General Information on Bitstring Functions
Bitstring functions manipulate encoded bit-based data. Although a bitstring can be used with any ObjectScript command or function, it is generally meaningful only within the context of the bit functions.
The $BIT bitstring functions perform atomic operations. Therefore, no locking is required when performing bitstring operations.
The $BIT bitstring functions perform internal compression of bitstrings. Therefore, the actual data length of a bitstring and its physical space allocation may differ. $BIT bitstring functions use the data length of bitstrings. In most circumstances, the physical space allocation should be invisible to the user. bitstring compression is invisible to users of the $BIT functions.
However, because this compressed binary representation is optimized for each bitstring, one cannot assume that two "identical" bitstrings (which were created differently) have identical internal representations. InterSystems IRIS selects from four separate bitstring internal representations to optimize for both sparse bitstrings and non-sparse bitstrings. Therefore, while matching operations on individual bits yield predictable results, comparisons of entire bitstrings may not.
$BIT bitstring functions support a maximum bitstring length of 262,104 bits (32763 x 8) for InterSystems IRIS. (Unlike with certain InterSystems legacy products, it is not an error in InterSystems IRIS to perform an operation on a bit that is beyond the bitstring length.) However, it is strongly recommended for performance reasons that you divide long bitstrings into chunks of less than 65,280 bits. This is the maximum number of bits that can fit in a single 8KB database block.
Bits in a bitstring are numbered with the first (leftmost) bit as position 1. All bitstring comparisons are performed left-to-right.
In the examples, bitstrings are shown within matching square brackets ([...]), with the bits delimited by commas. For example, a bitstring of four 1 bits is shown as [1,1,1,1], with the least significant bits to the right.
In all the bitstring functions, variables named bitstring are bitstrings that can be specified as values, variables, or expressions.
