EnchantmentHelper#generateEnchantments.java

    public static List<EnchantmentLevelEntry> generateEnchantments(Random random, ItemStack stack, int level, boolean treasureAllowed) {
      List<EnchantmentLevelEntry> list = Lists.newArrayList();
      Item item = stack.getItem();
      int i = item.getEnchantability();
      if (i <= 0) {
         return list;
      } else {
         level += 1 + random.nextInt(i / 4 + 1) + random.nextInt(i / 4 + 1);
         float f = (random.nextFloat() + random.nextFloat() - 1.0F) * 0.15F;
         level = MathHelper.clamp(Math.round((float)level + (float)level * f), 1, Integer.MAX_VALUE);
         List<EnchantmentLevelEntry> list2 = getPossibleEntries(level, stack, treasureAllowed);
         if (!list2.isEmpty()) {
            list.add(WeightedPicker.getRandom(random, list2));

            while(random.nextInt(50) <= level) {
               removeConflicts(list2, (EnchantmentLevelEntry)Util.getLast(list));
               if (list2.isEmpty()) {
                  break;
               }

               list.add(WeightedPicker.getRandom(random, list2));
               level /= 2;
            }
         }

         return list;
      }
   }