Owners of the iPhone and the iPod Touch can now access Wikipedia at any time. Independent of connectivity or wireless signal, nearly three million articles are being boasted to fit into Encyclopedia the mobile app. Patrick Collison wrote the application after spending some time with a new iPhone and no connection in Japan. After finishing the iPhone project, he adapted it for use on the OLPC (One Laptop Per Child) notebook.
Collison says in his O'Reilly interview from James Turner that not only does a mobile handset user spend quite a lot of time without connectivity, using a cached version of the application ends up being faster. Skipping browser launch and download time can really add up, and that is the bonus for having everything directly on the hard drive.
Originally only available for jail-broken iPhones, popular demand spurred Collison to finish the legitimate port for the App Store, which was released in October 2008. The app is barely under two gigabytes in size, which is Apple's stated limit.
Because of infrastructure limits and bugs, it took three months for application to actually be a functioning part of the App Store - most apps are a couple of hundred kilobytes. So the initial application actually downloads itself after it is installed, allowing the user to pause and resume downloading entries until the entirety is finally local to the device.
While most iPhone Apps are made by developers that are also creating the content, in this case the content is already available. Instead, Collison's main job was to pare down the Wikipedia source data, which came out to an uncompressed twelve or thirteen gigabytes.
The custom format of Encyclopedia has limited metadata, paring the "verbose XML format" that Collison describes into an efficient binary. This process alone strips out twenty to thirty percent. The rest of the shrinking is performed by cutting out images and video, removing references, info boxes and templates.