getrandomg

.clang-format-ignore

@@ -0,0 +1,2 @@
+src/
+include/

.gitignore

@@ -1,3 +1,8 @@
+a.out
+*.exe
+*.o
+*.a
+core
 .vscode
 notchian
 bareiron

compile_flags.txt

@@ -0,0 +1 @@
+-Iinclude

include/globals.h

@@ -34,20 +34,12 @@
 // Max render distance, determines how many chunks to send
 #define VIEW_DISTANCE 2
 
-// Time between server ticks in microseconds (default = 1s)
+// Time between server ticks in microseconds (default = 0.05s)
-#define TIME_BETWEEN_TICKS 1000000
+#define TIME_BETWEEN_TICKS 50000
 
 // Calculated from TIME_BETWEEN_TICKS
 #define TICKS_PER_SECOND ((float)1000000 / TIME_BETWEEN_TICKS)
 
-// Initial world generation seed, will be hashed on startup
-// Used in generating terrain and biomes
-#define INITIAL_WORLD_SEED 0xA103DE6C
-
-// Initial general RNG seed, will be hashed on startup
-// Used in random game events like item drops and mob behavior
-#define INITIAL_RNG_SEED 0xE2B9419
-
 // Size of each bilinearly interpolated area ("minichunk")
 // For best performance, CHUNK_SIZE should be a power of 2
 #define CHUNK_SIZE 8
@@ -74,6 +66,10 @@
 // Determines the fixed amount of memory allocated to blocks
 #define MAX_BLOCK_CHANGES 20000
 
+// How many "deferred" (happen at a later time than triggered) block updates to store.
+// Determines the fixed amount of memory allocated to that list
+#define MAX_DEFERRED_BLOCK_UPDATES 64
+
 // If defined, writes and reads world data to/from disk (or flash).
 // This is a synchronous operation, and can cause performance issues if
 // frequent random disk access is slow. Data is still stored in and
@@ -170,7 +166,7 @@
 #define STATE_PLAY 5
 
 extern ssize_t recv_count;
-extern uint8_t recv_buffer[256];
+extern uint8_t recv_buffer[MAX_RECV_BUF_LEN];
 
 extern uint32_t world_seed;
 extern uint32_t rng_seed;
@@ -195,6 +191,24 @@ typedef struct {
   uint8_t block;
 } BlockChange;
 
+#define UPDATE_BASIC              (1 << 0)
+// the sand at this position will be moved down immediately when this is processed
+#define UPDATE_FALL_SAND          (1 << 1)
+#define UPDATE_FLOW_WATER         (1 << 2)
+// the sand below this block will fall soon
+#define UPDATE_CHECK_SAND_FALL    (1 << 3)
+#define UPDATE_CHECK_WATER        (1 << 4)
+
+#define UPDATE_NOW (UPDATE_BASIC | UPDATE_CHECK_SAND_FALL | UPDATE_CHECK_WATER)
+
+typedef struct {
+  short update_kind;
+  short x;
+  short z;
+  uint8_t y;
+  uint8_t await_ticks;
+} DeferredBlockUpdate;
+
 #pragma pack(push, 1)
 
 typedef struct {
@@ -267,6 +281,10 @@ typedef struct {
 extern BlockChange block_changes[MAX_BLOCK_CHANGES];
 extern int block_changes_count;
 
+extern DeferredBlockUpdate deferred_block_updates[MAX_DEFERRED_BLOCK_UPDATES];
+extern int deferred_block_updates_count;
+extern uint8_t is_processing_deferred_block_updates;
+
 extern PlayerData player_data[MAX_PLAYERS];
 extern int player_data_count;
 

include/procedures.h

@@ -32,9 +32,11 @@ uint8_t makeBlockChange (short x, uint8_t y, short z, uint8_t block);
 
 uint8_t isInstantlyMined (PlayerData *player, uint8_t block);
 uint8_t isColumnBlock (uint8_t block);
+uint8_t isFallingBlock (uint8_t block);
 uint8_t isPassableBlock (uint8_t block);
 uint8_t isPassableSpawnBlock (uint8_t block);
 uint8_t isReplaceableBlock (uint8_t block);
+uint8_t getFluid (uint8_t block);
 uint32_t isCompostItem (uint16_t item);
 uint8_t getItemStackSize (uint16_t item);
 
@@ -43,7 +45,10 @@ void bumpToolDurability (PlayerData *player);
 void handlePlayerAction (PlayerData *player, int action, short x, short y, short z);
 void handlePlayerUseItem (PlayerData *player, short x, short y, short z, uint8_t face);
 
-void checkFluidUpdate (short x, uint8_t y, short z, uint8_t block);
+void processBlockUpdate (short x, uint8_t y, short z, uint8_t block, short update_kind);
+void updateXZNeighbors (short x, uint8_t y, short z, short update_kind);
+void updateXYZNeighbors (short x, uint8_t y, short z, short update_kind);
+void deferBlockUpdate (short x, uint8_t y, short z, uint8_t await_ticks, short update_kind);
 
 void spawnMob (uint8_t type, short x, uint8_t y, short z, uint8_t health);
 void interactEntity (int entity_id, int interactor_id);

src/globals.c

@@ -31,8 +31,8 @@
 ssize_t recv_count;
 uint8_t recv_buffer[MAX_RECV_BUF_LEN] = {0};
 
-uint32_t world_seed = INITIAL_WORLD_SEED;
+uint32_t world_seed;
-uint32_t rng_seed = INITIAL_RNG_SEED;
+uint32_t rng_seed;
 
 uint16_t world_time = 0;
 uint32_t server_ticks = 0;
@@ -50,6 +50,10 @@ uint16_t client_count;
 BlockChange block_changes[MAX_BLOCK_CHANGES];
 int block_changes_count = 0;
 
+DeferredBlockUpdate deferred_block_updates[MAX_DEFERRED_BLOCK_UPDATES];
+int deferred_block_updates_count = 0;
+uint8_t is_processing_deferred_block_updates = 0;
+
 PlayerData player_data[MAX_PLAYERS];
 int player_data_count = 0;
 

src/main.c

@@ -26,6 +26,8 @@
     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <arpa/inet.h>
+    #include <sys/select.h>
+    #include <sys/random.h>
   #endif
   #include <unistd.h>
   #include <time.h>
@@ -505,15 +507,16 @@ int main () {
       }
   #endif
 
-  // Hash the seeds to ensure they're random enough
+  if (0) { // TODO: manual seed
-  world_seed = splitmix64(world_seed);
+    world_seed = splitmix64(world_seed);
-  printf("World seed (hashed): ");
+  } else {
+    getrandom(&world_seed, 4, GRND_RANDOM);
+  }
+  printf("World seed: ");
   for (int i = 3; i >= 0; i --) printf("%X", (unsigned int)((world_seed >> (8 * i)) & 255));
+  printf("\n");
 
-  rng_seed = splitmix64(rng_seed);
+  getrandom(&rng_seed, 4, GRND_RANDOM);
-  printf("\nRNG seed (hashed): ");
-  for (int i = 3; i >= 0; i --) printf("%X", (unsigned int)((rng_seed >> (8 * i)) & 255));
-  printf("\n\n");
 
   // Initialize block changes entries as unallocated
   for (int i = 0; i < MAX_BLOCK_CHANGES; i ++) {
@@ -586,137 +589,168 @@ int main () {
   // Track time of last server tick (in microseconds)
   int64_t last_tick_time = get_program_time();
 
-  /**
+  // Main server loop: use select() to sleep until I/O or next tick
-   * Cycles through all connected clients, handling one packet at a time
-   * from each player. With every iteration, attempts to accept a new
-   * client connection.
-   */
   while (true) {
-    // Check if it's time to yield to the idle task
+    fd_set read_fds;
-    task_yield();
+    FD_ZERO(&read_fds);
+    FD_SET(server_fd, &read_fds);
 
-    // Attempt to accept a new connection
+    if (deferred_block_updates_count == MAX_DEFERRED_BLOCK_UPDATES) {
-    for (int i = 0; i < MAX_PLAYERS; i ++) {
+      printf("WARNING: Deferred block update queue maxed out\n");
-      if (clients[i] != -1) continue;
-      clients[i] = accept(server_fd, (struct sockaddr *)&client_addr, &addr_len);
-      // If the accept was successful, make the client non-blocking too
-      if (clients[i] != -1) {
-        printf("New client, fd: %d\n", clients[i]);
-      #ifdef _WIN32
-        u_long mode = 1;
-        ioctlsocket(clients[i], FIONBIO, &mode);
-      #else
-        int flags = fcntl(clients[i], F_GETFL, 0);
-        fcntl(clients[i], F_SETFL, flags | O_NONBLOCK);
-      #endif
-        client_count ++;
-      }
-      break;
     }
 
-    // Look for valid connected clients
+    // Attempt to accept a new connection
-    client_index ++;
+    int max_fd = server_fd;
-    if (client_index == MAX_PLAYERS) client_index = 0;
+    for (int i = 0; i < MAX_PLAYERS; i ++) {
-    if (clients[client_index] == -1) continue;
+      if (clients[i] != -1) {
+        FD_SET(clients[i], &read_fds);
+        if (clients[i] > max_fd) max_fd = clients[i];
+      }
+    }
 
-    // Handle periodic events (server ticks)
+    // Compute timeout until next server tick
-    int64_t time_since_last_tick = get_program_time() - last_tick_time;
+    struct timeval timeout;
-    if (time_since_last_tick > TIME_BETWEEN_TICKS) {
+    int64_t now = get_program_time();
-      handleServerTick(time_since_last_tick);
+    int64_t elapsed = now - last_tick_time;
+    int64_t to_next = TIME_BETWEEN_TICKS - elapsed;
+    if (to_next < 0) to_next = 0;
+    timeout.tv_sec = (time_t)(to_next / 1000000);
+    timeout.tv_usec = (suseconds_t)(to_next % 1000000);
+
+    int activity;
+  #ifdef _WIN32
+    activity = select(0, &read_fds, NULL, NULL, &timeout);
+    if (activity == SOCKET_ERROR) {
+      int err = WSAGetLastError();
+      if (err == WSAEINTR) continue;
+      if (err == WSAETIMEDOUT) activity = 0;
+    }
+  #else
+    activity = select(max_fd + 1, &read_fds, NULL, NULL, &timeout);
+    if (activity < 0) {
+      if (errno == EINTR) continue; // interrupted, retry
+    }
+  #endif
+
+    now = get_program_time();
+    elapsed = now - last_tick_time;
+    if (elapsed >= TIME_BETWEEN_TICKS) {
+      handleServerTick(elapsed);
       last_tick_time = get_program_time();
     }
 
-    // Handle this individual client
+    if (activity <= 0) {
-    int client_fd = clients[client_index];
+      // timeout or error already handled; continue loop
-
-    // Check if at least 2 bytes are available for reading
-    #ifdef _WIN32
-    recv_count = recv(client_fd, recv_buffer, 2, MSG_PEEK);
-    if (recv_count == 0) {
-      disconnectClient(&clients[client_index], 1);
       continue;
     }
-    if (recv_count == SOCKET_ERROR) {
+
-      int err = WSAGetLastError();
+    if (FD_ISSET(server_fd, &read_fds)) {
-      if (err == WSAEWOULDBLOCK) {
+      while (true) {
-        continue; // no data yet, keep client alive
+        int slot = -1;
-      } else {
+        for (int i = 0; i < MAX_PLAYERS; i ++) {
-        disconnectClient(&clients[client_index], 1);
+          if (clients[i] == -1) { slot = i; break; }
+        }
+        if (slot == -1) {
+          int tmp = accept(server_fd, (struct sockaddr *)&client_addr, &addr_len);
+          if (tmp == -1) break;
+        #ifdef _WIN32
+          closesocket(tmp);
+        #else
+          close(tmp);
+        #endif
+          break;
+        }
+
+        clients[slot] = accept(server_fd, (struct sockaddr *)&client_addr, &addr_len);
+        if (clients[slot] == -1) break; // no more pending
+        printf("New client, fd: %d\n", clients[slot]);
+      #ifdef _WIN32
+        u_long mode = 1;
+        ioctlsocket(clients[slot], FIONBIO, &mode);
+      #else
+        int flags = fcntl(clients[slot], F_GETFL, 0);
+        fcntl(clients[slot], F_SETFL, flags | O_NONBLOCK);
+      #endif
+        client_count ++;
+      }
+    }
+
+    for (int i = 0; i < MAX_PLAYERS; i ++) {
+      int client_fd = clients[i];
+      if (client_fd == -1) continue;
+      if (!FD_ISSET(client_fd, &read_fds)) continue;
+
+    #ifdef _WIN32
+      recv_count = recv(client_fd, recv_buffer, 2, MSG_PEEK);
+      if (recv_count == 0) {
+        disconnectClient(&clients[i], 1);
+        continue;
+      }
+      if (recv_count == SOCKET_ERROR) {
+        int err = WSAGetLastError();
+        if (err == WSAEWOULDBLOCK) {
+          continue; // no data yet, keep client alive
+        } else {
+          disconnectClient(&clients[i], 1);
+          continue;
+        }
+      }
+    #else
+      recv_count = recv(client_fd, &recv_buffer, 2, MSG_PEEK);
+      if (recv_count < 2) {
+        if (recv_count == 0 || (recv_count < 0 && errno != EAGAIN && errno != EWOULDBLOCK)) {
+          disconnectClient(&clients[i], 1);
+        }
+        continue;
+      }
+    #endif
+
+    #ifdef DEV_ENABLE_BEEF_DUMPS
+      if (recv_buffer[0] == 0xBE && recv_buffer[1] == 0xEF && getClientState(client_fd) == STATE_NONE) {
+        send_all(client_fd, block_changes, sizeof(block_changes));
+        send_all(client_fd, player_data, sizeof(player_data));
+        shutdown(client_fd, SHUT_WR);
+        recv_all(client_fd, recv_buffer, sizeof(recv_buffer), false);
+        disconnectClient(&clients[i], 6);
+        continue;
+      }
+      if (recv_buffer[0] == 0xFE && recv_buffer[1] == 0xED && getClientState(client_fd) == STATE_NONE) {
+        recv_all(client_fd, recv_buffer, 2, false);
+        recv_all(client_fd, block_changes, sizeof(block_changes), false);
+        recv_all(client_fd, player_data, sizeof(player_data), false);
+        for (int j = 0; j < MAX_BLOCK_CHANGES; j ++) {
+          if (block_changes[j].block == 0xFF) continue;
+          if (block_changes[j].block == B_chest) j += 14;
+          if (j >= block_changes_count) block_changes_count = j + 1;
+        }
+        writeBlockChangesToDisk(0, block_changes_count);
+        writePlayerDataToDisk();
+        disconnectClient(&clients[i], 7);
+        continue;
+      }
+    #endif
+
+      int length = readVarInt(client_fd);
+      if (length == VARNUM_ERROR) {
+        disconnectClient(&clients[i], 2);
+        continue;
+      }
+      int packet_id = readVarInt(client_fd);
+      if (packet_id == VARNUM_ERROR) {
+        disconnectClient(&clients[i], 3);
+        continue;
+      }
+      int state = getClientState(client_fd);
+      if (state == STATE_NONE && length == 254 && packet_id == 122) {
+        disconnectClient(&clients[i], 5);
+        continue;
+      }
+      handlePacket(client_fd, length - sizeVarInt(packet_id), packet_id, state);
+      if (recv_count == 0 || (recv_count == -1 && errno != EAGAIN && errno != EWOULDBLOCK)) {
+        disconnectClient(&clients[i], 4);
         continue;
       }
     }
-    #else
-    recv_count = recv(client_fd, &recv_buffer, 2, MSG_PEEK);
-    if (recv_count < 2) {
-      if (recv_count == 0 || (recv_count < 0 && errno != EAGAIN && errno != EWOULDBLOCK)) {
-        disconnectClient(&clients[client_index], 1);
-      }
-      continue;
-    }
-    #endif
-    // Handle 0xBEEF and 0xFEED packets for dumping/uploading world data
-    #ifdef DEV_ENABLE_BEEF_DUMPS
-    // Received BEEF packet, dump world data and disconnect
-    if (recv_buffer[0] == 0xBE && recv_buffer[1] == 0xEF && getClientState(client_fd) == STATE_NONE) {
-      // Send block changes and player data back to back
-      // The client is expected to know (or calculate) the size of these buffers
-      send_all(client_fd, block_changes, sizeof(block_changes));
-      send_all(client_fd, player_data, sizeof(player_data));
-      // Flush the socket and receive everything left on the wire
-      shutdown(client_fd, SHUT_WR);
-      recv_all(client_fd, recv_buffer, sizeof(recv_buffer), false);
-      // Kick the client
-      disconnectClient(&clients[client_index], 6);
-      continue;
-    }
-    // Received FEED packet, load world data from socket and disconnect
-    if (recv_buffer[0] == 0xFE && recv_buffer[1] == 0xED && getClientState(client_fd) == STATE_NONE) {
-      // Consume 0xFEED bytes (previous read was just a peek)
-      recv_all(client_fd, recv_buffer, 2, false);
-      // Write full buffers straight into memory
-      recv_all(client_fd, block_changes, sizeof(block_changes), false);
-      recv_all(client_fd, player_data, sizeof(player_data), false);
-      // Recover block_changes_count
-      for (int i = 0; i < MAX_BLOCK_CHANGES; i ++) {
-        if (block_changes[i].block == 0xFF) continue;
-        if (block_changes[i].block == B_chest) i += 14;
-        if (i >= block_changes_count) block_changes_count = i + 1;
-      }
-      // Update data on disk
-      writeBlockChangesToDisk(0, block_changes_count);
-      writePlayerDataToDisk();
-      // Kick the client
-      disconnectClient(&clients[client_index], 7);
-      continue;
-    }
-    #endif
-
-    // Read packet length
-    int length = readVarInt(client_fd);
-    if (length == VARNUM_ERROR) {
-      disconnectClient(&clients[client_index], 2);
-      continue;
-    }
-    // Read packet ID
-    int packet_id = readVarInt(client_fd);
-    if (packet_id == VARNUM_ERROR) {
-      disconnectClient(&clients[client_index], 3);
-      continue;
-    }
-    // Get client connection state
-    int state = getClientState(client_fd);
-    // Disconnect on legacy server list ping
-    if (state == STATE_NONE && length == 254 && packet_id == 122) {
-      disconnectClient(&clients[client_index], 5);
-      continue;
-    }
-    // Handle packet data
-    handlePacket(client_fd, length - sizeVarInt(packet_id), packet_id, state);
-    if (recv_count == 0 || (recv_count == -1 && errno != EAGAIN && errno != EWOULDBLOCK)) {
-      disconnectClient(&clients[client_index], 4);
-      continue;
-    }
-
   }
 
   close(server_fd);

src/procedures.c

@@ -1,3 +1,4 @@
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -761,6 +762,13 @@ uint8_t isInstantlyMined (PlayerData *player, uint8_t block) {
 
 }
 
+// Checks whether the given block can fall down (like sand, anvils, ...)
+uint8_t isFallingBlock (uint8_t block) {
+  return (
+    block == B_sand
+  );
+}
+
 // Checks whether the given block has to have something beneath it
 uint8_t isColumnBlock (uint8_t block) {
   return (
@@ -790,8 +798,7 @@ uint8_t isPassableBlock (uint8_t block) {
 }
 // Checks whether the given block is non-solid and spawnable
 uint8_t isPassableSpawnBlock (uint8_t block) {
-    if ((block >= B_water && block < B_water + 8) ||
+    if (getFluid(block))
-        (block >= B_lava && block < B_lava + 4))
     {
         return 0;
     }
@@ -1037,6 +1044,17 @@ void handleFluidMovement (short x, uint8_t y, short z, uint8_t fluid, uint8_t bl
   // a higher fluid "level" means the fluid has traveled farther
   uint8_t level = block - fluid;
 
+  uint8_t max_level;
+  switch (fluid) {
+    case B_lava:
+      max_level = 3;
+      break;
+
+    case B_water:
+      max_level = 7;
+      break;
+  }
+
   // Query blocks adjacent to this fluid stream
   uint8_t adjacent[4] = {
     getBlockAt(x + 1, y, z),
@@ -1058,10 +1076,7 @@ void handleFluidMovement (short x, uint8_t y, short z, uint8_t fluid, uint8_t bl
     // If not connected, clear this block and recalculate surrounding flow
     if (!connected) {
       makeBlockChange(x, y, z, B_air);
-      checkFluidUpdate(x + 1, y, z, adjacent[0]);
+      updateXYZNeighbors(x, y, z, UPDATE_NOW);
-      checkFluidUpdate(x - 1, y, z, adjacent[1]);
-      checkFluidUpdate(x, y, z + 1, adjacent[2]);
-      checkFluidUpdate(x, y, z - 1, adjacent[3]);
       return;
     }
   }
@@ -1070,42 +1085,38 @@ void handleFluidMovement (short x, uint8_t y, short z, uint8_t fluid, uint8_t bl
   uint8_t block_below = getBlockAt(x, y - 1, z);
   if (isReplaceableBlock(block_below)) {
     makeBlockChange(x, y - 1, z, fluid);
-    return handleFluidMovement(x, y - 1, z, fluid, fluid);
+    updateXYZNeighbors(x, y, z, UPDATE_NOW);
+    return;
   }
 
   // Stop flowing laterally at the maximum level
-  if (level == 3 && fluid == B_lava) return;
+  if (level == max_level) return;
-  if (level == 7) return;
 
   // Handle lateral water flow, increasing level by 1
   if (isReplaceableFluid(adjacent[0], level, fluid)) {
     makeBlockChange(x + 1, y, z, block + 1);
-    handleFluidMovement(x + 1, y, z, fluid, block + 1);
+    updateXYZNeighbors(x, y, z, UPDATE_NOW);
   }
   if (isReplaceableFluid(adjacent[1], level, fluid)) {
     makeBlockChange(x - 1, y, z, block + 1);
-    handleFluidMovement(x - 1, y, z, fluid, block + 1);
+    updateXYZNeighbors(x, y, z, UPDATE_NOW);
   }
   if (isReplaceableFluid(adjacent[2], level, fluid)) {
     makeBlockChange(x, y, z + 1, block + 1);
-    handleFluidMovement(x, y, z + 1, fluid, block + 1);
+    updateXYZNeighbors(x, y, z, UPDATE_NOW);
   }
   if (isReplaceableFluid(adjacent[3], level, fluid)) {
     makeBlockChange(x, y, z - 1, block + 1);
-    handleFluidMovement(x, y, z - 1, fluid, block + 1);
+    updateXYZNeighbors(x, y, z, UPDATE_NOW);
   }
-
 }
 
-void checkFluidUpdate (short x, uint8_t y, short z, uint8_t block) {
+uint8_t getFluid(uint8_t block) {
-
+  if (block >= B_water && block < B_water + 8)
-  uint8_t fluid;
+    return B_water;
-  if (block >= B_water && block < B_water + 8) fluid = B_water;
+  else if (block >= B_lava && block < B_lava + 4)
-  else if (block >= B_lava && block < B_lava + 4) fluid = B_lava;
+    return B_lava;
-  else return;
+  return 0;
-
-  handleFluidMovement(x, y, z, fluid, block);
-
 }
 
 #ifdef ENABLE_PICKUP_ANIMATION
@@ -1143,6 +1154,111 @@ void playPickupAnimation (PlayerData *player, uint16_t item, double x, double y,
 }
 #endif
 
+// can not be zero!!
+#define SAND_FALL_T 15
+#define SAND_FALL_PROPAGATE_T 2
+
+void processBlockUpdate (short x, uint8_t y, short z, uint8_t block, short update_kind) {
+  if (update_kind & UPDATE_BASIC) {
+    // "normal" block update
+  }
+
+  if (update_kind & UPDATE_CHECK_WATER) {
+    uint8_t fluid = getFluid(block);
+    if (fluid) {
+      uint8_t flow_speed;
+      switch (fluid) {
+        case B_lava:
+          flow_speed = 30;
+          break;
+
+        case B_water:
+          flow_speed = 5;
+          break;
+      }
+
+      deferBlockUpdate(x, y, z, flow_speed, UPDATE_FLOW_WATER);
+    }
+  }
+
+  if (update_kind & UPDATE_FLOW_WATER) {
+    uint8_t fluid = getFluid(block);
+    if (fluid) {
+      handleFluidMovement(x, y, z, fluid, block);
+    }
+  }
+
+  if (update_kind & UPDATE_CHECK_SAND_FALL) {
+    // we have a separate UPDATE_CHECK_SAND_FALL,
+    // to make chains of falling sand fall as one,
+    // with less delay between each
+
+    if (isFallingBlock(block) && y) {
+      uint8_t below = getBlockAt(x, y - 1, z);
+      if (isReplaceableBlock(below)) {
+        // move the sand down in 15 ticks
+        deferBlockUpdate(x, y, z, SAND_FALL_T - 1, UPDATE_FALL_SAND);
+
+        if (y != 255 && isFallingBlock(getBlockAt(x, y + 1, z))) {
+          // also tell the block above that the sand will be gone soon
+          deferBlockUpdate(x, y + 1, z, SAND_FALL_PROPAGATE_T, UPDATE_CHECK_SAND_FALL);
+        }
+      }
+    }
+  }
+
+  if (update_kind & UPDATE_FALL_SAND) {
+    // make sure that is a fallable:tm: block, and we are not in the floor
+    if (isFallingBlock(block) && y) {
+      uint8_t below = getBlockAt(x, y - 1, z);
+      // TODO: what to do if below block breaks sand
+      if (isReplaceableBlock(below)) {
+        // TODO: drop item of below block
+        makeBlockChange(x, y, z, B_air);
+        makeBlockChange(x, y - 1, z, B_air);
+        makeBlockChange(x, y - 1, z, block);
+        // update this block after moved
+        processBlockUpdate (x, y - 1, z, block, UPDATE_NOW);
+        // also update the neighbors
+        updateXZNeighbors(x, y, z, UPDATE_NOW);
+        updateXZNeighbors(x, y - 1, z, UPDATE_NOW);
+        if (y != 255) {
+          processBlockUpdate(x, y + 1, z, getBlockAt(x, y + 1, z), UPDATE_NOW);
+        }
+      }
+    }
+  }
+}
+
+void updateXZNeighbors (short x, uint8_t y, short z, short update_kind) {
+  processBlockUpdate(x - 1, y, z, getBlockAt(x - 1, y, z), update_kind);
+  processBlockUpdate(x + 1, y, z, getBlockAt(x + 1, y, z), update_kind);
+  processBlockUpdate(x, y, z - 1, getBlockAt(x, y, z - 1), update_kind);
+  processBlockUpdate(x, y, z + 1, getBlockAt(x, y, z + 1), update_kind);
+}
+
+void updateXYZNeighbors (short x, uint8_t y, short z, short update_kind) {
+  processBlockUpdate(x - 1, y, z, getBlockAt(x - 1, y, z), update_kind);
+  processBlockUpdate(x + 1, y, z, getBlockAt(x + 1, y, z), update_kind);
+  processBlockUpdate(x, y, z - 1, getBlockAt(x, y, z - 1), update_kind);
+  processBlockUpdate(x, y, z + 1, getBlockAt(x, y, z + 1), update_kind);
+  processBlockUpdate(x, y - 1, z, getBlockAt(x, y - 1, z), update_kind);
+  processBlockUpdate(x, y + 1, z, getBlockAt(x, y + 1, z), update_kind);
+}
+
+void deferBlockUpdate (short x, uint8_t y, short z, uint8_t await_ticks, short update_kind) {
+  if (deferred_block_updates_count == MAX_DEFERRED_BLOCK_UPDATES) {
+    return;
+  }
+
+  DeferredBlockUpdate *u = &deferred_block_updates[deferred_block_updates_count ++];
+  u->x = x;
+  u->y = y;
+  u->z = z;
+  u->await_ticks = await_ticks + is_processing_deferred_block_updates;
+  u->update_kind = update_kind;
+}
+
 void handlePlayerAction (PlayerData *player, int action, short x, short y, short z) {
 
   // Re-sync slot when player drops an item
@@ -1166,55 +1282,34 @@ void handlePlayerAction (PlayerData *player, int action, short x, short y, short
   // Ignore further actions not pertaining to mining blocks
   if (action != 0 && action != 2) return;
 
+  uint8_t block = getBlockAt(x, y, z);
+
   // In creative, only the "start mining" action is sent
   // No additional verification is performed, the block is simply removed
   if (action == 0 && GAMEMODE == 1) {
-    makeBlockChange(x, y, z, 0);
+    makeBlockChange(x, y, z, B_air);
-    return;
+  }
+  else {
+    // If this is a "start mining" packet, the block must be instamine
+    if (action == 0 && !isInstantlyMined(player, block)) return;
+
+    // Don't continue if the block change failed
+    if (makeBlockChange(x, y, z, B_air)) return;
+
+    uint16_t held_item = player->inventory_items[player->hotbar];
+    uint16_t item = getMiningResult(held_item, block);
+    bumpToolDurability(player);
+
+    if (item) {
+      #ifdef ENABLE_PICKUP_ANIMATION
+        playPickupAnimation(player, item, x, y, z);
+      #endif
+      givePlayerItem(player, item, 1);
+    }
   }
 
-  uint8_t block = getBlockAt(x, y, z);
+  // Update nearby blocks
-
+  updateXYZNeighbors(x, y, z, UPDATE_NOW);
-  // If this is a "start mining" packet, the block must be instamine
-  if (action == 0 && !isInstantlyMined(player, block)) return;
-
-  // Don't continue if the block change failed
-  if (makeBlockChange(x, y, z, 0)) return;
-
-  uint16_t held_item = player->inventory_items[player->hotbar];
-  uint16_t item = getMiningResult(held_item, block);
-  bumpToolDurability(player);
-
-  if (item) {
-    #ifdef ENABLE_PICKUP_ANIMATION
-    playPickupAnimation(player, item, x, y, z);
-    #endif
-    givePlayerItem(player, item, 1);
-  }
-
-  // Update nearby fluids
-  uint8_t block_above = getBlockAt(x, y + 1, z);
-  #ifdef DO_FLUID_FLOW
-    checkFluidUpdate(x, y + 1, z, block_above);
-    checkFluidUpdate(x - 1, y, z, getBlockAt(x - 1, y, z));
-    checkFluidUpdate(x + 1, y, z, getBlockAt(x + 1, y, z));
-    checkFluidUpdate(x, y, z - 1, getBlockAt(x, y, z - 1));
-    checkFluidUpdate(x, y, z + 1, getBlockAt(x, y, z + 1));
-  #endif
-
-  // Check if any blocks above this should break, and if so,
-  // iterate upward over all blocks in the column and break them
-  uint8_t y_offset = 1;
-  while (isColumnBlock(block_above)) {
-    // Destroy the next block
-    makeBlockChange(x, y + y_offset, z, 0);
-    // Check for item drops *without a tool*
-    uint16_t item = getMiningResult(0, block_above);
-    if (item) givePlayerItem(player, item, 1);
-    // Select the next block in the column
-    y_offset ++;
-    block_above = getBlockAt(x, y + y_offset, z);
-  }
 }
 
 void handlePlayerUseItem (PlayerData *player, short x, short y, short z, uint8_t face) {
@@ -1350,8 +1445,7 @@ void handlePlayerUseItem (PlayerData *player, short x, short y, short z, uint8_t
       (y == player->y || y == player->y + 1) &&
       z == player->z
     ) &&
-    isReplaceableBlock(getBlockAt(x, y, z)) &&
+    isReplaceableBlock(getBlockAt(x, y, z))
-    (!isColumnBlock(block) || getBlockAt(x, y - 1, z) != B_air)
   ) {
     // Apply server-side block change
     if (makeBlockChange(x, y, z, block)) return;
@@ -1359,14 +1453,9 @@ void handlePlayerUseItem (PlayerData *player, short x, short y, short z, uint8_t
     *count -= 1;
     // Clear item id in slot if amount is zero
     if (*count == 0) *item = 0;
-    // Calculate fluid flow
+    // Send updates
-    #ifdef DO_FLUID_FLOW
+    processBlockUpdate(x, y, z, block, UPDATE_NOW);
-      checkFluidUpdate(x, y + 1, z, getBlockAt(x, y + 1, z));
+    updateXYZNeighbors(x, y, z, UPDATE_NOW);
-      checkFluidUpdate(x - 1, y, z, getBlockAt(x - 1, y, z));
-      checkFluidUpdate(x + 1, y, z, getBlockAt(x + 1, y, z));
-      checkFluidUpdate(x, y, z - 1, getBlockAt(x, y, z - 1));
-      checkFluidUpdate(x, y, z + 1, getBlockAt(x, y, z + 1));
-    #endif
   }
 
   // Sync hotbar contents to player
@@ -1685,6 +1774,22 @@ void handleServerTick (int64_t time_since_last_tick) {
    */
   if (rng_seed == 0) rng_seed = world_seed;
 
+  // block updates might add more deferred block updates,
+  // so we temporarily make all new block updates add one more tick to the defer tick counter
+  is_processing_deferred_block_updates = 1;
+  int next_update_idx = 0;
+  for (int i = 0; i < deferred_block_updates_count; i ++) {
+    DeferredBlockUpdate *u = &deferred_block_updates[i];
+    if (u->await_ticks) {
+      u->await_ticks --;
+      deferred_block_updates[next_update_idx ++] = *u;
+    } else {
+      processBlockUpdate(u->x, u->y, u->z, getBlockAt(u->x, u->y, u->z), u->update_kind);
+    }
+  }
+  deferred_block_updates_count = next_update_idx;
+  is_processing_deferred_block_updates = 0;
+
   // Tick mob behavior
   for (int i = 0; i < MAX_MOBS; i ++) {
     if (mob_data[i].type == 0) continue;

src/serialize.c

@@ -16,6 +16,8 @@
 
 int64_t last_disk_sync_time = 0;
 
+// TODO: store deferred block updates
+
 // Restores world data from disk, or writes world file if it doesn't exist
 int initSerializer () {