From cfff95bff2eaefacd1461074f483cc2de4af8a09 Mon Sep 17 00:00:00 2001
From: Christian Vidal Wolf <christianvidalwolf@gmail.com>
Date: Wed, 28 Jan 2026 13:24:29 +0100
Subject: [PATCH] Fix Global Weights: Fallback to flat distribution if
 reference data is sparse (<4 months)

---
 services/dataProcessor.ts | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/services/dataProcessor.ts b/services/dataProcessor.ts
index 89c33a8..591b8da 100644
--- a/services/dataProcessor.ts
+++ b/services/dataProcessor.ts
@@ -1658,15 +1658,26 @@ export const calculateForecastViewData = (
     const getGlobalWeightsInner = (records: SalesRecord[]) => {
         const weights = new Array(12).fill(0);
         let total = 0;
+        const seenMonths = new Set<string>();
+
         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;
+                if (r.units > 0) seenMonths.add(m);
             }
         });
-        return total === 0 ? new Array(12).fill(1 / 12) : weights.map(w => w / total);
+
+        // SAFETY NET: Even for Global Weights, if the reference file (2025 Sales) 
+        // has fewer than 4 months of data (e.g. user only uploaded Jan 2025),
+        // we should NOT assume 100% seasonality in those months. Fallback to flat.
+        if (total === 0 || seenMonths.size < 4) {
+            return new Array(12).fill(1 / 12);
+        }
+
+        return weights.map(w => w / total);
     };
 
     const panEuWeights = getGlobalWeightsInner(panEuData2025);