Add pokeblock slot filtering

since there's 300 calls to rand to shuffle, I wouldn't be surprised if
there are vblanks messing up the frames

judging from my data 6 years ago (wow, still wrong, just look at the
frame offsets)

http://www.smogon.com/forums/threads/past-gen-rng-research.61090/page-35#post-4021415

looks like it happens quite erratically. may have to just override this
to return true for all gen3 safari mons as it's entirely unpredictable.

PKHeX.Core/Legality/RNG/Frame/FrameFinder.cs

@@ -185,6 +185,18 @@ private static IEnumerable<Frame> FilterNatureSync(IEnumerable<SeedInfo> seeds,
             foreach (var seed in seeds)
             {
                 var s = seed.Seed;
+
+                if (info.Safari3)
+                {
+                    // successful pokeblock activation
+                    bool result = IsValidPokeBlockNature(s, info.Nature, out uint blockSeed);
+                    if (result)
+                        yield return info.GetFrame(blockSeed, seed.Charm3 ? LeadRequired.CuteCharm : LeadRequired.None);
+
+                    // if no pokeblocks present (or failed call), fall out of the safari specific code and generate via the other scenarios
+                    s = info.RNG.Prev(s); // wasted RNG call
+                }
+
                 var rand = s >> 16;
                 bool sync = info.AllowLeads && !seed.Charm3 && (info.DPPt ? rand >> 15 : rand & 1) == 0;
                 bool reg = (info.DPPt ? rand / 0xA3E : rand % 25) == info.Nature;
@@ -209,6 +221,60 @@ private static IEnumerable<Frame> FilterNatureSync(IEnumerable<SeedInfo> seeds,
                 }
             }
         }
+        private static bool IsValidPokeBlockNature(uint seed, uint nature, out uint natureOrigin)
+        {
+            if (nature % 6 == 0) // neutral
+            {
+                natureOrigin = 0;
+                return false;
+            }
+
+            // unroll the RNG to a stack of seeds
+            var stack = new Stack<uint>();
+            for (uint i = 0; i < 25; i++)
+            for (uint j = 1 + i; j < 25; j++)
+                stack.Push(seed = RNG.LCRNG.Prev(seed));
+
+            natureOrigin = RNG.LCRNG.Prev(stack.Peek());
+            if (natureOrigin >> 16 % 100 >= 80) // failed proc
+                return false;
+
+            // init natures
+            uint[] natures = new uint[25];
+            for (uint i = 0; i < 25; i++)
+                natures[i] = i;
+
+            // shuffle nature list
+            for (uint i = 0; i < 25; i++)
+            for (uint j = 1 + i; j < 25; j++)
+            {
+                var s = stack.Pop();
+                if ((s >> 16 & 1) == 0)
+                    continue; // only swap if 1
+
+                var temp = natures[i];
+                natures[i] = natures[j];
+                natures[j] = temp;
+            }
+            
+            var likes = Pokeblock.GetLikedBlockFlavor(nature);
+            // best case scenario is a perfect flavored pokeblock for the nature.
+            // has liked flavor, and all other non-disliked flavors are present.
+            // is it possible to skip this step?
+            for (int i = 0; i < 25; i++)
+            {
+                var n = natures[i];
+                if (n == nature)
+                    break;
+
+                var nl = Pokeblock.GetLikedBlockFlavor(natures[i]);
+                if (nl == likes) // next random nature likes the same block as the desired nature
+                    return false; // current nature is chosen instead, fail!
+            }
+            // unroll once more to hit the level calc (origin with respect for beginning the nature calcs)
+            natureOrigin = RNG.LCRNG.Prev(natureOrigin);
+            return true;
+        }
 
         /// <summary>
         /// Filters the input <see cref="SeedInfo"/> according to a Cute Charm frame generation pattern.

PKHeX.Core/Legality/RNG/Frame/FrameGenerator.cs

@@ -10,6 +10,8 @@ public class FrameGenerator
         public readonly bool AllowLeads;
         public readonly FrameType FrameType = FrameType.None;
         public readonly RNG RNG;
+        public readonly bool Safari3;
+
         public Frame GetFrame(uint seed, LeadRequired lead) => new Frame(seed, FrameType, RNG, lead);
         public Frame GetFrame(uint seed, LeadRequired lead, uint esv) => new Frame(seed, FrameType, RNG, lead) {ESV = esv};
 
@@ -33,6 +35,7 @@ public FrameGenerator(PIDIV pidiv, PKM pk)
                     DPPt = false;
                     FrameType = FrameType.MethodH;
                     RNG = pidiv.RNG;
+                    Safari3 = pk.Ball == 5;
 
                     if (ver != GameVersion.E)
                         return;

PKHeX.Core/Legality/RNG/Frame/Pokeblock.cs

@@ -0,0 +1,16 @@
+namespace PKHeX.Core
+{
+    public static class Pokeblock
+    {
+        public static Flavor GetLikedBlockFlavor(uint nature) => (Flavor)(nature/5);
+        public static Flavor GetDislikedBlockFlavor(uint nature) => (Flavor)(nature%5);
+        public enum Flavor
+        {
+            Spicy,
+            Sour,
+            Sweet,
+            Dry,
+            Bitter,
+        }
+    }
+}