Practical Common Lisp

However, the maximum Unicode code point is #x10ffff, so a UTF-8 encoding of Unicode requires at most four bytes per code point.

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If you need to parse a file format that uses other character codes, or if you need to parse files containing arbitrary Unicode strings using a non-Unicode-Common-Lisp implementation, you can always represent such strings in memory as vectors of integer code points. They won't be Lisp strings, so you won't be able to manipulate or compare them with the string functions, but you'll still be able to do anything with them that you can with arbitrary vectors.

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Unfortunately, the language itself doesn't always provide a good model in this respect: the macro DEFSTRUCT, which I don't discuss since it has largely been superseded by DEFCLASS, generates functions with names that it generates based on the name of the structure it's given. DEFSTRUCT's bad example leads many new macro writers astray.

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Technically there's no possibility of type or object conflicting with slot names—at worst they'd be shadowed within the WITH-SLOTS form. But it doesn't hurt anything to simply GENSYM all local variable names used within a macro template.

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Using ASSOC to extract the :reader and :writer elements of spec allows users of define-binary-type to include the elements in either order; if you required the :reader element to be always be first, you could then have used (rest (first spec)) to extract the reader and (rest (second spec)) to extract the writer. However, as long as you require the :reader and :writer keywords to improve the readability of define-binary-type forms, you might as well use them to extract the correct data.

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The ID3 format doesn't require the parent-of-type function since it's a relatively flat structure. This function comes into its own when you need to parse a format made up of many deeply nested structures whose parsing depends on information stored in higher-level structures. For example, in the Java class file format, the top-level class file structure contains a constant pool that maps numeric values used in other substructures within the class file to constant values that are needed while parsing those substructures. If you were writing a class file parser, you could use parent-of-type in the code that reads and writes those substructures to get at the top-level class file object and from there to the constant pool.

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Ripping is the process by which a song on an audio CD is converted to an MP3 file on your hard drive. These days most ripping software also automatically retrieves information about the songs being ripped from online databases such as Gracenote (nee the Compact Disc Database [CDDB]) or FreeDB, which it then embeds in the MP3 files as ID3 tags.

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Almost all file systems provide the ability to overwrite existing bytes of a file, but few, if any, provide a way to add or remove data at the beginning or middle of a file without having to rewrite the rest of the file. Since ID3 tags are typically stored at the beginning of a file, to rewrite an ID3 tag without disturbing the rest of the file you must replace the old tag with a new tag of exactly the same length. By writing ID3 tags with a certain amount of padding, you have a better chance of being able to do so—if the new tag has more data than the original tag, you use less padding, and if it's shorter, you use more.

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The frame data following the ID3 header could also potentially contain the illegal sequence. That's prevented using a different scheme that's turned on via one of the flags in the tag header. The code in this chapter doesn't account for the possibility that this flag might be set; in practice it's rarely used.

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In ID3v2.4, UCS-2 is replaced by the virtually identical UTF-16, and UTF-16BE and UTF-8 are added as additional encodings.

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The 2.4 version of the ID3 format also supports placing a footer at the end of a tag, which makes it easier to find a tag appended to the end of a file.