gistfile1.java

    public void shiftClick(GlowPlayer player, InventoryView view, int clickedSlot, ItemStack clickedItem) {
        // try to fill partial, then empty stacks
        // first main inventory then hotbar
        clickedItem = tryToFillSlots(clickedItem, player.getInventory(), 9, 36, 0, 9);

        view.setItem(clickedSlot, clickedItem);
    }

    public static ItemStack tryToFillSlots(ItemStack stack, GlowInventory target, int...slots) {
        if (slots.length % 2 != 0) {
            throw new IllegalArgumentException("Slots must be pairs.");
        }
        // First empty slot, -1 if no empty slot was found yet
        int firstEmpty = -1;
        for (int s = 0; s < slots.length && stack.getAmount() > 0; s+=2) {
            // Iterate through all pairs of start and end slots
            int start = slots[s];
            int end = slots[s+1];
            int delta = start < end ? 1 : -1;
            for (int i = start; i != end && stack.getAmount() > 0; i += delta) {
                // Check whether shift clicking is allowed in that slot of the inventory
                if (!target.itemShiftClickAllowed(slot, stack)) {
                    continue;
                }

                ItemStack currentStack = target.getItem(i);
                if (currentStack == null) {
                    if (firstEmpty == -1) {
                        firstEmpty = i; // Found first empty slot
                    }
                } else {
                    tryToFillSlot(stack, currentStack, target, i); // Non empty slot, try to fill stack
                }
            }
        }
        if (stack.getAmount() <= 0) {
            stack = null;
        }
        if (stack != null && firstEmpty != -1) {
            target.setItem(firstEmpty, stack);
            stack = null;
        }
        return stack;
    }

    private static ItemStack tryToFillSlot(ItemStack stack, ItemStack currentStack, GlowInventory target, int slot) {
        if (currentStack.isSimilar(stack)) {
            // If current item and new item are similar, calculate the amount of transferable items
            int amount = currentStack.getAmount();
            int maxStackSize = Math.min(currentStack.getMaxStackSize(), target.getMaxStackSize());
            int transfer = Math.min(stack.getAmount(), maxStackSize - amount);
            if (transfer > 0) {
                // And if there are any, transfer them
                currentStack.setAmount(amount + transfer);
                stack.setAmount(stack.getAmount() - transfer);
            }
            target.setItem(slot, currentStack);
        }
    }