        {
            return;
        }

        switch (column.Role)
        {
            case MappingRole.Reading:
                StageReading(column, row, cell, time, rowIndex, staged, previousByMeter, detectSwaps);
                break;

            case MappingRole.Delivery:
                if (GermanNumber.TryParse(cell, out var litres) && litres > 0)
                {
                    staged.Events.Add(new MeterEvent
                    {
                        MeterId = column.MeterId!.Value,
                        Time = time,
                        EventType = MeterEventType.Delivery,
                        Amount = litres,
                        Unit = column.Unit ?? "L",
                    });
                }

                break;

            case MappingRole.TankLevel:
                var split = ValueCell.Split(cell);
                if (split is not null)
                {
                    staged.Events.Add(new MeterEvent
                    {
                        MeterId = column.MeterId!.Value,
                        Time = time,
                        EventType = MeterEventType.TankLevel,
                        Amount = split.Value.Value,
                        Unit = column.Unit ?? split.Value.Unit,
                    });
                }

                break;

            case MappingRole.ManualCost:
                if (GermanMoney.TryParse(cell, out var amount, out var currency))
                {
                    staged.ManualCosts.Add(new ManualCost
                    {
                        CategoryId = column.CategoryId,
                        PeriodStart = DateOnly.FromDateTime(time.UtcDateTime),
                        PeriodEnd = DateOnly.FromDateTime(time.UtcDateTime.AddMonths(1).AddDays(-1)),
                        Amount = (double)amount,
                        Currency = currency,
                    });
                }

                break;

            case MappingRole.Ignore:
            default:
                break;
        }
    }

    private static void StageReading(ColumnMapping column, IReadOnlyList<string> row, string cell,
        DateTimeOffset time, int rowIndex, StagedImport staged, Dictionary<int, double> previousByMeter, bool detectSwaps)
    {
        var split = ValueCell.Split(cell);
        if (split is null)
        {
            return;
        }

        var meterId = column.MeterId!.Value;
        var value = split.Value.Value;

        if (column.Unit is not null && split.Value.Unit is not null
            && !string.Equals(column.Unit, split.Value.Unit, StringComparison.OrdinalIgnoreCase))
        {
            staged.Warnings.Add(
                $"Row {rowIndex + 1}: expected unit '{column.Unit}' but found '{split.Value.Unit}'.");
        }

        if (detectSwaps && previousByMeter.TryGetValue(meterId, out var previous) && value < previous)
        {
            var override_ = ReadSwapOverride(column, row);
            staged.Events.Add(new MeterEvent
            {
                MeterId = meterId,
                Time = time,
                EventType = MeterEventType.MeterSwap,
                PrevValue = previous,
                NewValue = value,
                Amount = override_,
                Notes = "Auto-detected register decrease (meter swap / reset candidate).",
            });
            staged.Warnings.Add(
                $"Row {rowIndex + 1}: register for meter {meterId} dropped {previous}→{value}; " +
                (override_ is null ? "swap event created (verify)." : $"swap consumption set to {override_}."));
        }

        staged.Readings.Add(new Reading
        {
            MeterId = meterId,
            Time = time,
            Value = value,
            Quality = ReadingQuality.Imported,
        });
        previousByMeter[meterId] = value;
    }

    private static double? ReadSwapOverride(ColumnMapping column, IReadOnlyList<string> row)
    {
        if (column.SwapConsumptionColumn is not { } source || source >= row.Count)
        {
            return null;
        }

        return GermanNumber.TryParse(row[source], out var value) ? value : null;
    }

    private static bool TryGetDate(IReadOnlyList<string> row, MappingProfile profile, out DateTimeOffset time)
    {
        time = default;
        if (profile.DateColumn >= row.Count)
        {
            return false;
        }

        var raw = row[profile.DateColumn];
        DateOnly date;
        var parsed = profile.DateKind switch
        {
            DateKind.MonthName => TryParseMonthAnchored(raw, profile.AnchorMonthsToEnd, out date),
            DateKind.DayDotMonthYear => GermanDate.TryParseDay(raw, out date),
            _ => ImportDate.TryResolve(raw, profile.AnchorMonthsToEnd, out date),
        };

        if (!parsed)
        {
            return false;
        }

        time = new DateTimeOffset(date.Year, date.Month, date.Day, 0, 0, 0, TimeSpan.Zero);
        return true;
    }

    private static bool TryParseMonthAnchored(string? raw, bool anchorToEnd, out DateOnly date)
    {
        if (!GermanDate.TryParseMonth(raw, out var month))
        {
            date = default;
            return false;
        }

        date = anchorToEnd
            ? new DateOnly(month.Year, month.Month, DateTime.DaysInMonth(month.Year, month.Month))
            : month;
        return true;
    }

    /// <summary>
    /// Reads the raw CSV grid (no profile applied) so the import wizard can show a column preview
    /// and let the user map columns before staging. Rows are ragged (their own field counts).
    /// </summary>
    public static IReadOnlyList<string[]> ReadRawRows(TextReader reader) => ReadRows(reader);

    internal static List<string[]> ReadRows(TextReader reader)
    {
        var config = new CsvConfiguration(CultureInfo.InvariantCulture)
        {
            HasHeaderRecord = false,
            DetectColumnCountChanges = false,
            MissingFieldFound = null,
            BadDataFound = null,
        };

        using var parser = new CsvParser(reader, config);
        var rows = new List<string[]>();
        while (parser.Read())
        {
            rows.Add(parser.Record ?? []);
        }

        return rows;
    }
}