Program: 81213
Project: 81213
Buggy source:
@Override public boolean mergeItemStack(ItemStack par1ItemStack,int par2,int par3,boolean par4){
  boolean flag1=false;
  int k=par2;
  if (par4) {
    k=par3 - 1;
  }
  Slot slot;
  ItemStack itemstack1;
  if (par1ItemStack.isStackable()) {
    while (par1ItemStack.stackSize > 0 && (!par4 && k < par3 || par4 && k >= par2)) {
      slot=(Slot)this.inventorySlots.get(k);
      itemstack1=slot.getStack();
      if (itemstack1 != null && itemstack1.getItem() == par1ItemStack.getItem() && (!par1ItemStack.getHasSubtypes() || par1ItemStack.getItemDamage() == itemstack1.getItemDamage()) && ItemStack.areItemStackTagsEqual(par1ItemStack,itemstack1) && slot.isItemValid(par1ItemStack)) {
        int l=itemstack1.stackSize + par1ItemStack.stackSize;
        if (l <= par1ItemStack.getMaxStackSize()) {
          par1ItemStack.stackSize=0;
          itemstack1.stackSize=l;
          slot.onSlotChanged();
          flag1=true;
        }
 else         if (itemstack1.stackSize < par1ItemStack.getMaxStackSize()) {
          par1ItemStack.stackSize-=par1ItemStack.getMaxStackSize() - itemstack1.stackSize;
          itemstack1.stackSize=par1ItemStack.getMaxStackSize();
          slot.onSlotChanged();
          flag1=true;
        }
      }
      if (par4) {
        --k;
      }
 else {
        ++k;
      }
    }
  }
  if (par1ItemStack.stackSize > 0) {
    if (par4) {
      k=par3 - 1;
    }
 else {
      k=par2;
    }
    while (!par4 && k < par3 || par4 && k >= par2) {
      slot=(Slot)this.inventorySlots.get(k);
      itemstack1=slot.getStack();
      if (itemstack1 == null && slot.isItemValid(par1ItemStack)) {
        if (1 < par1ItemStack.stackSize) {
          ItemStack copy=par1ItemStack.copy();
          copy.stackSize=1;
          slot.putStack(copy);
          par1ItemStack.stackSize-=1;
          flag1=true;
          break;
        }
 else {
          slot.putStack(par1ItemStack.copy());
          slot.onSlotChanged();
          par1ItemStack.stackSize=0;
          flag1=true;
          break;
        }
      }
      if (par4) {
        --k;
      }
 else {
        ++k;
      }
    }
  }
  return flag1;
}

Fixed version (ground truth):
@Override public boolean mergeItemStack(ItemStack par1ItemStack,int par2,int par3,boolean par4){
  boolean flag1=false;
  int k=par2;
  if (par4) {
    k=par3 - 1;
  }
  Slot slot;
  ItemStack itemstack1;
  if (par1ItemStack.isStackable()) {
    while (par1ItemStack.stackSize > 0 && (!par4 && k < par3 || par4 && k >= par2)) {
      slot=(Slot)inventorySlots.get(k);
      itemstack1=slot.getStack();
      if (itemstack1 != null && itemstack1.getItem() == par1ItemStack.getItem() && (!par1ItemStack.getHasSubtypes() || par1ItemStack.getItemDamage() == itemstack1.getItemDamage()) && ItemStack.areItemStackTagsEqual(par1ItemStack,itemstack1) && slot.isItemValid(par1ItemStack)) {
        int l=itemstack1.stackSize + par1ItemStack.stackSize;
        if (l <= par1ItemStack.getMaxStackSize()) {
          par1ItemStack.stackSize=0;
          itemstack1.stackSize=l;
          slot.onSlotChanged();
          flag1=true;
        }
 else         if (itemstack1.stackSize < par1ItemStack.getMaxStackSize()) {
          par1ItemStack.stackSize-=par1ItemStack.getMaxStackSize() - itemstack1.stackSize;
          itemstack1.stackSize=par1ItemStack.getMaxStackSize();
          slot.onSlotChanged();
          flag1=true;
        }
      }
      if (par4) {
        --k;
      }
 else {
        ++k;
      }
    }
  }
  if (par1ItemStack.stackSize > 0) {
    if (par4) {
      k=par3 - 1;
    }
 else {
      k=par2;
    }
    while (!par4 && k < par3 || par4 && k >= par2) {
      slot=(Slot)inventorySlots.get(k);
      itemstack1=slot.getStack();
      if (itemstack1 == null && slot.isItemValid(par1ItemStack)) {
        if (1 < par1ItemStack.stackSize) {
          ItemStack copy=par1ItemStack.copy();
          copy.stackSize=1;
          slot.putStack(copy);
          par1ItemStack.stackSize-=1;
          flag1=true;
          break;
        }
 else {
          slot.putStack(par1ItemStack.copy());
          slot.onSlotChanged();
          par1ItemStack.stackSize=0;
          flag1=true;
          break;
        }
      }
      if (par4) {
        --k;
      }
 else {
        ++k;
      }
    }
  }
  return flag1;
}