alias ~ Number, length, count.
idea ~ Quantity before quality.
context ~ A data element with some length or another numeric property.
motivation ~ Data elements with known size can be arranged and compared independent from their actual content. Size is relevant also because processing and storing data is always limited by size.
implementations ~ - Use a special element as end-marker such as the null byte for null-terminated strings (prohibition). - Explicitly encode size value and content in an embedding. - Use fixed size elements only.
examples
~ - The size of a byte is 8 bit.
- All finite data types have limited size.
- Numeric data types represent a size.
- UTF-8 is a variable width encoding that expresses the number of
bytes of a character with the number of 1 bits in the first
byte.
- The possible number of occurrences of a data element can be
expressed in schema languages, for instance with :n-m
(BNF), minOccurs/maxOccurrs (XSD), and
maxCardinality/minCardinality (OWL).
difficulties
~ - The prohibited end-marker may be allowed to be escaped. For
instance the ending double quote in a string may be encoded as
\". Such encoding requires to parse the full content,
contradicting the motivation of the size pattern.
- The conceptual difference between number (multiple elements) and
size (one element) is not clear in all data.
- Counting requires to detect boundaries between elements (see
sequence and separator patterns).
related patterns ~ - Sizes can be constrained by a schema. - The number of elements in a container is a size. - The size pattern includes the idea of (natural) numbers. Ordinal numbers in contrast are covered by the sequence pattern.
specialized patterns ~ Knowing the exact size of an element allows to skip its internal structure (atomicity).