Alph design -- the interfaces

Alph is an implementation of xanalogical hypertext (see e.g. Nelson's writings). The fundamental point in xanalogical hypertext is that each smallest unit of media has a unique identity.

Spans and Media types

The most user-visible objects in Alph are spans. A span is a contiguous block of permanent media, e.g. "the 5 characters 'xyahb' typed on 5 May 2003 by Janne V. Kujala" or "the 23x12 block of pixels in a particular photo snapshot of a custom controller built from lego bricks by Asko Soukka". Spans are operated on much as any immutable media objects, like Java's own String 's.

The media types handled by Alph are

There additionally exist interfaces for

but these have not yet been implemented and will not be discussed further in this document.

The inheritance hierarchy between the core media type interfaces is as follows:

Span Span1D TextSpan PageSpan ImageSpan PageImageSpan Span Span1D TextSpan PageSpan ImageSpan PageImageSpan alph_mediatypes

The interfaces in the lowest row are those that are actually implemented by some classes.

Note: Spans are immutable objects - the append() method returns a new span, if the the object and the parameter were consecutive in the right order.

Manipulating text: Enfilades, Span makers


Alph does not use the tumbler model in previous implementations, but is rather based on scrollblocks - fixed units of fluid media. This allows Alph to exist on top of Storm.

Each span can contain media only inside one scrollblock:

Span ScrollBlock Span ScrollBlock alph_scrollblock

The scrollblock model assumes that given a span it is possible to access all the fluid media units inside the same block. However, this is not always true: there are some span types that do not have a real scroll block - FakeTextSpan and URN5TextSpan.

Access to the original media: block files

Alph does not currently provide access methods for data in images and pagespans. This is because the programs may use their own methods for accessing these - e.g. Fenfire uses either Java AWT or the Libvob OpenGL system implemented in C++. Forcing access using AWT images would make things inefficient for the OpenGL system.

Because of this, Alph provides direct access to the underlying data block by allowing the access through the BlockFile class.

ScrollBlock BlockFile ScrollBlock BlockFile alph_blockfile

The blockfile is created by calling ScrollBlock.getBlockFile(), and can then be used to obtain access to the file. The file may be temporary, which is why it is important to explicitly close() the BlockFile after use.

XXX getBlockInputStream, delegation to Alph?

The central media repository class: Alph

Finally, there is a central class Alph which takes care of finding ScrollBlocks based on URIs, adding files etc.

Alph ScrollBlock Alph ScrollBlock alph_alph