BoxPacker is 10!

Documenting the project history

Posted by Doug Wright on August 01, 2023 · 17 mins read

Ten years ago, in August 2013, I made the first release of the BoxPacker project on GitHub. This is a reconstructed record of the project's origins and subsequent development.

Origins of the project

I used to work for an educational non-profit that was focused on the area of nurseries and pre-schools. The organisation ran about 100 of these directly, and provided advice and support to approx 13,000 others. As part of my role in looking after the technical side of the organisation's website, I was responsible for our online bookstore where all sorts of books were offered for sale - some advice type, some practical (e.g. blank attendance registers). The system went through all kinds of evolutions whilst I was there, but one common factor was that it was not easily possible to use an off-the-shelf solution due to some unique business requirements, including the requirement to offer discounts to members of the organisation (which were not a fixed % off). This requirement to have a dual-price list caused havoc, resulting eventually in a decision of mine to just write everything from scratch rather than try to maintain a forked copy of any existing open source ecommerce solution.

For many years the organisation had outsourced the non-online portions (operating an order phone line, pick, pack dispatch) to external companies but this had always proved challenging - there were an endless series of customer complaints about being charged the regular price instead of the discounted one, and the split of the phone numbers (and staff) between those who could offer advice and those who could sell things was also frustrating.

In 2011, the decision was made to in-source almost everything - the printing firm that physically made most of the items that were sold kindly agreed to handle dispatching to individual customers (something they'd never done before) and I was tasked with building everything to enable that. That involved some interesting challenges, e.g. I'd never had to care about stock levels before (something was either eligible for purchase or not) and making sure that customer details and Royal Mail pre-printed postage insignia ended up on the right place on address labels.

Another postage-related challenge that I found I needed to tackle was sending customer orders via the most cost-efficient route. For example, many orders could go 2nd Class post but over a certain weight 2nd Class is not available anymore, only 1st Class. Certain over-size items (e.g. posters in poster tubes) couldn't be sent via regular post at all, but needed to be sent via a parcel service.

“Obviously” (haha), we couldn't be the first people doing ecommerce to need to figure out the box size for an order, there must be well-known solutions to that problem that Google would reveal. Some PHP library I could just use. Right?

No. Not even a little.

For several weeks, I researched every academic paper I could trying to find one that included source code. Eventually I found one that included some C. I ported it to PHP. It didn't work. I spent days looking for the reason, couldn't find a typo. Deleted, started over. It still didn't work.

Eventually, I gave up - I'd already spent more time than was justifiable, so I wrote up a quick bodge that simply calculated the order weight, and then special cased the few annoyingly large items. On the whole, it worked pretty well. The only downside was that where we got large orders, the person doing the packing had to manually go back into the system to print off duplicate labels for any boxes >=2. We didn't get many large orders though, so this was a really just a minor annoyance.

There was one use case where the bodge wasn't enough, which was international orders. About once a week we'd get an order to be sent overseas. Whereas for UK-based orders we charged flat-rate postage, international orders were priced up individually - historically these were only taken over the phone, someone would physically prepare the order up to the point of sealing the box, weigh + measure the box(s), calculate the shipping costs, then go back to the customer and see if they happy with the price and still wanted to proceed.

Our international courier had an API to calculate rates, so in theory we could start doing online orders too but the required fields included box dimensions. I found a different company online that had a free API that would do the virtual packing necessary and hooked that up so that we could get the correct information for the courier to price things up. It felt like a decision that was going to backfire one day, but as a 1-person development team on a short deadline I went with it.

 * Parcel packing (aka 3D bin packing problem)
 * i.e. how many/how big will the parcels be
 * XXX uses free API to do it. Should re-implement ourselves for speed/control

A couple of years passed. Then in July 2013, the free API that we'd been using for those international orders suddenly went offline. The world's worst quick-fix was promptly implemented as a virtual packing replacement (just stack items on top of each other) which restored somewhat-normal service although at inflated shipping prices.

However, a proper solution was needed, and needed urgently.

Introducing BoxPacker

The inability of myself in 2011 during working time to get any kind of in-house solution working by porting other people's work to PHP had annoyed me to the extent that during 2012 I did start toying with some code to try and do something myself. I really hadn't gotten very far though, really all I'd done before getting bored with it was creating some interfaces. These Box and Item interfaces were the very first lines of code I ever wrote for the project, and they remain the absolute core of the project even today - I have no recollection of exactly why I chose to make them interfaces back then rather than concrete classes, but looking back it remains a decision I think I got exactly right - allowing consuming applications to pass in their own objects rather than convert to/from BoxPacker specific data structures is one my favourite features.

So, 2013 hit and I had a sudden professional need for a virtual packing solution. So I wrote up a more robust version of my just-stack-items-on-top-of-each-other approach, and released v0.1, followed the next day by v0.2 which could split an order into multiple boxes and v0.3 shortly thereafter which could pack items side by side and not just stacked vertically. By this point, I was confident that the calculations were all physically correct (if dumb) and integrated my solution back into my actual work project.

Having a code-based solution now, I also integrated into the UK dispatch process too, so that we could automatically print the correct number of address labels for large orders, and also create customised dispatch notes for each box.

For completely unrelated reasons, that week with the emergency integration of my personal project happened to be my last week at the non-profit. In fact, I haven't worked in the ecommerce space since! All subsequent releases of BoxPacker have been driven purely by self-motivation and the feature requests from users, and not an actual business need originating from any of my then-employers. Despite that, BoxPacker has moved from strength to strength.


Weight-(re)distribution (preferring 2 medium-weight boxes rather than 1 large, heavy box and 1 small, light box) followed 3 months after all that in the first officially stable release v1.0 and is one of my favourite features. As far as I know, BoxPacker is the only open-source library (of any language) that incorporates the real-world aspect that people don't like lifting heavy things into its packing solutions. Of course this can be turned off for maximum packing efficiency if desired.

A series of v1.x minor releases followed over 2014 and 2015 as other people started to use the library and reported issues or requested features. I also made some improvements to packing efficiency over this time period

2016 saw an API-breaking change to the Item interface in v2.0 to indicate whether an item should be shipped-flat or could be rotated arbitrarily. Although the library had always worked in full 3D space, as it had originally been conceived to ship books it assumed everything would always be kept flat and didn't ever consider turning items onto their sides. The initial v2.x releases didn't actually do anything with this information, this changed with v2.2 in 2017.

That year also saw the introduction of the first callback system to allow for produced packing solutions to take into account custom constraints such as capping the number of dangerous items (e.g. batteries) in a single box.


Finally 2017 also had the release of v3.0. The main user-facing change was the detailed tracking of the placement of each individual item within a box - previous the library had simply kept a running total internally of how far along each axis the "cursor" was. This allowed for more sophisticated customs constraints to be handled, and also helped with debugging. The BC break also enabled me to change the declared API of the ItemList and BoxList classes which had previously used (and directly extended from) PHP's built-in SplHeap.

Back in 2012 when I had first created those classes I thought I had been doing the right thing by using the SplHeap data structures rather than a simple array - as data structures explicitly designed for working with ordered lists, I simply assumed they'd be faster and quicker.

I was wrong. Very wrong. 🤦‍♂️

It actually turns out that the operations BoxPacker needs to do (and frequently) mean that using a SplHeap has terrible, awful performance. For a start, you can't simply iterate over one because there's no concept of an internal pointer - iteration actually involves removing the top element on each invocation of ->next(). That means to do a non-destructive iteration you have to operate on a cloned copy of the SplHeap. Secondly, because SplHeap is a classic textbook heap, each time that top item was removed so the library could get a look at the next item the heap had to be rebalanced which meant calling the internal sort functions over and over again. Simple iteration actually meant sorting the heap over and over and over again. For small lists this impact was fairly small, but when investigating performance on larger packing problems I found that BoxPacker was sometimes spending more time in sorting a list than on actually doing any of the more complex decisioning work!

So from v3.0, BoxPacker uses a simple wrapped array that can be sorted just once and then iterated over easily and quickly.


2018 saw the introduction of the optional Infallible packing mode where if BoxPacker couldn't pack an item for any reason (e.g. no boxes large enough) it would simply keep track of that rather than throw and exception and crash as some users of the library had items that they would make custom arrangements for if ordered.

2019's main feature was improving the custom constraint system so that the constraints could take into account the proposed placement of an item and not just its inherent properties (e.g. the constraint could now be "don't stack batteries" rather than just "no more than 2 per box").

2020 saw the introduction of another feature, the LimitedSupplyBox. Until then, BoxPacker had always assumed that there were an effectively unlimited number of each box size available for use, with the assumption that if any particular type was running low then more would simply be ordered in. LimitedSupplyBox allowed BoxPacker to work better in situations were stock was more tightly constrained, ensuring that it wouldn't deliver a packing solution that couldn't actually be acted upon.

2021's main implementation-level feature was improved support for using BoxPacker in side-loading scenarios (e.g. trucks), which adds some additional restrictions to the proposed placements that aren't applicable when loading from the top (i.e. a box). However, the main feature of 2021 for me was the introduction of an often-requested visualiser, to actually "see" the packing. Until then, I'd not actually even developed a rough-and-ready one for myself, any/all visualisation when debugging something I'd just done in my head, limited to just the placement of the 1 or 2 individual items I was trying to troubleshoot. With the new visualiser, it became a lot easier to see patterns and improve the heuristics.

BoxPacker Visualiser

2022 saw hooks added to BoxPacker to allow library users to control every aspect of how things are sorted. The default rules are still perfect for most ecommerce usecases, but non-ecommerce usecases kept climbing into the feature requests 😅, and the new hooks allow for every user to tailor things to their precise needs.

2023's BoxPacker highlight will be the release of v4.0, cleaning up various deprecated features, using modern PHP in the core and replacing the simple keepFlat rotation control toggle with a more sophisticated set of rotation enums Never, KeepFlat and BestFit.

What's next

There is no perfect algorithm to do box packing other than brute force try-every-permutation but there are a few examples in the GitHub issues of the project that I feel the library should be able to handle smarter. The nature of heuristics though means that it's really easy to get any specific example packing better by tuning a decision, the problem is that making that one reported case better tends to regress many more examples meaning that fixes for specific testcases are becoming increasingly more challenging as the library gets better.

There are also some feature requests that involve challenging fundamental assumptions that both items and boxes can be modelled as simple cuboids - bags/envelopes have varying geometry depending on how close to the seam you are, and that's a whole new challenge!