CHAPTER 3
162
Syntax
•
The values associated with the keys may be objects of any type. Stream objects
are required to be specified by indirect object references. It is recommended,
though not required, that dictionary, array, and string objects be specified by
indirect object references, and other PDF objects (nulls, numbers, booleans,
and names) be specified as direct objects.
•
The data structure can represent an arbitrarily large collection of key-value
pairs, which can be looked up efficiently without requiring the entire data
structure to be read from the PDF file. (In contrast, a dictionary is subject to an
implementation limit on the number of entries it can contain.)
A name tree is constructed of
nodes,
each of which is a dictionary object.
depending on the specific entries they contain. The tree always has exactly one
root node,
which contains a single entry: either
Kids
or
Names
but not both. If the
root node has a
Names
entry, it is the only node in the tree. If it has a
Kids
entry,
each of the remaining nodes is either an
intermediate node,
containing a
Limits
entry and a
Kids
entry, or a
leaf node,
containing a
Limits
entry and a
Names
entry.
TABLE 3.33 Entries in a name tree node dictionary
KEY
TYPE
VALUE
Kids
array
(Root and intermediate nodes only; required in intermediate nodes; present in the root
node if and only if
Names
is not present)
An array of indirect references to the immediate
children of this node. The children may be intermediate or leaf nodes.
(Root and leaf nodes only; required in leaf nodes; present in the root node if and only if
Kids
is not present)
An array of the form
[
key
1
value
1
key
2
value
2
…
key
n
value
n
]
Names
array
where each
key
i
is a string and the corresponding
value
i
is the object associated with that
key. The keys are sorted in lexical order, as described below.
Limits
array
(Intermediate and leaf nodes only; required)
An array of two strings, specifying the (lexi-
cally) least and greatest keys included in the
Names
array of a leaf node or in the
Names
arrays of any leaf nodes that are descendants of an intermediate node.
The
Kids
entries in the root and intermediate nodes define the tree’s structure by
identifying the immediate children of each node. The
Names
entries in the leaf
(or root) nodes contain the tree’s keys and their associated values, arranged in
key-value pairs and sorted lexically in ascending order by key. Shorter keys