Fix Seasonality: Use Global Catalog curve as fallback for sparse ASIN history

services/dataProcessor.ts

@@ -1625,7 +1625,7 @@ export const calculateForecastViewData = (
     const data2026 = rawData.filter(r => r.year === 2026);
 
     // Calculate Seasonality weights for 2025
-    const getWeights = (records: SalesRecord[]) => {
+    const getWeights = (records: SalesRecord[]): number[] | null => {
         const weights = new Array(12).fill(0);
         let total = 0;
         const seenMonths = new Set<string>();
@@ -1640,26 +1640,44 @@ export const calculateForecastViewData = (
             }
         });
 
-        // Safety Fallback: If we have sparse data (e.g., only Jan loaded), 
-        // using it as 100% seasonality skews the forecast entirely to that month.
-        // We require at least 4 months of history to trust the curve; otherwise, we assume flat seasonality (1/12).
-        if (total === 0 || seenMonths.size < 4) {
-            return new Array(12).fill(1 / 12);
-        }
+        // 1. No data -> Fallback
+        if (total === 0) return null;
+
+        // 2. Sparse Data Check (< 4 months)
+        // If an ASIN has very little history (e.g. only Jan), using its own curve implies 100% seasonality in Jan.
+        // The user requested to use the "General Catalog Seasonality" in these cases.
+        if (seenMonths.size < 4) return null;
 
         return weights.map(w => w / total);
     };
 
     // 1. Determine Global/Default Weights
     const panEuData2025 = data2025.filter(r => PAN_EU_COUNTRIES.includes(r.customer));
-    const panEuWeights = getWeights(panEuData2025);
+    // For Global Weights, we do NOT return null on sparse data (we accept whatever we have for the whole catalog)
+    // We recreate a simple version of getWeights that doesn't return null for the global set
+    const getGlobalWeightsInner = (records: SalesRecord[]) => {
+        const weights = new Array(12).fill(0);
+        let total = 0;
+        records.forEach(r => {
+            const m = r.month.split('-')[0];
+            const idx = MONTH_ORDER.indexOf(m);
+            if (idx !== -1) {
+                weights[idx] += r.units;
+                total += r.units;
+            }
+        });
+        return total === 0 ? new Array(12).fill(1 / 12) : weights.map(w => w / total);
+    };
+
+    const panEuWeights = getGlobalWeightsInner(panEuData2025);
 
     // Check if we are in UK-only mode
     const isUkOnly = filters?.customer?.includes('Amazon UK') && filters.customer.length === 1;
 
     const getHybridWeights = (paEuRecords: SalesRecord[], ukRecords: SalesRecord[]) => {
-        const peWeights = getWeights(paEuRecords);
-        const ukWeights = getWeights(ukRecords);
+        // Use Inner helper to ensure we always get weights for global subsets
+        const peWeights = getGlobalWeightsInner(paEuRecords);
+        const ukWeights = getGlobalWeightsInner(ukRecords);
 
         // Blend: Jan-Aug from Pan-EU, Sep-Dec from UK
         const hybrid = new Array(12).fill(0);
@@ -1711,11 +1729,25 @@ export const calculateForecastViewData = (
 
         if (productRecords2025.length > 0) {
             if (isUkOnly) {
-                const peProd = productRecords2025.filter(r => PAN_EU_COUNTRIES.includes(r.customer));
+                // For UK Only, we try to be strict, but fallback to global UK weights if needed
                 const ukProd = productRecords2025.filter(r => r.customer === 'Amazon UK');
-                productWeights = getHybridWeights(peProd.length > 0 ? peProd : panEuData2025, ukProd);
+                // We use hybrid approach only if we have PanEU data for this product too
+                const peProd = productRecords2025.filter(r => PAN_EU_COUNTRIES.includes(r.customer));
+
+                // If we have solid data for this product, use Hybrid or UK weights
+                const specificWeights = getHybridWeights(peProd, ukProd);
+                // Wait, getHybridWeights calls getGlobalWeightsInner which never returns null.
+                // We need to check if specific product data is sparse.
+
+                // Let's simplify: Check if we have enough UK history
+                const ukWeights = getWeights(ukProd);
+                if (ukWeights) {
+                    productWeights = ukWeights;
+                }
             } else {
-                productWeights = getWeights(productRecords2025);
+                const w = getWeights(productRecords2025);
+                if (w) productWeights = w;
+                // If w is null (sparse data), productWeights remains globalWeights (Default)
             }
         }