Programming Perl, 4th Edition

vec

vec EXPR, OFFSET, BITS

The vec function provides compact storage of lists of unsigned integers. These integers are packed as tightly as possible within an ordinary Perl string. The string in EXPR is treated as a bit string made up of some arbitrary number of elements, depending on the length of the string.

OFFSET specifies the index of the particular element you’re interested in. The syntaxes for reading and writing the element are the same, since vec stores or returns the value of the element depending on whether you use it in an lvalue or an rvalue context.

BITS specifies how wide each element is in bits, which must be a power of two: 1, 2, 4, 8, 16, or 32 (and also 64 on some platforms). (An exception is raised if any other value is used.) Each element can therefore contain an integer in the range 0..(2BITS)–1. For the smaller sizes, as many elements as possible are packed into each byte. When BITS is 1, there are eight elements per byte. When BITS is 2, there are four elements per byte. When BITS is 4, there are two elements (traditionally called nybbles) per byte. And so on. Integers larger than a byte are stored in big-endian order.

A list of unsigned integers can be stored in a single scalar variable by assigning them individually to the vec function. (If EXPR is not a valid lvalue, an exception is raised.) In the following example, the elements are each 4 bits wide:

$bitstring ...