Introduce uid.dat (offline PlayerUIDs), fix multiplayer save data persistence (#536)

* fix: fix multiplayer player data mix between different players bug

Fixes a Win64 multiplayer issue where player data (`players/*.dat`) could be mismatched because identity was effectively tied to connection-order `smallId` XUIDs.
Introduces a deterministic username-derived persistent XUID and integrates it into the existing XUID-based save pipeline.

- Added `Windows64_NameXuid` for deterministic `name -> persistent xuid` resolution
- On Win64 login (`PlayerList`), set `ServerPlayer::xuid` from username-based resolver
- Aligned local player `xuid` assignment (`Minecraft`) for create/init/respawn paths to use the same resolver
- Added Win64 local-self guard in `ClientConnection::handleAddPlayer` using name match to avoid duplicate local remote-player creation
- Kept `IQNet::GetPlayerByXuid` compatibility fallback behavior, while extending lookup to also resolve username-based XUIDs
- Moved implementation to `Minecraft.Client/Windows64/Windows64_NameXuid.h`; kept legacy `Win64NameXuid.h` as compatibility include

Rename migration is intentionally out of scope (same-name identity only).

* fix: preserve legacy host xuid (base xuid + 0) for existing world compatibility

- Add legacy embedded host XUID helper (base + 0).
- When Minecraft.Client is hosting, force only the first host player to use legacy host XUID.
- Keep name-based XUID for non-host players.
- Prevent old singleplayer/hosted worlds from losing/mismatching host player data.

* update: migrate Win64 player uid to `uid.dat`-backed XUID and add XUID based duplicate login guards

- Replace Win64 username-derived XUID resolution with persistent `uid.dat`-backed identity (`Windows64_Xuid` / `Win64Xuid`).
- Persist a per-client XUID next to the executable, with first-run generation, read/write, and process-local caching.
- Keep legacy host compatibility by pinning host self to legacy embedded `base + 0` XUID for existing world/playerdata continuity.
- Propagate packet-authoritative XUIDs into QNet player slots via `m_resolvedXuid`, and use it for `GetXuid`/`GetPlayerByXuid` with legacy fallback.
- Update Win64 profile/network paths to use persistent XUID for non-host clients and clear resolved identity on disconnect.
- Add login-time duplicate checks: reject connections when the same XUID is already connected (in addition to existing duplicate-name checks on Win64).
- Add inline compatibility comments around legacy/new identity coexistence paths for easier future maintenance.

* update: ensure uid.dat exists at startup in client mode for multiplayer

2 files changed, 221 insertions, 0 deletions

diff --git a/Minecraft.Client/Windows64/Windows64_Minecraft.cpp b/Minecraft.Client/Windows64/Windows64_Minecraft.cpp
index 1146b86d..fa4b8366 100644
--- a/Minecraft.Client/Windows64/Windows64_Minecraft.cpp
+++ b/Minecraft.Client/Windows64/Windows64_Minecraft.cpp
@@ -42,6 +42,7 @@
 #include "..\..\Minecraft.World\OldChunkStorage.h"
 #include "Common/PostProcesser.h"
 #include "Network\WinsockNetLayer.h"
+#include "Windows64_Xuid.h"
 
 #include "Xbox/resource.h"
 
@@ -1221,6 +1222,12 @@ int APIENTRY _tWinMain(_In_ HINSTANCE hInstance,
 	Win64LaunchOptions launchOptions = ParseLaunchOptions();
 	ApplyScreenMode(launchOptions.screenMode);
 
+	// Ensure uid.dat exists from startup in client mode (before any multiplayer/login path).
+	if (!launchOptions.serverMode)
+	{
+		Win64Xuid::ResolvePersistentXuid();
+	}
+
 	// If no username, let's fall back
 	if (g_Win64Username[0] == 0)
 	{
diff --git a/Minecraft.Client/Windows64/Windows64_Xuid.h b/Minecraft.Client/Windows64/Windows64_Xuid.h
new file mode 100644
index 00000000..93577751
--- /dev/null
+++ b/Minecraft.Client/Windows64/Windows64_Xuid.h
@@ -0,0 +1,214 @@
+#pragma once
+
+#ifdef _WINDOWS64
+
+#include <string>
+#include <cstdio>
+#include <cstdlib>
+#include <cerrno>
+#include <cstring>
+#include <Windows.h>
+
+namespace Win64Xuid
+{
+	inline PlayerUID GetLegacyEmbeddedBaseXuid()
+	{
+		return (PlayerUID)0xe000d45248242f2eULL;
+	}
+
+	inline PlayerUID GetLegacyEmbeddedHostXuid()
+	{
+		// Legacy behavior used "embedded base + smallId"; host was always smallId 0.
+		// We intentionally keep this value for host/self compatibility with pre-migration worlds.
+		return GetLegacyEmbeddedBaseXuid();
+	}
+
+	inline bool IsLegacyEmbeddedRange(PlayerUID xuid)
+	{
+		// Old Win64 XUIDs were not persistent and always lived in this narrow base+smallId range.
+		// Treat them as legacy/non-persistent so uid.dat values never collide with old slot IDs.
+		const PlayerUID base = GetLegacyEmbeddedBaseXuid();
+		return xuid >= base && xuid < (base + MINECRAFT_NET_MAX_PLAYERS);
+	}
+
+	inline bool IsPersistedUidValid(PlayerUID xuid)
+	{
+		return xuid != INVALID_XUID && !IsLegacyEmbeddedRange(xuid);
+	}
+
+
+	// ./uid.dat
+	inline bool BuildUidFilePath(char* outPath, size_t outPathSize)
+	{
+		if (outPath == NULL || outPathSize == 0)
+			return false;
+
+		outPath[0] = 0;
+
+		char exePath[MAX_PATH] = {};
+		DWORD len = GetModuleFileNameA(NULL, exePath, MAX_PATH);
+		if (len == 0 || len >= MAX_PATH)
+			return false;
+
+		char* lastSlash = strrchr(exePath, '\\');
+		if (lastSlash != NULL)
+		{
+			*(lastSlash + 1) = 0;
+		}
+
+		if (strcpy_s(outPath, outPathSize, exePath) != 0)
+			return false;
+		if (strcat_s(outPath, outPathSize, "uid.dat") != 0)
+			return false;
+
+		return true;
+	}
+
+	inline bool ReadUid(PlayerUID* outXuid)
+	{
+		if (outXuid == NULL)
+			return false;
+
+		char path[MAX_PATH] = {};
+		if (!BuildUidFilePath(path, MAX_PATH))
+			return false;
+
+		FILE* f = NULL;
+		if (fopen_s(&f, path, "rb") != 0 || f == NULL)
+			return false;
+
+		char buffer[128] = {};
+		size_t readBytes = fread(buffer, 1, sizeof(buffer) - 1, f);
+		fclose(f);
+
+		if (readBytes == 0)
+			return false;
+
+		// Compatibility: earlier experiments may have written raw 8-byte uid.dat.
+		if (readBytes == sizeof(unsigned __int64))
+		{
+			unsigned __int64 raw = 0;
+			memcpy(&raw, buffer, sizeof(raw));
+			PlayerUID parsed = (PlayerUID)raw;
+			if (IsPersistedUidValid(parsed))
+			{
+				*outXuid = parsed;
+				return true;
+			}
+		}
+
+		buffer[readBytes] = 0;
+		char* begin = buffer;
+		while (*begin == ' ' || *begin == '\t' || *begin == '\r' || *begin == '\n')
+		{
+			++begin;
+		}
+
+		errno = 0;
+		char* end = NULL;
+		unsigned __int64 raw = _strtoui64(begin, &end, 0);
+		if (begin == end || errno != 0)
+			return false;
+
+		while (*end == ' ' || *end == '\t' || *end == '\r' || *end == '\n')
+		{
+			++end;
+		}
+		if (*end != 0)
+			return false;
+
+		PlayerUID parsed = (PlayerUID)raw;
+		if (!IsPersistedUidValid(parsed))
+			return false;
+
+		*outXuid = parsed;
+		return true;
+	}
+
+	inline bool WriteUid(PlayerUID xuid)
+	{
+		char path[MAX_PATH] = {};
+		if (!BuildUidFilePath(path, MAX_PATH))
+			return false;
+
+		FILE* f = NULL;
+		if (fopen_s(&f, path, "wb") != 0 || f == NULL)
+			return false;
+
+		int written = fprintf_s(f, "0x%016llX\n", (unsigned long long)xuid);
+		fclose(f);
+		return written > 0;
+	}
+
+	inline unsigned __int64 Mix64(unsigned __int64 x)
+	{
+		x += 0x9E3779B97F4A7C15ULL;
+		x = (x ^ (x >> 30)) * 0xBF58476D1CE4E5B9ULL;
+		x = (x ^ (x >> 27)) * 0x94D049BB133111EBULL;
+		return x ^ (x >> 31);
+	}
+
+	inline PlayerUID GeneratePersistentUid()
+	{
+		// Avoid rand_s dependency: mix several Win64 runtime values into a 64-bit seed.
+		FILETIME ft = {};
+		GetSystemTimeAsFileTime(&ft);
+		unsigned __int64 t = (((unsigned __int64)ft.dwHighDateTime) << 32) | ft.dwLowDateTime;
+
+		LARGE_INTEGER qpc = {};
+		QueryPerformanceCounter(&qpc);
+
+		unsigned __int64 seed = t;
+		seed ^= (unsigned __int64)qpc.QuadPart;
+		seed ^= ((unsigned __int64)GetCurrentProcessId() << 32);
+		seed ^= (unsigned __int64)GetCurrentThreadId();
+		seed ^= (unsigned __int64)GetTickCount();
+		seed ^= (unsigned __int64)(size_t)&qpc;
+		seed ^= (unsigned __int64)(size_t)GetModuleHandleA(NULL);
+
+		unsigned __int64 raw = Mix64(seed) ^ Mix64(seed + 0xA0761D6478BD642FULL);
+		raw ^= 0x8F4B2D6C1A93E705ULL;
+		raw |= 0x8000000000000000ULL;
+
+		PlayerUID xuid = (PlayerUID)raw;
+		if (!IsPersistedUidValid(xuid))
+		{
+			raw ^= 0x0100000000000001ULL;
+			xuid = (PlayerUID)raw;
+		}
+
+		if (!IsPersistedUidValid(xuid))
+		{
+			// Last-resort deterministic fallback for pathological cases.
+			xuid = (PlayerUID)0xD15EA5E000000001ULL;
+		}
+
+		return xuid;
+	}
+
+	inline PlayerUID ResolvePersistentXuid()
+	{
+		// Process-local cache: uid.dat is immutable during runtime and this path is hot.
+		static bool s_cached = false;
+		static PlayerUID s_xuid = INVALID_XUID;
+
+		if (s_cached)
+			return s_xuid;
+
+		PlayerUID fileXuid = INVALID_XUID;
+		if (ReadUid(&fileXuid))
+		{
+			s_xuid = fileXuid;
+			s_cached = true;
+			return s_xuid;
+		}
+
+		// First launch on this client: generate once and persist to uid.dat.
+		s_xuid = GeneratePersistentUid();
+		WriteUid(s_xuid);
+		s_cached = true;
+		return s_xuid;
+	}
+}
+
+#endif