From a255dc718d7f3dc173dcc84c294a0b6b9219f4e0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Lema=C5=84czyk?= <marek@lemanczyk.eu>
Date: Mon, 13 Oct 2025 18:21:22 +0200
Subject: [PATCH] Restore snapshot missing-cycle guard

---
 PerfectNumbers.Core/DivisorCycleCache.cs | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/PerfectNumbers.Core/DivisorCycleCache.cs b/PerfectNumbers.Core/DivisorCycleCache.cs
index 4ca014d..bcacd5c 100644
--- a/PerfectNumbers.Core/DivisorCycleCache.cs
+++ b/PerfectNumbers.Core/DivisorCycleCache.cs
@@ -80,11 +80,13 @@ public sealed class DivisorCycleCache
             if (divisor < (ulong)snapshot.Length)
             {
                 ulong cached = snapshot[divisor];
-                if (cached != 0UL)
+                if (cached == 0UL)
                 {
-                    cycles[i] = cached;
-                    continue;
+                    throw new InvalidDataException($"Divisor cycle is missing for {divisor}");
                 }
+
+                cycles[i] = cached;
+                continue;
             }
 
             missingBuffer[missingCount++] = i;