using System.Text.Json;
using Distribution.Features.Item;
using Distribution.Features.ShopPO;
using Distribution.Features.Stock;
using Distribution.Features.Store;
using Distribution.Infra.UoW;
using Distribution.Models;

namespace Distribution.Features.Scheme;

public class SchemeService(
    ISchemeRepository repo,
    IShopPORepository shopPORepo,
    IItemRepository itemRepo,
    IStoreRepository storeRepo,
    IStockRepository stockRepo,
    IUnitOfWork uow)
{
    private static readonly JsonSerializerOptions JsonOptions = new(JsonSerializerDefaults.Web);

    private static SchemeResponse ToResponse(Models.Scheme s) =>
        new(s.Id, s.Name, s.RuleType, s.RuleConfig,
            s.FK_TriggerItemId, s.TriggerItem?.Name, s.TriggerQty,
            s.FK_FreeItemId, s.FreeItem?.Name, s.FreeQty,
            s.BundleComponents.Select(c => new BundleComponentResponse(c.FK_ItemId, c.Item.Sku, c.Item.Name, c.Qty)).ToList(),
            s.FK_RegionId, s.Region?.Name, s.EligibleSegment,
            s.DateRangeFrom, s.DateRangeTo, s.AgentLimitExempt, s.BaselineExpectedSales, s.IsActive, s.CreatedAt);

    private static SchemeRedemptionResponse ToRedemptionResponse(SchemeRedemption r) =>
        new(r.Id, r.FK_SchemeId, r.FK_ShopPOId, r.DiscountAmount, r.FK_FreeItemId, r.FreeItem?.Name,
            r.FreeQty, r.FreeItemCostValue, r.RedeemedAt);

    // ── CRUD ───────────────────────────────────────────────────────────────

    public async Task<List<SchemeResponse>> GetAllAsync(Guid? regionId = null)
    {
        var schemes = await repo.GetAllAsync(regionId);
        return schemes.Select(ToResponse).ToList();
    }

    public async Task<SchemeResponse?> GetByIdAsync(Guid id)
    {
        var scheme = await repo.GetByIdAsync(id);
        return scheme is null ? null : ToResponse(scheme);
    }

    /// <summary>
    /// createScheme(schemeRules, eligibility, dateRange). RuleConfig is only meaningful for
    /// SlabDiscount/FlatPercent. For BuyXGetY/FreeBundle, the free-goods payout (and bundle
    /// components) can be supplied inline here to create a fully-populated scheme in one call, or
    /// added afterwards via SetFreeGoodsRuleAsync / UpsertBundleComponentAsync.
    /// </summary>
    public async Task<SchemeResponse> CreateAsync(CreateSchemeRequest request, CancellationToken ct = default)
    {
        ValidatePercentRuleConfig(request.RuleType, request.RuleConfig);
        if (request.DateRangeFrom > request.DateRangeTo)
            throw new InvalidOperationException("DateRangeFrom must not be after DateRangeTo.");

        var hasFreeGoodsRule = request.FreeItemId.HasValue || request.FreeQty.HasValue ||
            request.TriggerItemId.HasValue || request.TriggerQty.HasValue;
        if (hasFreeGoodsRule)
            await ValidateFreeGoodsRuleAsync(request.RuleType, request.TriggerItemId, request.TriggerQty, request.FreeItemId, request.FreeQty);

        if (request.BundleComponents is { Count: > 0 })
        {
            if (request.RuleType != SchemeRuleType.FreeBundle)
                throw new InvalidOperationException("BundleComponents only applies to FreeBundle schemes.");
            foreach (var c in request.BundleComponents)
            {
                if (c.Qty <= 0)
                    throw new InvalidOperationException("Bundle component Qty must be positive.");
                if (await itemRepo.GetByIdAsync(c.ItemId) is null)
                    throw new InvalidOperationException($"Bundle component item '{c.ItemId}' not found.");
            }
        }

        var scheme = new Models.Scheme
        {
            Name = request.Name,
            RuleType = request.RuleType,
            RuleConfig = request.RuleConfig,
            FK_RegionId = request.RegionId,
            EligibleSegment = request.EligibleSegment,
            DateRangeFrom = request.DateRangeFrom,
            DateRangeTo = request.DateRangeTo,
            AgentLimitExempt = request.AgentLimitExempt,
            BaselineExpectedSales = request.BaselineExpectedSales
        };

        if (hasFreeGoodsRule)
        {
            scheme.FK_TriggerItemId = request.RuleType == SchemeRuleType.BuyXGetY ? request.TriggerItemId : null;
            scheme.TriggerQty = request.RuleType == SchemeRuleType.BuyXGetY ? request.TriggerQty : null;
            scheme.FK_FreeItemId = request.FreeItemId;
            scheme.FreeQty = request.FreeQty;
        }

        await uow.BeginAsync(ct);
        try
        {
            await repo.AddAsync(scheme);
            if (request.BundleComponents is { Count: > 0 })
            {
                foreach (var c in request.BundleComponents)
                    await repo.AddBundleComponentAsync(new SchemeBundleComponent { FK_SchemeId = scheme.Id, FK_ItemId = c.ItemId, Qty = c.Qty });
            }
            await uow.CommitAsync(ct);
        }
        catch { await uow.RollbackAsync(ct); throw; }

        return (await GetByIdAsync(scheme.Id))!;
    }

    public async Task<SchemeResponse?> UpdateAsync(Guid id, UpdateSchemeRequest request, CancellationToken ct = default)
    {
        var scheme = await repo.GetByIdAsync(id);
        if (scheme is null) return null;

        ValidatePercentRuleConfig(scheme.RuleType, request.RuleConfig);
        if (request.DateRangeFrom > request.DateRangeTo)
            throw new InvalidOperationException("DateRangeFrom must not be after DateRangeTo.");

        scheme.Name = request.Name;
        scheme.RuleConfig = request.RuleConfig;
        scheme.FK_RegionId = request.RegionId;
        scheme.EligibleSegment = request.EligibleSegment;
        scheme.DateRangeFrom = request.DateRangeFrom;
        scheme.DateRangeTo = request.DateRangeTo;
        scheme.AgentLimitExempt = request.AgentLimitExempt;
        scheme.BaselineExpectedSales = request.BaselineExpectedSales;
        scheme.IsActive = request.IsActive;

        await uow.BeginAsync(ct);
        try { repo.Update(scheme); await uow.CommitAsync(ct); }
        catch { await uow.RollbackAsync(ct); throw; }

        return ToResponse(scheme);
    }

    public async Task<bool> DeleteAsync(Guid id, CancellationToken ct = default)
    {
        var scheme = await repo.GetByIdAsync(id);
        if (scheme is null || !scheme.IsActive) return false;

        await uow.BeginAsync(ct);
        try { repo.Remove(scheme); await uow.CommitAsync(ct); }
        catch { await uow.RollbackAsync(ct); throw; }

        return true;
    }

    // ── Free-goods rule CRUD (BuyXGetY trigger + shared payout) ──────────────

    public async Task<SchemeResponse?> SetFreeGoodsRuleAsync(Guid schemeId, SetFreeGoodsRuleRequest request, CancellationToken ct = default)
    {
        var scheme = await repo.GetByIdAsync(schemeId);
        if (scheme is null || !scheme.IsActive) return null;

        await ValidateFreeGoodsRuleAsync(scheme.RuleType, request.TriggerItemId, request.TriggerQty, request.FreeItemId, request.FreeQty);

        scheme.FK_TriggerItemId = scheme.RuleType == SchemeRuleType.BuyXGetY ? request.TriggerItemId : null;
        scheme.TriggerQty = scheme.RuleType == SchemeRuleType.BuyXGetY ? request.TriggerQty : null;
        scheme.FK_FreeItemId = request.FreeItemId;
        scheme.FreeQty = request.FreeQty;

        await uow.BeginAsync(ct);
        try { repo.Update(scheme); await uow.CommitAsync(ct); }
        catch { await uow.RollbackAsync(ct); throw; }

        return (await GetByIdAsync(schemeId))!;
    }

    public async Task<SchemeResponse?> ClearFreeGoodsRuleAsync(Guid schemeId, CancellationToken ct = default)
    {
        var scheme = await repo.GetByIdAsync(schemeId);
        if (scheme is null || !scheme.IsActive) return null;

        scheme.FK_TriggerItemId = null;
        scheme.TriggerQty = null;
        scheme.FK_FreeItemId = null;
        scheme.FreeQty = null;

        await uow.BeginAsync(ct);
        try { repo.Update(scheme); await uow.CommitAsync(ct); }
        catch { await uow.RollbackAsync(ct); throw; }

        return (await GetByIdAsync(schemeId))!;
    }

    // ── Bundle component CRUD (FreeBundle required items) ────────────────────

    public async Task<SchemeResponse?> UpsertBundleComponentAsync(Guid schemeId, Guid itemId, UpsertBundleComponentRequest request, CancellationToken ct = default)
    {
        var scheme = await repo.GetByIdAsync(schemeId);
        if (scheme is null || !scheme.IsActive) return null;

        if (scheme.RuleType != SchemeRuleType.FreeBundle)
            throw new InvalidOperationException("Bundle components only apply to FreeBundle schemes.");
        if (request.Qty <= 0)
            throw new InvalidOperationException("Qty must be positive.");

        var item = await itemRepo.GetByIdAsync(itemId)
            ?? throw new InvalidOperationException("Item not found.");

        var existing = await repo.GetBundleComponentAsync(schemeId, itemId);

        await uow.BeginAsync(ct);
        try
        {
            if (existing is null)
            {
                await repo.AddBundleComponentAsync(new SchemeBundleComponent
                {
                    FK_SchemeId = schemeId,
                    FK_ItemId = item.Id,
                    Qty = request.Qty
                });
            }
            else
            {
                existing.Qty = request.Qty;
                repo.UpdateBundleComponent(existing);
            }
            await uow.CommitAsync(ct);
        }
        catch { await uow.RollbackAsync(ct); throw; }

        return (await GetByIdAsync(schemeId))!;
    }

    public async Task<SchemeResponse?> RemoveBundleComponentAsync(Guid schemeId, Guid itemId, CancellationToken ct = default)
    {
        var scheme = await repo.GetByIdAsync(schemeId);
        if (scheme is null || !scheme.IsActive) return null;

        var existing = await repo.GetBundleComponentAsync(schemeId, itemId);
        if (existing is null) return null;

        await uow.BeginAsync(ct);
        try { repo.RemoveBundleComponent(existing); await uow.CommitAsync(ct); }
        catch { await uow.RollbackAsync(ct); throw; }

        return (await GetByIdAsync(schemeId))!;
    }

    // ── evaluateSchemeEligibility(storeId, shopPOId) ──────────────────────────

    public async Task<List<SchemeEligibilityResponse>?> EvaluateEligibilityAsync(Guid storeId, Guid shopPOId)
    {
        var store = await storeRepo.GetByIdWithRegionAsync(storeId);
        if (store is null || !store.IsActive) return null;

        var shopPO = await shopPORepo.GetByIdAsync(shopPOId);
        if (shopPO is null || !shopPO.IsActive) return null;

        var items = await shopPORepo.GetActiveItemsByShopPOIdAsync(shopPOId);
        var regionId = store.Route?.FK_RegionId;

        var candidates = (await repo.GetAllAsync()).Where(s => s.IsActive);
        var result = new List<SchemeEligibilityResponse>();

        foreach (var scheme in candidates)
        {
            if (scheme.FK_RegionId.HasValue && scheme.FK_RegionId != regionId)
                continue;
            if (!string.IsNullOrEmpty(scheme.EligibleSegment) &&
                !string.Equals(scheme.EligibleSegment, store.Segment, StringComparison.OrdinalIgnoreCase))
                continue;
            if (shopPO.CreatedAt < scheme.DateRangeFrom || shopPO.CreatedAt > scheme.DateRangeTo)
                continue;

            result.Add(EvaluateRule(scheme, items));
        }

        return result;
    }

    /// <summary>
    /// Scope-only check (region, segment, date range as of today) with no ShopPO to evaluate line
    /// items against — used by Store Management's `checkStoreSchemeEligibility(storeId, schemeId)`
    /// (12_store_management.md), which has no order context. Returns null if the store or scheme
    /// doesn't exist; otherwise (Eligible, Reason) — Eligible here means "in scope for this store",
    /// not "this ShopPO's items satisfy the rule" (use EvaluateEligibilityAsync for that).
    /// </summary>
    public async Task<(bool Eligible, string? Reason)?> CheckStoreScopeEligibilityAsync(Guid storeId, Guid schemeId)
    {
        var store = await storeRepo.GetByIdWithRegionAsync(storeId);
        if (store is null || !store.IsActive) return null;

        var scheme = await repo.GetByIdAsync(schemeId);
        if (scheme is null || !scheme.IsActive) return null;

        var regionId = store.Route?.FK_RegionId;
        if (scheme.FK_RegionId.HasValue && scheme.FK_RegionId != regionId)
            return (false, "Scheme is not available in the store's region.");

        if (!string.IsNullOrEmpty(scheme.EligibleSegment) &&
            !string.Equals(scheme.EligibleSegment, store.Segment, StringComparison.OrdinalIgnoreCase))
            return (false, $"Scheme is only available to the '{scheme.EligibleSegment}' segment.");

        var today = DateTime.Now;
        if (today < scheme.DateRangeFrom || today > scheme.DateRangeTo)
            return (false, "Scheme is not currently active (outside its date range).");

        return (true, null);
    }

    private static SchemeEligibilityResponse EvaluateRule(Models.Scheme scheme, List<ShopPOItem> items)
    {
        switch (scheme.RuleType)
        {
            case SchemeRuleType.BuyXGetY:
            {
                if (scheme.FK_TriggerItemId is null || scheme.TriggerQty is null or <= 0 ||
                    scheme.FK_FreeItemId is null || scheme.FreeQty is null or <= 0)
                    return new SchemeEligibilityResponse(scheme.Id, scheme.Name, scheme.RuleType, false,
                        "Scheme has no free-goods rule configured.", null, null, null);

                var boughtQty = items.Where(i => i.FK_ItemId == scheme.FK_TriggerItemId).Sum(i => i.Quantity);
                if (boughtQty < scheme.TriggerQty)
                    return new SchemeEligibilityResponse(scheme.Id, scheme.Name, scheme.RuleType, false,
                        $"Requires {scheme.TriggerQty} of the trigger item; ShopPO has {boughtQty}.", null, null, null);

                var multiplier = boughtQty / scheme.TriggerQty.Value;
                return new SchemeEligibilityResponse(scheme.Id, scheme.Name, scheme.RuleType, true, null,
                    null, scheme.FK_FreeItemId, scheme.FreeQty * multiplier);
            }
            case SchemeRuleType.FreeBundle:
            {
                if (scheme.BundleComponents.Count == 0 || scheme.FK_FreeItemId is null || scheme.FreeQty is null or <= 0)
                    return new SchemeEligibilityResponse(scheme.Id, scheme.Name, scheme.RuleType, false,
                        "Scheme has no bundle components or free-goods payout configured.", null, null, null);

                var missing = scheme.BundleComponents
                    .Where(c => items.Where(i => i.FK_ItemId == c.FK_ItemId).Sum(i => i.Quantity) < c.Qty)
                    .ToList();
                if (missing.Count > 0)
                    return new SchemeEligibilityResponse(scheme.Id, scheme.Name, scheme.RuleType, false,
                        "ShopPO is missing required bundle components.", null, null, null);

                return new SchemeEligibilityResponse(scheme.Id, scheme.Name, scheme.RuleType, true, null,
                    null, scheme.FK_FreeItemId, scheme.FreeQty);
            }
            case SchemeRuleType.FlatPercent:
            {
                if (string.IsNullOrEmpty(scheme.RuleConfig))
                    return new SchemeEligibilityResponse(scheme.Id, scheme.Name, scheme.RuleType, false, "Scheme has no RuleConfig.", null, null, null);
                var cfg = JsonSerializer.Deserialize<FlatPercentConfig>(scheme.RuleConfig, JsonOptions)!;
                var orderTotal = items.Sum(i => i.Quantity * i.UnitPrice);
                return new SchemeEligibilityResponse(scheme.Id, scheme.Name, scheme.RuleType, true, null,
                    Math.Round(orderTotal * cfg.DiscountPercent / 100m, 2), null, null);
            }
            case SchemeRuleType.SlabDiscount:
            {
                if (string.IsNullOrEmpty(scheme.RuleConfig))
                    return new SchemeEligibilityResponse(scheme.Id, scheme.Name, scheme.RuleType, false, "Scheme has no RuleConfig.", null, null, null);
                var cfg = JsonSerializer.Deserialize<SlabDiscountConfig>(scheme.RuleConfig, JsonOptions)!;
                var orderTotal = items.Sum(i => i.Quantity * i.UnitPrice);
                var tier = cfg.Tiers.Where(t => orderTotal >= t.MinOrderAmount)
                    .OrderByDescending(t => t.MinOrderAmount).FirstOrDefault();
                if (tier is null)
                    return new SchemeEligibilityResponse(scheme.Id, scheme.Name, scheme.RuleType, false,
                        "ShopPO total does not meet any slab threshold.", null, null, null);

                return new SchemeEligibilityResponse(scheme.Id, scheme.Name, scheme.RuleType, true, null,
                    Math.Round(orderTotal * tier.DiscountPercent / 100m, 2), null, null);
            }
            default:
                return new SchemeEligibilityResponse(scheme.Id, scheme.Name, scheme.RuleType, false, "Unknown rule type.", null, null, null);
        }
    }

    // ── applySchemeDiscount(shopPOId, schemeId) ───────────────────────────────

    public async Task<SchemeRedemptionResponse?> ApplySchemeAsync(Guid shopPOId, Guid schemeId, CancellationToken ct = default)
    {
        var scheme = await repo.GetByIdAsync(schemeId);
        if (scheme is null || !scheme.IsActive) return null;

        var shopPO = await shopPORepo.GetByIdAsync(shopPOId);
        if (shopPO is null || !shopPO.IsActive) return null;

        if (await repo.HasRedemptionAsync(schemeId, shopPOId))
            throw new InvalidOperationException("This scheme has already been redeemed against this ShopPO.");

        if (await repo.HasLockedInvoiceAsync(shopPOId))
            throw new InvalidOperationException("This order's invoice is already printed or finalized, so a scheme can no longer be applied. Apply schemes before the invoice is printed.");

        var items = await shopPORepo.GetActiveItemsByShopPOIdAsync(shopPOId);
        var eligibility = EvaluateRule(scheme, items);
        if (!eligibility.Eligible)
            throw new InvalidOperationException(eligibility.Reason ?? "ShopPO is not eligible for this scheme.");

        var redemption = new SchemeRedemption
        {
            FK_SchemeId = schemeId,
            FK_ShopPOId = shopPOId,
            DiscountAmount = eligibility.DiscountAmount
        };

        var newItems = new List<ShopPOItem>();

        if (eligibility.FreeItemId.HasValue && eligibility.FreeQty is > 0)
        {
            var freeItem = await itemRepo.GetByIdAsync(eligibility.FreeItemId.Value)
                ?? throw new InvalidOperationException("Free item not found.");

            var availableQty = (await stockRepo.GetBalancesByItemAsync(freeItem.Id)).Sum(b => b.Quantity);
            if (availableQty < eligibility.FreeQty.Value)
                throw new InvalidOperationException($"Insufficient stock of '{freeItem.Sku}' to fulfil the free-goods scheme.");

            newItems.Add(new ShopPOItem
            {
                FK_ShopPOId = shopPOId,
                FK_ItemId = freeItem.Id,
                Quantity = (int)eligibility.FreeQty.Value,
                UnitPrice = 0m
            });

            redemption.FK_FreeItemId = freeItem.Id;
            redemption.FreeQty = eligibility.FreeQty;
            redemption.FreeItemCostValue = freeItem.CostPrice ?? 0m;
        }

        await uow.BeginAsync(ct);
        try
        {
            if (newItems.Count > 0) await shopPORepo.AddItemsAsync(newItems);
            await repo.AddRedemptionAsync(redemption);
            await uow.CommitAsync(ct);
        }
        catch { await uow.RollbackAsync(ct); throw; }

        var reloaded = (await repo.GetRedemptionsAsync(schemeId)).First(r => r.Id == redemption.Id);
        return ToRedemptionResponse(reloaded);
    }

    // ── trackSchemeSpend(schemeId) ───────────────────────────────────────────

    public async Task<SchemeSpendResponse?> GetSpendAsync(Guid schemeId)
    {
        var scheme = await repo.GetByIdAsync(schemeId);
        if (scheme is null) return null;

        var redemptions = await repo.GetRedemptionsAsync(schemeId);
        var discountSpend = redemptions.Sum(r => r.DiscountAmount ?? 0m);
        var freeGoodsSpend = redemptions.Sum(r => (r.FreeItemCostValue ?? 0m) * (r.FreeQty ?? 0m));

        return new SchemeSpendResponse(schemeId, discountSpend, freeGoodsSpend, discountSpend + freeGoodsSpend, redemptions.Count);
    }

    // ── getSchemeROI(schemeId) ───────────────────────────────────────────────

    public async Task<SchemeROIResponse?> GetROIAsync(Guid schemeId)
    {
        var scheme = await repo.GetByIdAsync(schemeId);
        if (scheme is null) return null;

        var redemptions = await repo.GetRedemptionsAsync(schemeId);
        var spend = await GetSpendAsync(schemeId);

        var actualSales = redemptions
            .Where(r => r.ShopPO is not null)
            .Select(r => r.ShopPO!)
            .Distinct()
            .Sum(o => o.TotalAmount);

        var baseline = scheme.BaselineExpectedSales ?? 0m;
        var incremental = actualSales - baseline;
        decimal? roi = spend!.TotalSpend > 0 ? incremental / spend.TotalSpend : null;

        return new SchemeROIResponse(schemeId, baseline, actualSales, incremental, spend.TotalSpend, roi);
    }

    private async Task ValidateFreeGoodsRuleAsync(SchemeRuleType ruleType, Guid? triggerItemId, int? triggerQty, Guid? freeItemId, decimal? freeQty)
    {
        if (ruleType is not (SchemeRuleType.BuyXGetY or SchemeRuleType.FreeBundle))
            throw new InvalidOperationException($"Free-goods rule only applies to BuyXGetY or FreeBundle schemes, not '{ruleType}'.");

        if (ruleType == SchemeRuleType.BuyXGetY)
        {
            if (triggerItemId is null || triggerQty is null or <= 0)
                throw new InvalidOperationException("BuyXGetY requires TriggerItemId and a positive TriggerQty.");
            if (await itemRepo.GetByIdAsync(triggerItemId.Value) is null)
                throw new InvalidOperationException("Trigger item not found.");
        }

        if (freeItemId is null || freeQty is null or <= 0)
            throw new InvalidOperationException("FreeItemId and a positive FreeQty are required.");
        if (await itemRepo.GetByIdAsync(freeItemId.Value) is null)
            throw new InvalidOperationException("Free item not found.");
    }

    private static void ValidatePercentRuleConfig(SchemeRuleType ruleType, string? ruleConfig)
    {
        if (ruleType is not (SchemeRuleType.SlabDiscount or SchemeRuleType.FlatPercent)) return;

        if (string.IsNullOrWhiteSpace(ruleConfig))
            throw new InvalidOperationException($"RuleConfig is required for RuleType '{ruleType}'.");

        try
        {
            if (ruleType == SchemeRuleType.SlabDiscount)
            {
                var slab = JsonSerializer.Deserialize<SlabDiscountConfig>(ruleConfig, JsonOptions) ?? throw new JsonException();
                if (slab.Tiers.Count == 0)
                    throw new InvalidOperationException("SlabDiscount scheme requires at least one tier.");
            }
            else
            {
                _ = JsonSerializer.Deserialize<FlatPercentConfig>(ruleConfig, JsonOptions) ?? throw new JsonException();
            }
        }
        catch (JsonException)
        {
            throw new InvalidOperationException($"RuleConfig is not valid JSON for RuleType '{ruleType}'.");
        }
    }
}