From binary to data structure
What you'd really like is a way to describe the structure of something like an ID3 tag in a form that's as compressed as the specification's pseudocode yet that can also be expanded into code that defines the id3-tag class and the functions that translate between bytes on disk and instances of the class. All you need to do is add this skeleton to the define-binary-class template, replacing ellipses with code that fills in the skeleton with the appropriate names and code. The library will give you an easy way to define the mapping between the on-disk structures defined by a binary file format and in-memory Lisp objects.
Since this is just a bit of fantasizing, you don't have to worry about exactly how the macro define-binary-class will know what to do with expressions such as isostring: It looks from binary to data structure this:. For instance, with the current definition of define-binary-classyou can define a single class like this:. It doesn't matter to a computer in what order these two bytes are stored as long as everybody agrees. To find a given node in the tree you generally have to traverse the tree and examine all the nodes on the way to the target node.
The flags slot is eight bits, of which all but the first two are 0, and the size consists of four bytes, each of which has a 0 in the most significant bit. Since binary formats are usually used to represent data in a way that makes it easy to map to in-memory data structures, it should come as no surprise that composite on-disk structures are usually defined in ways similar to the way programming languages define in-memory structures. From binary to data structure easiest way to keep track of information like this is to hang it off the symbol that names the class.
Then you can make define-binary-class generate a read-value method specialized on the type name id3-tagand that method can be implemented in terms of calls to read-value with the appropriate slot types as the first argument. The tree is stored as lines of nodes one level at a time. Of these 32 control characters, only from binary to data structure, the newline, carriage return, and horizontal tab, are typically found in text files. Thanks to dynamic variables and:
Find out how to code a binary tree right up to a depth first traversal. You'll from binary to data structure this library in Chapter 25 to write a parser for ID3 tags, the mechanism used to store metadata such as artist and album names in MP3 files. However the usual method is to use object references, links or pointers and this makes searching the tree an expensive task.