Bukkit

• Updated for 1.12, depends more heavily on the Bukkit API and should be less likely to break in the future

From 1.6.5 (unreleased):

• Modified how Efficiency works: fuel burn time now decreases proportionate to the speed increase. Effectively, 1 coal will always smelt 8 items. If it's 1 second per smelt, coal will only last 8 seconds instead of the default 80. Silk touch becomes more useful because higher efficiency means more fuel wasted when changing smelted items, and unbreaking is no longer completely left by the wayside due to how amazing(ly broken) efficiency is.