From 4d702d261d7f4f629ffcc748c0c13624a074a4cd Mon Sep 17 00:00:00 2001
From: Kurt <kwsch@users.noreply.github.com>
Date: Wed, 5 Mar 2025 22:45:41 -0600
Subject: [PATCH] Add ivs->channel seed recovery

Still lack IVs/nature/shiny request for the BACD/M2 variants, maybe in the future those can be added.
---
 .../Templates/Gen3/Gifts/EncounterGift3.cs    | 18 +++++
 PKHeX.Core/Legality/RNG/Algorithms/XDRNG.cs   | 76 ++++++++++++++++++-
 2 files changed, 92 insertions(+), 2 deletions(-)

diff --git a/PKHeX.Core/Legality/Encounters/Templates/Gen3/Gifts/EncounterGift3.cs b/PKHeX.Core/Legality/Encounters/Templates/Gen3/Gifts/EncounterGift3.cs
index 7d0a5ac36..51602f87c 100644
--- a/PKHeX.Core/Legality/Encounters/Templates/Gen3/Gifts/EncounterGift3.cs
+++ b/PKHeX.Core/Legality/Encounters/Templates/Gen3/Gifts/EncounterGift3.cs
@@ -269,6 +269,24 @@ private static uint GetMethod2(ref uint seed)
 
     private static uint SetPINGAChannel(PK3 pk, EncounterCriteria criteria)
     {
+        if (criteria.IsSpecifiedIVsAll())
+        {
+            Span<uint> seeds = stackalloc uint[XDRNG.MaxCountSeedsChannel];
+            var count = XDRNG.GetSeedsChannel(seeds, (uint)criteria.IV_HP, (uint)criteria.IV_ATK, (uint)criteria.IV_DEF, (uint)criteria.IV_SPA, (uint)criteria.IV_SPD, (uint)criteria.IV_SPE);
+            foreach (var seed in seeds[..count])
+            {
+                if (!ChannelJirachi.IsPossible(seed))
+                    continue;
+                PIDGenerator.SetValuesFromSeedChannel(pk, seed);
+                var pid = pk.EncryptionConstant;
+                if (criteria.IsSpecifiedNature() && !criteria.IsSatisfiedNature((Nature)(pid % 25)))
+                    continue; // try again
+                if (criteria.Shiny.IsShiny() != ShinyUtil.GetIsShiny(pk.ID32, pid, 8))
+                    continue; // try again
+                return seed;
+            }
+        }
+
         while (true)
         {
             uint seed = Util.Rand32();
diff --git a/PKHeX.Core/Legality/RNG/Algorithms/XDRNG.cs b/PKHeX.Core/Legality/RNG/Algorithms/XDRNG.cs
index b9e321d82..84c1e1265 100644
--- a/PKHeX.Core/Legality/RNG/Algorithms/XDRNG.cs
+++ b/PKHeX.Core/Legality/RNG/Algorithms/XDRNG.cs
@@ -105,6 +105,17 @@ public static uint Next16(ref uint seed)
         return seed >> 16;
     }
 
+    /// <summary>
+    /// Gets the upper 5 bits of the next RNG seed.
+    /// </summary>
+    /// <param name="seed">Seed to advance one step.</param>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static uint Next5(ref uint seed)
+    {
+        seed = Next(seed);
+        return seed >> 27;
+    }
+
     /// <summary>
     /// Gets the upper 0x7FFF bits of the next RNG seed.
     /// </summary>
@@ -177,8 +188,7 @@ public static uint GetSequentialIV32(uint seed)
         uint result = 0;
         for (int i = 0; i < 6; i++)
         {
-            seed = Next(seed);
-            var value = seed >> 27; // extract top 5 bits
+            var value = Next5(ref seed); // extract top 5 bits
             result |= value << (i * 5);
         }
         return result;
@@ -190,6 +200,7 @@ public static uint GetSequentialIV32(uint seed)
     // Instead of using yield and iterators, we calculate all results in a tight loop and return the count found.
     public const int MaxCountSeedsPID = 2;
     public const int MaxCountSeedsIV = 6;
+    public const int MaxCountSeedsChannel = 12;
 
     // Euclidean division constants
     private const uint Sub = Add - 0xFFFF;
@@ -272,6 +283,67 @@ public static int GetSeedsIVs(Span<uint> result, uint first, uint second)
         return ctr;
     }
 
+    public static int GetSeedsChannel(Span<uint> result, uint hp, uint atk, uint def, uint spa, uint spd, uint spe)
+    {
+        // Recovers seeds from 6 successive calls to rand() that returns the top 5 bits of each.
+        // Mult(j) = Mult^j
+        // Add(j) = Add * (Mult^0 + Mult^1 + ... + Mult^(j-1))
+        // Using j = 3 and XDRNG gives Mult = 0x45c82be5 and Add = 0xd2f65b55
+        const uint mult = Mult3; // Modified mult (3 advances)
+        const uint sub = 0xcaf65b56; // Modified add - 0x7ffffff
+        const ulong b = 0x22e415e_ea37d41a; // (Modified mult + 1) * 0x7ffffff
+
+        const ulong prime = 3;
+        const ulong add = 0x3_0000_0000; // prime * 0x1_0000_0000
+        const uint rmax = 0x__1800_0000; // prime * 0x__0800_0000
+        const ulong skip = 0x661D29; // prime * 2^32 % mult
+
+        uint first = hp << 27;
+        uint t = (((spe << 27) - mult * first) - sub);
+        uint kmax = (uint)((b - t) >> 32);
+        ulong x = (t * prime) % mult;
+
+        int ctr = 0;
+        for (ulong k = 0; k <= kmax;)
+        {
+            var r = (x + skip * k) % mult;
+            var m = r % prime;
+            if (m != 0)
+            {
+                m = m == 1 ? 2u : 1u;
+                r += m * skip;
+                k += (byte)m;
+            }
+
+            var tmp = k << 32 | t;
+            while (r < rmax && k <= kmax)
+            {
+                uint seed = first | (uint)(tmp / mult); // hp
+                if (Next5(ref seed) == atk)
+                {
+                    if (Next5(ref seed) == def)
+                    {
+                        _ = Next5(ref seed); // spe
+                        if (Next5(ref seed) == spa)
+                        {
+                            if (Next5(ref seed) == spd)
+                                result[ctr++] = Prev12(seed); // unroll to origin
+                        }
+                    }
+                }
+
+                r += prime * skip;
+                k += prime;
+                tmp += add;
+            }
+
+            // Rounding up without using floats
+            k += ((mult - r) + skip - 1) / skip;
+        }
+
+        return ctr;
+    }
+
     /// <summary>
     /// Multiplication constants for jumping 2^(index) frames forward.
     /// </summary>