In this post I wanted to briefly write about some of the challenges faced during the creation of the Rebrickable database and how they were resolved. It is by no means a complete list

Part Numbers

One of the challenges in creating this site was the initial lack of matching part numbers between LEGO sets. To understand what I mean, consider the following three parts which look similar but have slight differences:

The only difference is in the thickness and strength of the claw. Over the years LEGO make a lot of these subtle changes to address weaknesses in the mold designs. The parts above have IDs 4085a, 4085b and 4085c to indicate the evolution of this design.

All three of these parts are functionally equivalent – so swapping one with a different one makes no difference (other than the part strength issue that was addressed by LEGO).

A search on Bricklink tells me that all three of these parts are still used in sets, even seeing the oldest part as recent as 2010:

• 4085a = 101 sets
• 4085b = 157 sets
• 4085c = 615 sets

What this does to the database is to prevent a set that contains 4085b from reusing part 4085c from another set.

The solution to this problem was to build a large mapping database that converts any occurances of these parts to a single part number. This mapping database will continue to improve over time so the matching % of your sets may gradually increase each time you check on them!

Colors

There are other things done to massage the data into a more usable format, such as using a more consistent color scheme. For example I don’t care about the difference between light gray and light bluish gray in the Exact Colours matching:
 

When you choose Close Colours matching, it goes one step further and equates light and dark gray colours:

It should be apparent by now that Rebrickable is not about maintaining a pure pristine database of historical LEGO parts, but is a tool to facilitate building of sets. If you want the pure data, I recommend Bricklink.

Minifigs

There are a LOT of people which seem to have an obsession with minifigs. Sadly, they are one of the least reusable parts between sets so I ignore them in all calculations For this reason, even though you will see minifigs in the total part counts and part breakdown summary, you will not see any in the actual part inventories on Rebrickable.