Bit Strings
A bit string represents a logical set of numbered bits with boolean values. Bits in a string are numbered starting with bit number 1. Any numbered bit that has not been explicitly set to boolean value 1 evaluates as 0. Therefore, referencing any numbered bit beyond those explicitly set returns a bit value of 0.
A bit string has a logical length, which is the highest bit position explicitly set to either 0 or 1. This logical length is only accessible using the $BITCOUNT function, and usually should not be used in application logic. To the bit string functions, an undefined global or local variable is equivalent to a bitstring with any specified numbered bit returning a bit value 0, and a $BITCOUNT value of 0.
A bit string is stored as a normal ObjectScript string with an internal format. This internal string representation is not accessible with the bit string functions. Because of this internal format, the string length of a bit string is not meaningful in determining anything about the number of bits in the string.
Because of the bit string internal format, you cannot use the concatenate operator with bit strings. Attempting to do so results in an <INVALID BIT STRING> error.
Two bit strings in the same state (with the same boolean values) may have different internal string representations, and therefore string representations should not be inspected or compared in application logic.
To the bit string functions, a bitstring specified as an undefined variable is equivalent to a bitstring with all bits 0, and a length of 0.
Unlike an ordinary string, a bit string treats the empty string and the character $CHAR(0) to be equivalent to each other and to represent a 0 bit. This is because $BIT treats any non-numeric string as 0. Therefore:
SET $BIT(bstr1,1)=""
SET $BIT(bstr2,1)=$CHAR(0)
SET $BIT(bstr3,1)=0
IF $BIT(bstr1,1)=$BIT(bstr2,1) {WRITE "bitstrings are the same"} ELSE {WRITE "bitstrings different"}
WRITE $BITCOUNT(bstr1),$BITCOUNT(bstr2),$BITCOUNT(bstr3)
A bit set in a global variable during a transaction will be reverted to its previous value following transaction rollback. However, rollback does not return the global variable bit string to its previous string length or previous internal string representation. Local variables are not reverted by a rollback operation.
A logical bitmap structure can be represented by an array of bit strings, where each element of the array represents a "chunk" with a fixed number of bits. Since undefined is equivalent to a chunk with all 0 bits, the array can be sparse, where array elements representing a chunk of all 0 bits need not exist at all. For this reason, and due to the rollback behavior above, application logic should avoid depending on the length of a bit string or the count of 0-valued bits accessible using $BITCOUNT(str) or $BITCOUNT(str,0).
