For loss-less data compression, only redundant or insignificant data may be discarded. For example, the numbers 45 and 000000045 have the same value but in the second case there are seven redundant zeros before the significant data. A policy that aligns data on the least-significant bit (LSB) allows redundant data on the most-significant part to be discovered and discarded. This is the policy best suited to whole numbers.
Fractions can also have redundant data. Consider 0.125 versus 0.1250000000. In this example the seven trailing zeros are insignificant and may be discarded. A policy that aligns data on the most-significant bit (MSB) allows redundant data on the least-significant part to be discovered and discarded. This is the policy best suited to fractions.
When a number contains both a whole number and a fraction; the bit on which the two numbers align is a fixed number of bits from the most significant bit and is therefore msb-aligned. The alignment policies divide as follows:
- MSB Alignment:
64 #IEEE-754 real number policy
65 ^Angle policy
66 :Date and Timestamp policy
67 ~Logical set (bit-vector) policy
- LSB Alignment:
68 ?Boolean policy
69 -Twos-complement sign-extended integral value policy
70 +Unsigned integral value policy
71 @Indexed element policy
72 *Array dimension policy