using System.Text.Json;
using Data.SeaHavenIndustries;
using Data.SeaHavenIndustries.Enums;
using SeaHaven.DataServices.Helpers;
using SeaHaven.DataServices.Interfaces;
using SeaHaven.Services.Constants;
using SeaHaven.Services.DTOs;
using SeaHaven.Services.Exceptions;
using SeaHaven.Services.Helpers;
using SeaHaven.Services.Interfaces;

namespace SeaHaven.Services.Implementation
{
    public class WorkOrderBoardUpdateService : IWorkOrderBoardUpdateService
    {
        private readonly IWorkOrderBoardDataService _boardDataService;
        private readonly IWorkOrderBoardMutationDataService _mutationData;
        private readonly IWorkOrderAuditService _auditService;

        public WorkOrderBoardUpdateService(
            IWorkOrderBoardDataService boardDataService,
            IWorkOrderBoardMutationDataService mutationData,
            IWorkOrderAuditService auditService)
        {
            _boardDataService = boardDataService;
            _mutationData = mutationData;
            _auditService = auditService;
        }

        public async Task<WorkOrderBoardRowDto> PatchFieldAsync(
            int workOrderId,
            WorkOrderBoardPatchRequestDto request,
            string? actorId)
        {
            await _mutationData.ExecuteTransactionalAsync(async ct =>
            {
                if (string.IsNullOrWhiteSpace(request.Field))
                    throw new WorkOrderBoardValidationException("InvalidField", "Field is required.");

                var field = request.Field.Trim();
                var canonicalField = WorkOrderBoardFieldNames.Canonicalize(field);
                if (canonicalField == null)
                    throw new WorkOrderBoardValidationException("InvalidField", $"Field '{field}' is not editable.");

                var workOrderVersion = ParseRowVersion(request.WorkOrderVersion, "workOrderVersion");
                if (workOrderVersion == null)
                    throw new WorkOrderBoardValidationException("WorkOrderVersionRequired", "workOrderVersion is required.");

                var dispatchVersion = ParseRowVersion(request.DispatchVersion, "dispatchVersion");

                var workOrder = await _mutationData.GetTrackedWorkOrderAsync(workOrderId, ct);

                if (workOrder == null)
                    throw new WorkOrderBoardValidationException("NotFound", "Work order not found.");

                if (!RowVersionsMatch(workOrder.RowVersion, workOrderVersion))
                {
                    var staleState = await LoadBoardRowAsync(workOrderId);
                    throw new WorkOrderBoardConcurrencyException(staleState);
                }

                if (!WorkOrderBoardMutationRules.CanPatchBoardField(workOrder.LifecycleStatus, canonicalField))
                {
                    var code = WorkOrderBoardMutationRules.IsFullyLocked(workOrder.LifecycleStatus)
                        ? "CanceledReadOnly"
                        : "ReadOnly";
                    throw new WorkOrderBoardValidationException(code, "Work order is read-only in its current status.");
                }

                var isPastDue = WorkOrderDerivedFields.IsPastDue(workOrder.ScheduledDate, workOrder.LifecycleStatus);
                if (WorkOrderBoardMutationRules.ShouldBlockStatusChangeWhenPastDue(canonicalField, isPastDue))
                    throw new WorkOrderBoardValidationException("PastDueStatusBlocked", "Status cannot be changed while work order is past due. Update Schedule On first.");

                _mutationData.SetExpectedWorkOrderVersion(workOrder, workOrderVersion);

                Dispatch? dispatch = null;
                ResolvedDispatch? resolved = null;
                if (WorkOrderBoardFieldNames.IsDispatchField(canonicalField))
                {
                    var vendorIdHint = canonicalField.Equals(WorkOrderBoardFieldNames.VendorId, StringComparison.OrdinalIgnoreCase)
                        ? ParseVendorIdHint(request.Value)
                        : null;

                    var hadPrimaryDispatch = workOrder.PrimaryDispatchId.HasValue;
                    var replaceInactivePrimary = vendorIdHint.HasValue;
                    resolved = await ResolveDispatchAsync(
                        workOrder,
                        request.PrimaryDispatchId,
                        vendorIdHint,
                        replaceInactivePrimary,
                        ct);
                    dispatch = resolved.Dispatch;

                    var versionSource = resolved.Created ? resolved.Previous : dispatch;
                    if (dispatchVersion != null)
                    {
                        if (versionSource != null
                            && !RowVersionsMatch(versionSource.RowVersion, dispatchVersion))
                        {
                            var staleState = await LoadBoardRowAsync(workOrderId);
                            throw new WorkOrderBoardConcurrencyException(staleState);
                        }

                        if (!resolved.Created)
                            _mutationData.SetExpectedDispatchVersion(dispatch, dispatchVersion);
                    }
                    else if (hadPrimaryDispatch && versionSource?.RowVersion != null)
                    {
                        throw new WorkOrderBoardValidationException(
                            "DispatchVersionRequired",
                            "dispatchVersion is required for dispatch fields.");
                    }
                }

                var auditField = WorkOrderBoardFieldNames.ToAuditFieldName(canonicalField);
                var changes = await ApplyFieldMutationAsync(canonicalField, workOrder, dispatch, request.Value, auditField);
                if (resolved is { Created: true, Dispatch: var createdDispatch }
                    && canonicalField.Equals(WorkOrderBoardFieldNames.VendorId, StringComparison.OrdinalIgnoreCase))
                {
                    var previousVendor = resolved.Previous?.VendorId.ToString() ?? "";
                    changes = new List<FieldChange>
                    {
                        FieldChange.ForField(
                            auditField,
                            previousVendor,
                            createdDispatch.VendorId.ToString(),
                            createdDispatch.Id)
                    };
                }
                else
                {
                    changes = changes.Where(c => c.HasChanged).ToList();
                }

                if (changes.Count == 0 && !_mutationData.HasPendingChanges())
                    return;

                if (changes.Count == 0)
                {
                    await SaveTrackedOrThrowAsync(workOrderId, ct);
                    return;
                }

                foreach (var change in changes)
                {
                    var dispatchId = change.DispatchId ?? (dispatch?.Id > 0 ? dispatch.Id : null);
                    switch (change.Action)
                    {
                        case AuditActionType.StatusChanged:
                            await _auditService.StageStatusChangedAsync(workOrderId, change.OldValue, change.NewValue, actorId);
                            break;
                        case AuditActionType.AssignmentChanged:
                            await _auditService.StageAssignmentChangedAsync(workOrderId, change.OldValue, change.NewValue, actorId);
                            break;
                        default:
                            await _auditService.StageFieldChangedAsync(workOrderId, change.FieldName, change.OldValue, change.NewValue, actorId, dispatchId);
                            break;
                    }
                }

                await SaveTrackedOrThrowAsync(workOrderId, ct);
            }, CancellationToken.None);

            var row = await LoadBoardRowAsync(workOrderId);
            return row ?? throw new WorkOrderBoardValidationException("NotFound", "Work order not found.");
        }

        private async Task<List<FieldChange>> ApplyFieldMutationAsync(
            string field,
            WorkOrder workOrder,
            Dispatch? dispatch,
            string? value,
            string auditField)
        {
            return field switch
            {
                WorkOrderBoardFieldNames.WoNumber => new List<FieldChange> { await ApplyWoNumber(workOrder, value, auditField) },
                WorkOrderBoardFieldNames.WorkOrderType => new List<FieldChange> { ApplyWorkOrderType(workOrder, value, auditField) },
                WorkOrderBoardFieldNames.SiteCode => new List<FieldChange> { ApplyStringField(value, auditField, v => workOrder.SiteCode = v, () => workOrder.SiteCode) },
                WorkOrderBoardFieldNames.LifecycleStatus => new List<FieldChange> { ApplyLifecycleStatus(workOrder, value, auditField) },
                WorkOrderBoardFieldNames.AssignTo => ApplyAssignTo(workOrder, value, auditField),
                WorkOrderBoardFieldNames.DueDate => new List<FieldChange> { ApplyDateField(value, auditField, v => workOrder.DueDate = v, () => workOrder.DueDate) },
                WorkOrderBoardFieldNames.ScheduledDate => ApplyScheduledDate(workOrder, value, auditField),
                WorkOrderBoardFieldNames.TargetWeek => ApplyTargetWeek(workOrder, value, auditField),
                WorkOrderBoardFieldNames.ScheduleWeekOnly => ApplyScheduleWeekOnly(workOrder, value, auditField),
                WorkOrderBoardFieldNames.VendorId => new List<FieldChange> { await ApplyVendorIdAsync(dispatch!, value, auditField) },
                WorkOrderBoardFieldNames.ApptDate => new List<FieldChange> { ApplyApptDate(dispatch!, value, auditField) },
                WorkOrderBoardFieldNames.ApptTime => ApplyApptTime(workOrder, dispatch!, value),
                WorkOrderBoardFieldNames.DocStatus => new List<FieldChange> { ApplyDocStatus(workOrder, value, auditField) },
                WorkOrderBoardFieldNames.CompletedDate => new List<FieldChange> { ApplyDateField(value, auditField, v => workOrder.CompletedDate = v, () => workOrder.CompletedDate) },
                WorkOrderBoardFieldNames.Pm => new List<FieldChange> { ApplyStringField(value, auditField, v => workOrder.Trade = v, () => workOrder.Trade) },
                WorkOrderBoardFieldNames.ServiceNotes => new List<FieldChange> { ApplyServiceNotes(workOrder, value, auditField) },
                WorkOrderBoardFieldNames.ExtraServices => new List<FieldChange> { ApplyExtraServices(workOrder, value, auditField) },
                WorkOrderBoardFieldNames.AdditionalContacts => new List<FieldChange> { ApplyAdditionalContacts(workOrder, value, auditField) },
                WorkOrderBoardFieldNames.FlagColor => new List<FieldChange> { ApplyFlagColor(workOrder, value, auditField) },
                _ => throw new WorkOrderBoardValidationException("InvalidField", $"Field '{field}' is not editable.")
            };
        }

        private sealed record ResolvedDispatch(Dispatch Dispatch, bool Created, Dispatch? Previous);

        private async Task<ResolvedDispatch> ResolveDispatchAsync(
            WorkOrder workOrder,
            int? primaryDispatchId,
            int? vendorIdForCreate,
            bool replaceInactivePrimary,
            CancellationToken cancellationToken)
        {
            var existing = await LoadExistingDispatchAsync(workOrder, primaryDispatchId, cancellationToken);
            if (existing != null
                && !(replaceInactivePrimary
                    && WorkOrderBoardDispatchAssignment.IsInactivePrimaryDispatch(existing.Status)))
            {
                workOrder.PrimaryDispatchId = existing.Id;
                return new ResolvedDispatch(existing, Created: false, Previous: null);
            }

            if (!vendorIdForCreate.HasValue || vendorIdForCreate.Value <= 0)
                throw new WorkOrderBoardValidationException("DispatchRequired", "A primary dispatch is required. Set vendorId first or provide primaryDispatchId.");

            await EnsureVendorExistsAsync(vendorIdForCreate.Value);

            var created = new Dispatch
            {
                WorkOrderId = workOrder.Id,
                VendorId = vendorIdForCreate.Value,
                Status = "Pending",
                CreatedDate = DateTime.UtcNow
            };
            _mutationData.TrackNewDispatch(created);
            await SaveTrackedOrThrowAsync(workOrder.Id, cancellationToken);
            workOrder.PrimaryDispatchId = created.Id;
            return new ResolvedDispatch(created, Created: true, Previous: existing);
        }

        private async Task<Dispatch?> LoadExistingDispatchAsync(
            WorkOrder workOrder,
            int? primaryDispatchId,
            CancellationToken cancellationToken)
        {
            if (primaryDispatchId.HasValue)
            {
                var dispatch = await _mutationData.GetTrackedDispatchAsync(
                    primaryDispatchId.Value,
                    workOrder.Id,
                    cancellationToken);

                if (dispatch == null)
                    throw new WorkOrderBoardValidationException(
                        "InvalidDispatch",
                        "Primary dispatch does not belong to this work order.");

                return dispatch;
            }

            if (!workOrder.PrimaryDispatchId.HasValue)
                return null;

            return await _mutationData.GetTrackedDispatchByIdAsync(
                workOrder.PrimaryDispatchId.Value,
                cancellationToken);
        }

        private async Task EnsureVendorExistsAsync(int vendorId)
        {
            if (!await _mutationData.VendorExistsAsync(vendorId, CancellationToken.None))
                throw new WorkOrderBoardValidationException("VendorNotFound", "vendorId does not exist.");
        }

        private async Task SaveTrackedOrThrowAsync(int workOrderId, CancellationToken cancellationToken)
        {
            var outcome = await _mutationData.SaveAsync(cancellationToken);
            if (outcome != BoardSaveOutcome.ConcurrencyConflict)
                return;

            var currentState = await LoadBoardRowAsync(workOrderId);
            throw new WorkOrderBoardConcurrencyException(currentState);
        }

        private async Task<FieldChange> ApplyWoNumber(WorkOrder workOrder, string? value, string auditField)
        {
            if (!WorkOrderNumberNormalizer.TryNormalize(value, out var normalized, out var error))
                throw new WorkOrderBoardValidationException("InvalidWoNumber", error ?? "Invalid WO number.");

            if (await _boardDataService.InternalWoNumberExistsAsync(normalized, workOrder.Id))
                throw new WorkOrderBoardValidationException("DuplicateWoNumber", "WO number already exists.");

            var old = workOrder.InternalWONumber;
            if (old == normalized)
                return FieldChange.Unchanged(auditField);

            workOrder.InternalWONumber = normalized;
            workOrder.WorkerOrderNumber = normalized;
            return FieldChange.ForField(auditField, old, normalized);
        }

        private static FieldChange ApplyWorkOrderType(WorkOrder workOrder, string? value, string auditField)
        {
            if (string.IsNullOrWhiteSpace(value))
                throw new WorkOrderBoardValidationException("InvalidValue", "workOrderType value is required.");

            if (string.Equals(value.Trim(), "Overdue", StringComparison.OrdinalIgnoreCase))
                throw new WorkOrderBoardValidationException(
                    "InvalidValue",
                    "Overdue is not a work order type; it is derived from dueDate (isPastDue).");

            if (!WorkOrderTypeMapper.TryParseFeLabel(value, out var parsed))
                throw new WorkOrderBoardValidationException("InvalidValue", $"Invalid workOrderType: {value}");

            var old = workOrder.WorkOrderType?.ToString();
            if (workOrder.WorkOrderType == parsed)
                return FieldChange.Unchanged(auditField);

            workOrder.WorkOrderType = parsed;
            return FieldChange.ForField(auditField, old, parsed.ToString());
        }

        private static FieldChange ApplyStringField(
            string? value,
            string auditField,
            Action<string?> setter,
            Func<string?> getter)
        {
            var normalized = string.IsNullOrWhiteSpace(value) ? null : value.Trim();
            var old = getter();
            if (old == normalized)
                return FieldChange.Unchanged(auditField);

            setter(normalized);
            return FieldChange.ForField(auditField, old, normalized);
        }

        private static FieldChange ApplyServiceNotes(WorkOrder workOrder, string? value, string auditField)
        {
            var normalized = string.IsNullOrWhiteSpace(value) ? null : value.Trim();
            if (normalized != null && normalized.Length > 4000)
                throw new WorkOrderBoardValidationException("InvalidValue", "serviceNotes must be at most 4000 characters.");

            var old = workOrder.ServiceNotes;
            if (old == normalized)
                return FieldChange.Unchanged(auditField);

            workOrder.ServiceNotes = normalized;
            return FieldChange.ForField(auditField, old, normalized);
        }

        private static FieldChange ApplyExtraServices(WorkOrder workOrder, string? value, string auditField)
        {
            string? normalizedJson = null;
            if (!string.IsNullOrWhiteSpace(value))
            {
                List<string>? parsed;
                try
                {
                    parsed = JsonSerializer.Deserialize<List<string>>(value.Trim());
                }
                catch (JsonException)
                {
                    throw new WorkOrderBoardValidationException("InvalidValue", "extraServices must be a JSON string array.");
                }

                var items = NormalizeExtraServiceList(parsed);
                if (items.Count > 0)
                {
                    if (string.IsNullOrWhiteSpace(workOrder.Trade))
                    {
                        throw new WorkOrderBoardValidationException(
                            "InvalidValue",
                            "primaryService (pm/trade) is required when extraServices is provided.");
                    }

                    items = items
                        .Where(s => !string.Equals(s, workOrder.Trade, StringComparison.OrdinalIgnoreCase))
                        .ToList();

                    normalizedJson = items.Count > 0 ? JsonSerializer.Serialize(items) : null;
                    if (normalizedJson != null && normalizedJson.Length > 2000)
                    {
                        throw new WorkOrderBoardValidationException(
                            "InvalidValue",
                            "extraServices must serialize to at most 2000 characters.");
                    }
                }
            }

            var old = workOrder.ExtraServices;
            if (old == normalizedJson)
                return FieldChange.Unchanged(auditField);

            workOrder.ExtraServices = normalizedJson;
            return FieldChange.ForField(auditField, old, normalizedJson);
        }

        private static FieldChange ApplyAdditionalContacts(WorkOrder workOrder, string? value, string auditField)
        {
            var normalizedJson = WorkOrderAdditionalContactsMapper.SerializeForPatch(value);

            var old = workOrder.AdditionalContacts;
            if (old == normalizedJson)
                return FieldChange.Unchanged(auditField);

            workOrder.AdditionalContacts = normalizedJson;
            return FieldChange.ForField(auditField, old, normalizedJson);
        }

        private static List<string> NormalizeExtraServiceList(List<string>? extras)
        {
            var normalized = new List<string>();
            if (extras == null || extras.Count == 0)
                return normalized;

            var seen = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
            foreach (var item in extras)
            {
                if (string.IsNullOrWhiteSpace(item))
                    continue;
                var trimmed = item.Trim();
                if (trimmed.Length > 128)
                    throw new WorkOrderBoardValidationException("InvalidValue", "Each extra service must be at most 128 characters.");
                if (!seen.Add(trimmed))
                    continue;
                normalized.Add(trimmed);
            }

            return normalized;
        }

        private static FieldChange ApplyLifecycleStatus(WorkOrder workOrder, string? value, string auditField)
        {
            var parsed = LifecycleStatusMapper.ParseLifecycleStatus(value);
            if (parsed == null)
                throw new WorkOrderBoardValidationException("InvalidValue", $"Invalid lifecycleStatus: {value}");

            if (parsed == LifecycleStatus.Completed && WorkOrderNumberRules.IsProvisional(workOrder.InternalWONumber))
            {
                throw new WorkOrderBoardValidationException(
                    "ProvisionalWoNumber",
                    "Replace the provisional SH work order number before marking the work order Completed.");
            }

            var old = workOrder.LifecycleStatus?.ToString() ?? workOrder.Status;
            if (workOrder.LifecycleStatus == parsed)
                return FieldChange.Unchanged(auditField);

            workOrder.LifecycleStatus = parsed;
            workOrder.Status = WorkOrderDerivedFields.GetLifecycleStatusLabel(parsed);
            if (workOrder.LegacyStatus == null && workOrder.Status != null)
                workOrder.LegacyStatus = workOrder.Status;

            return FieldChange.StatusChanged(auditField, old, parsed.ToString());
        }

        private static List<FieldChange> ApplyAssignTo(WorkOrder workOrder, string? value, string auditField)
        {
            var normalized = string.IsNullOrWhiteSpace(value) ? null : value.Trim();
            var old = workOrder.AssignTo;
            var changes = new List<FieldChange>();

            if (old != normalized)
            {
                workOrder.AssignTo = normalized;
                changes.Add(FieldChange.AssignmentChanged(auditField, old, normalized));
            }

            changes.AddRange(ApplyAutoScheduleSideEffects(workOrder));
            return changes;
        }

        private static FieldChange ApplyDateField(
            string? value,
            string auditField,
            Action<DateTime?> setter,
            Func<DateTime?> getter)
        {
            DateTime? parsed = null;
            if (!string.IsNullOrWhiteSpace(value))
            {
                if (!DateTime.TryParse(value, out var dt))
                    throw new WorkOrderBoardValidationException("InvalidValue", $"Invalid date for {auditField}: {value}");
                parsed = dt.Date;
            }

            var old = FormatDate(getter());
            var newVal = FormatDate(parsed);
            if (old == newVal)
                return FieldChange.Unchanged(auditField);

            setter(parsed);
            return FieldChange.ForField(auditField, old, newVal);
        }

        private static List<FieldChange> ApplyScheduledDate(WorkOrder workOrder, string? value, string auditField)
        {
            var previousDate = workOrder.ScheduledDate;
            DateTime? parsed = null;
            if (!string.IsNullOrWhiteSpace(value))
            {
                if (!DateTime.TryParse(value, out var dt))
                    throw new WorkOrderBoardValidationException("InvalidValue", $"Invalid scheduledDate: {value}");
                parsed = dt.Date;
            }

            var changes = new List<FieldChange>();
            var oldFormatted = FormatDate(previousDate);
            var newFormatted = FormatDate(parsed);

            if (oldFormatted != newFormatted)
            {
                workOrder.ScheduledDate = parsed;
                changes.Add(FieldChange.ForField(auditField, oldFormatted, newFormatted));

                if (!workOrder.OriginalDate.HasValue && parsed.HasValue)
                {
                    workOrder.OriginalDate = DateOnly.FromDateTime(parsed.Value);
                    var originalWeek = WorkOrderBoardMutationRules.ComputeOriginalWeek(parsed);
                    if (originalWeek.HasValue)
                        workOrder.OriginalWeek = originalWeek;
                }

                if (WorkOrderBoardMutationRules.IsReschedule(previousDate, parsed))
                {
                    var oldCount = workOrder.RescheduleCount.ToString();
                    workOrder.RescheduleCount++;
                    changes.Add(FieldChange.ForField(WorkOrderBoardFieldNames.RescheduleCount, oldCount, workOrder.RescheduleCount.ToString()));
                }
            }

            changes.AddRange(ApplyAutoScheduleSideEffects(workOrder));
            AppendIsAddOnRecalculation(workOrder, changes);
            return changes;
        }

        private static List<FieldChange> ApplyTargetWeek(WorkOrder workOrder, string? value, string auditField)
        {
            DateOnly? parsed = null;
            if (!string.IsNullOrWhiteSpace(value))
            {
                if (!DateOnly.TryParse(value, out var d))
                    throw new WorkOrderBoardValidationException("InvalidValue", $"Invalid targetWeek: {value}");
                parsed = d;
            }

            var changes = new List<FieldChange>();
            var old = workOrder.TargetWeek?.ToString("yyyy-MM-dd");
            var newVal = parsed?.ToString("yyyy-MM-dd");
            if (old != newVal)
            {
                workOrder.TargetWeek = parsed;
                changes.Add(FieldChange.ForField(auditField, old, newVal));
            }

            AppendIsAddOnRecalculation(workOrder, changes);
            return changes;
        }

        private static List<FieldChange> ApplyScheduleWeekOnly(WorkOrder workOrder, string? value, string auditField)
        {
            var change = ApplyBoolField(value, auditField, v => workOrder.ScheduleWeekOnly = v, () => workOrder.ScheduleWeekOnly);
            var changes = new List<FieldChange> { change };
            if (change.HasChanged)
                AppendIsAddOnRecalculation(workOrder, changes);
            return changes;
        }

        private static void AppendIsAddOnRecalculation(WorkOrder workOrder, List<FieldChange> changes)
        {
            var createdAt = workOrder.CreatedDate ?? DateTime.UtcNow;
            var oldValue = workOrder.IsAddOn.ToString();
            var newIsAddOn = WorkOrderAddonCutoff.RecalculateIsAddOnFromSchedule(
                createdAt,
                workOrder.ScheduledDate,
                workOrder.TargetWeek);

            if (workOrder.IsAddOn == newIsAddOn)
                return;

            workOrder.IsAddOn = newIsAddOn;
            changes.Add(FieldChange.ForField("IsAddOn", oldValue, newIsAddOn.ToString()));
        }

        private static FieldChange ApplyBoolField(
            string? value,
            string auditField,
            Action<bool?> setter,
            Func<bool?> getter)
        {
            if (string.IsNullOrWhiteSpace(value))
                throw new WorkOrderBoardValidationException("InvalidValue", $"{auditField} value is required.");

            if (!bool.TryParse(value, out var parsed))
                throw new WorkOrderBoardValidationException("InvalidValue", $"Invalid boolean for {auditField}: {value}");

            var old = getter()?.ToString();
            if (getter() == parsed)
                return FieldChange.Unchanged(auditField);

            setter(parsed);
            return FieldChange.ForField(auditField, old, parsed.ToString());
        }

        private async Task<FieldChange> ApplyVendorIdAsync(Dispatch dispatch, string? value, string auditField)
        {
            var vendorId = ParseVendorIdHint(value);
            if (!vendorId.HasValue)
                throw new WorkOrderBoardValidationException("InvalidValue", "vendorId must be a positive integer.");

            await EnsureVendorExistsAsync(vendorId.Value);

            var old = dispatch.VendorId.ToString();
            if (dispatch.VendorId == vendorId.Value)
                return FieldChange.Unchanged(auditField, ResolveDispatchIdForAudit(dispatch));

            dispatch.VendorId = vendorId.Value;
            return FieldChange.ForField(auditField, old, vendorId.Value.ToString(), ResolveDispatchIdForAudit(dispatch));
        }

        private static FieldChange ApplyApptDate(Dispatch dispatch, string? value, string auditField)
        {
            DateTime? parsed = null;
            if (!string.IsNullOrWhiteSpace(value))
            {
                if (!DateTime.TryParse(value, out var dt))
                    throw new WorkOrderBoardValidationException("InvalidValue", $"Invalid apptDate: {value}");
                parsed = dt.Date;
            }

            var old = FormatDate(dispatch.ScheduledDate);
            var newVal = FormatDate(parsed);
            if (old == newVal)
                return FieldChange.Unchanged(auditField, ResolveDispatchIdForAudit(dispatch));

            dispatch.ScheduledDate = parsed;
            return FieldChange.ForField(auditField, old, newVal, ResolveDispatchIdForAudit(dispatch));
        }

        // apptTime is stored on WorkOrder (ScheduledStart/End). Lock those sync field names so
        // SyncFieldMergePolicy honors the manual edit (not a synthetic "ApptTime" lock).
        private static List<FieldChange> ApplyApptTime(WorkOrder workOrder, Dispatch dispatch, string? value)
        {
            if (!WorkOrderBoardApptTimeParser.TryParse(value, out var start, out var end, out var error))
                throw new WorkOrderBoardValidationException("InvalidValue", error ?? "Invalid apptTime.");

            var apptDate = dispatch.ScheduledDate ?? workOrder.ScheduledDate;
            var oldStart = FormatDateTime(workOrder.ScheduledStart);
            var oldEnd = FormatDateTime(workOrder.ScheduledEnd);
            var dispatchId = ResolveDispatchIdForAudit(dispatch);

            workOrder.ScheduledStart = WorkOrderBoardApptTimeParser.CombineDateAndTime(apptDate, start);
            workOrder.ScheduledEnd = WorkOrderBoardApptTimeParser.CombineDateAndTime(apptDate, end);

            var newStart = FormatDateTime(workOrder.ScheduledStart);
            var newEnd = FormatDateTime(workOrder.ScheduledEnd);
            var changes = new List<FieldChange>();

            if (oldStart != newStart)
                changes.Add(FieldChange.ForField(WorkOrderFieldNames.ScheduledStart, oldStart, newStart, dispatchId));
            if (oldEnd != newEnd)
                changes.Add(FieldChange.ForField(WorkOrderFieldNames.ScheduledEnd, oldEnd, newEnd, dispatchId));

            if (changes.Count == 0)
                changes.Add(FieldChange.Unchanged(WorkOrderFieldNames.ScheduledStart, dispatchId));

            return changes;
        }

        private static FieldChange ApplyDocStatus(WorkOrder workOrder, string? value, string auditField)
        {
            if (string.IsNullOrWhiteSpace(value))
                throw new WorkOrderBoardValidationException("InvalidValue", "docStatus value is required.");

            if (!Enum.TryParse<DocStatus>(value, true, out var parsed))
                throw new WorkOrderBoardValidationException("InvalidValue", $"Invalid docStatus: {value}");

            var old = workOrder.DocStatus?.ToString();
            if (workOrder.DocStatus == parsed)
                return FieldChange.Unchanged(auditField);

            workOrder.DocStatus = parsed;
            return FieldChange.ForField(auditField, old, parsed.ToString());
        }

        private static FieldChange ApplyFlagColor(WorkOrder workOrder, string? value, string auditField)
        {
            if (!WorkOrderFlagColors.TryNormalize(value, out var canonical))
                throw new WorkOrderBoardValidationException("InvalidValue", $"Invalid flagColor: {value}");

            if (canonical != null && !WorkOrderFlagColors.IsAllowed(canonical))
                throw new WorkOrderBoardValidationException(
                    "InvalidValue",
                    $"flagColor must be one of the allowed palette values or empty to clear.");

            var old = workOrder.FlagColor;
            if (string.Equals(old, canonical, StringComparison.OrdinalIgnoreCase))
                return FieldChange.Unchanged(auditField);

            workOrder.FlagColor = canonical;
            return FieldChange.ForField(auditField, old, canonical);
        }

        private static List<FieldChange> ApplyAutoScheduleSideEffects(WorkOrder workOrder)
        {
            var changes = new List<FieldChange>();
            if (!WorkOrderBoardMutationRules.ShouldAutoSchedule(
                    workOrder.LifecycleStatus,
                    workOrder.ScheduledDate,
                    workOrder.AssignTo,
                    workOrder.ScheduleWeekOnly))
                return changes;

            var old = workOrder.LifecycleStatus?.ToString();
            workOrder.LifecycleStatus = LifecycleStatus.Scheduled;
            workOrder.Status = WorkOrderDerivedFields.GetLifecycleStatusLabel(LifecycleStatus.Scheduled);
            changes.Add(FieldChange.StatusChanged("LifecycleStatus", old, LifecycleStatus.Scheduled.ToString()));
            return changes;
        }

        private static bool RowVersionsMatch(byte[]? current, byte[] expected)
            => current != null && current.AsSpan().SequenceEqual(expected);

        private async Task<WorkOrderBoardRowDto?> LoadBoardRowAsync(int workOrderId)
        {
            var raw = await _boardDataService.GetBoardRowByIdAsync(workOrderId);
            return raw == null ? null : WorkOrderBoardService.MapRawRow(raw, DateTime.UtcNow);
        }

        private static byte[]? ParseRowVersion(string? base64, string fieldName)
        {
            if (string.IsNullOrWhiteSpace(base64))
                return null;

            try
            {
                return Convert.FromBase64String(base64);
            }
            catch (FormatException)
            {
                throw new WorkOrderBoardValidationException("InvalidRowVersion", $"Invalid {fieldName} format.");
            }
        }

        private static int? ParseVendorIdHint(string? value)
        {
            if (string.IsNullOrWhiteSpace(value) || !int.TryParse(value, out var vendorId) || vendorId <= 0)
                return null;
            return vendorId;
        }

        private static int? ResolveDispatchIdForAudit(Dispatch dispatch)
            => dispatch.Id > 0 ? dispatch.Id : null;

        private static string? FormatDate(DateTime? value) => value?.ToString("yyyy-MM-dd");
        private static string? FormatDateTime(DateTime? value) => value?.ToString("O");
        private static string? FormatApptRange(string? start, string? end)
            => start == null && end == null ? null : $"{start}|{end}";

        private sealed record FieldChange(
            string FieldName,
            string? OldValue,
            string? NewValue,
            AuditActionType Action,
            int? DispatchId,
            bool HasChanged)
        {
            public static FieldChange Unchanged(string fieldName, int? dispatchId = null)
                => new(fieldName, null, null, AuditActionType.FieldChanged, dispatchId, false);

            public static FieldChange ForField(string fieldName, string? oldValue, string? newValue, int? dispatchId = null)
                => new(fieldName, oldValue, newValue, AuditActionType.FieldChanged, dispatchId, true);

            public static FieldChange StatusChanged(string fieldName, string? oldValue, string? newValue)
                => new(fieldName, oldValue, newValue, AuditActionType.StatusChanged, null, true);

            public static FieldChange AssignmentChanged(string fieldName, string? oldValue, string? newValue)
                => new(fieldName, oldValue, newValue, AuditActionType.AssignmentChanged, null, true);
        }
    }
}