From 91dd7f9811e6c627855bf7a47532e220da2b9032 Mon Sep 17 00:00:00 2001
From: Adam Macdonald <adam@2khz.xyz>
Date: Wed, 23 Apr 2025 23:06:42 +0100
Subject: [PATCH] Support replacing blocks when using your off-hand

Co-authored-by: termilu <47648082+termilu@users.noreply.github.com>
---
 .../java/xyz/twokilohertz/HotbarReplace.java  | 50 +++++++++----------
 1 file changed, 23 insertions(+), 27 deletions(-)

diff --git a/src/client/java/xyz/twokilohertz/HotbarReplace.java b/src/client/java/xyz/twokilohertz/HotbarReplace.java
index 0bfaac1..f607eaf 100644
--- a/src/client/java/xyz/twokilohertz/HotbarReplace.java
+++ b/src/client/java/xyz/twokilohertz/HotbarReplace.java
@@ -7,6 +7,8 @@ import net.minecraft.entity.player.PlayerInventory;
 import net.minecraft.item.Item;
 import net.minecraft.item.ItemPlacementContext;
 import net.minecraft.screen.slot.SlotActionType;
+import net.minecraft.util.Hand;
+
 import org.lwjgl.glfw.GLFW;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -26,49 +28,43 @@ public class HotbarReplace implements ClientModInitializer {
     }
 
     public static void tryReplaceSlot(ItemPlacementContext context, Item item) {
-        // Return immediately if player is a spectator
+        // Return immediately if player is a spectator or in creative
         PlayerEntity player = context.getPlayer();
-        if (player.isSpectator())
-            return;
-
-        // Creative inventories don't run out of blocks anyway
-        if (player.getAbilities().creativeMode)
+        if (player == null || player.isSpectator() || player.getAbilities().creativeMode)
             return;
 
         // Get reference to player's current inventory
         PlayerInventory inventory = player.getInventory();
-        if (inventory == null)
-            return;
 
         // Return if the inventory is empty
-        if (inventory.isEmpty())
-            return;
-
-        // If current screen handler is null, return
-        if (player.currentScreenHandler == null)
+        if (inventory == null || inventory.isEmpty() || player.currentScreenHandler == null)
             return;
 
         // Attempt to find a stack of matching items in the player's inventory
         for (int i = 0; i < player.currentScreenHandler.slots.size(); i++) {
             if (player.currentScreenHandler.slots.get(i).getStack().isOf(item)) {
                 // Simulate moving the stack from one slot to another
-                if (client != null) {
-                    // TODO: This still feels like a bit of a hack
-                    // I honestly do not know Minecraft internals enough to be sure that there won't
-                    // be de-sync issues.
 
-                    int current_fps = client.getCurrentFps();
-                    int click_delay = Math.round(1.0f / (float) current_fps) * 1000;
+                if (client == null)
+                    return;
 
-                    client.interactionManager.clickSlot(player.currentScreenHandler.syncId, i, GLFW.GLFW_MOUSE_BUTTON_1,
+                // TODO: This still feels like a bit of a hack
+                // I honestly do not know Minecraft internals enough to be sure that there won't
+                // be de-sync issues.
+
+                int current_fps = client.getCurrentFps();
+                int click_delay = Math.round(1.0f / (float) current_fps) * 1000;
+
+                client.interactionManager.clickSlot(player.currentScreenHandler.syncId, i, GLFW.GLFW_MOUSE_BUTTON_1,
+                        SlotActionType.PICKUP, player);
+
+                int slot = (context.getHand() == Hand.OFF_HAND ? 9 : inventory.getSelectedSlot());
+
+                scheduler.schedule(() -> {
+                    client.interactionManager.clickSlot(player.currentScreenHandler.syncId,
+                            slot + PlayerInventory.MAIN_SIZE, GLFW.GLFW_MOUSE_BUTTON_1,
                             SlotActionType.PICKUP, player);
-
-                    scheduler.schedule(() -> {
-                        client.interactionManager.clickSlot(player.currentScreenHandler.syncId,
-                                inventory.getSelectedSlot() + PlayerInventory.MAIN_SIZE, GLFW.GLFW_MOUSE_BUTTON_1,
-                                SlotActionType.PICKUP, player);
-                    }, click_delay, TimeUnit.MILLISECONDS);
-                }
+                }, click_delay, TimeUnit.MILLISECONDS);
 
                 return;
             }