NAME
bt_postprocess - post-processing of BibTeX strings, values, and entries
SYNOPSIS
void bt_postprocess_string (char * s,
ushort options)
char * bt_postprocess_value (AST * value,
ushort options,
boolean replace);
char * bt_postprocess_field (AST * field,
ushort options,
boolean replace);
void bt_postprocess_entry (AST * entry,
ushort options);
DESCRIPTION
When btparse parses a BibTeX entry, it initially stores the results in
an abstract syntax tree (AST), in a form exactly mirroring the parsed
data. For example, the entry
@Article{Jones:1997a,
AuThOr = "Bob Jones" # and # "Jim Smith ",
TITLE = "Feeding Habits of
the Common Cockroach",
JoUrNaL = j_ent,
YEAR = 1997
}
would parse to an AST that could be represented as follows:
(entry,"Article")
(key,"Jones:1997a")
(field,"AuThOr")
(string,"Bob Jones")
(macro,"and")
(string,"Jim Smith ")
(field,"TITLE")
(string,"Feeding Habits of the Common Cockroach")
(field,"JoUrNaL")
(macro,"j_ent")
(field,"YEAR")
(number,"1997")
The advantage of this form is that all the important information in the
entry is readily available by traversing the tree using the functions
described in bt_traversal. This obvious problem is that the data is a
little too raw to be immediately useful: entry types and field names
are inconsistently capitalized, strings are full of unwanted
whitespace, field values not reduced to single strings, and so forth.
All of these problems are addressed by btparse’s post-processing
functions, described here. Normally, you won’t have to call these
functions---the library does the Right Thing for you after parsing each
entry, and you can customize what exactly the Right Thing is for your
application. (For instance, you can tell it to expand macros, but not
to concatenate substrings together.) However, it’s conceivable that
you might wish to move the post-processing into your own code and out
of the library’s control. More likely, you could have strings that
come from something other than BibTeX files that you would like to have
treated as BibTeX strings; for that situation, the post-processing
functions are essential. Finally, you might just be curious about what
exactly happens to your data after it’s parsed. If so, you’ve come to
the right place for excruciatingly detailed explanations.
FUNCTIONS
btparse offers four points of entry to its post-processing code. Of
these, probably only the first and last---for processing individual
strings and whole entries---will be commonly used.
Post-processing entry points
To understand why four entry points are offered, an explanation of the
sample AST shown above will help. First of all, the whole entry is
represented by the "(entry,"Article")" node; this node has the entry
key and all its field/value pairs as children. Entry nodes are
returned by "bt_parse_entry()" and "bt_parse_entry_s()" (see bt_input)
as well as "bt_next_entry()" (which traverses a list of entries
returned from "bt_parse_file()"---see bt_traversal). Whole entries may
be post-processed with "bt_postprocess_entry()".
You may also need to post-process a single field, or just the value
associated with it. (The difference is that processing the field can
change the field name---e.g. to lowercase---in addition to the field
value.) The "(field,"AuThOr")" node above is an example of a field
sub-AST, and "(string,"Bob Jones")" is the first node in the list of
simple values representing that field’s value. (Recall that a field
value is, in general, a list of simple values.) Field nodes are
returned by "bt_next_field()", value nodes by "bt_next_value()". The
former may be passed to "bt_postprocess_field()" for post-processing,
the latter to "bt_postprocess_value()".
Finally, individual strings may wander into your program from many
places other than a btparse AST. For that reason,
"bt_postprocess_string()" is available for post-processing arbitrary
strings.
Post-processing options
All of the post-processing routines have an "options" parameter, which
you can use to fine-tune the post-processing. (This is just like the
per-metatype string-processing options that you can set before parsing
entries; see "bt_set_stringopts()" in bt_input.) Like elsewhere in the
library, "options" is a bitmap constructed by or’ing together various
predefined constants. These constants and their effects are documented
in "String processing option macros" in btparse.
bt_postprocess_string ()
void bt_postprocess_string (char * s,
ushort options)
Post-processes an individual string, "s", which is modified in
place. The only post-processing option that makes sense on
individual strings is whether to collapse whitespace according to
the BibTeX rules; thus, if "options & BTO_COLLAPSE" is false, this
function has no effect. (Although it makes a complete pass over
the string anyways. This is for future expansion.)
The exact rules for collapsing whitespace are simple: non-space
whitespace characters (tabs and newlines mainly) are converted to
space, any strings of more than one space within are collapsed to a
single space, and any leading or trailing spaces are deleted.
(Ensuring that all whitespace is spaces is actually done by
btparse’s lexical scanner, so strings in btparse ASTs will never
have whitespace apart from space. Likewise, any strings passed to
bt_postprocess_string() should not contain non-space whitespace
characters.)
bt_postprocess_value ()
char * bt_postprocess_value (AST * value,
ushort options,
boolean replace);
Post-processes a single field value, which is the head of a list of
simple values as returned by "bt_next_value()". All of the
relevant string-processing options come into play here: conversion
of numbers to strings ("BTO_CONVERT"), macro expansion
("BTO_EXPAND"), collapsing of whitespace ("BTO_COLLAPSE"), and
string pasting ("BTO_PASTE"). Since pasting substrings together
without first expanding macros and converting numbers would be
nonsensical, attempting to do so is a fatal error.
If "replace" is true, then the list headed by "value" will be
replaced by a list representing the processed value. That is, if
string pasting is turned on ("options & BTO_PASTE" is true), then
this list will be collapsed to a single node containing the single
string that results from pasting together all the substrings. If
string pasting is not on, then each node in the list will be left
intact, but will have its text replaced by processed text.
If "replace" is false, then a new string will be built on the fly
and returned by the function. Note that if pasting is not on in
this case, you will only get the last string in the list. (It
doesn’t really make a lot of sense to post-process a value without
pasting unless you’re replacing it with the new value, though.)
Returns the string that resulted from processing the whole value,
which only makes sense if pasting was on or there was only one
value in the list. If a multiple-value list was processed without
pasting, the last string in the list is returned (after
processing).
Consider what might be done to the value of the "author" field in
the above example, which is the concatenation of a string, a macro,
and another string. Assume that the macro "and" expands to " and
", and that the variable "value" points to the sub-AST for this
value. The original sub-AST corresponding to this value is
(string,"Bob Jones")
(macro,"and")
(string,"Jim Smith ")
To fully process this value in-place, you would call
bt_postprocess_value (value, BTO_FULL, TRUE);
This would convert the value to a single-element list,
(string,"Bob Jones and Jim Smith")
and return the fully-processed string "Bob Jones and Jim Smith".
Note that the "and" macro has been expanded, interpolated between
the two literal strings, everything pasted together, and finally
whitespace collapsed. (Collapsing whitespace before concatenating
the strings would be a bad idea.)
(Incidentally, "BTO_FULL" is just a macro for the combination of
all possible string-processing options, currently:
BTO_CONVERT │ BTO_EXPAND │ BTO_PASTE │ BTO_COLLAPSE
There are two other similar shortcut macros: "BTO_MACRO" to express
the special string-processing done on macro values, which is the
same as "BTO_FULL" except for the absence of "BTO_COLLAPSE"; and
"BTO_MINIMAL", which means no string-processing is to be done.)
Let’s say you’d rather preserve the list nature of the value, while
expanding macros and converting any numbers to strings. (This
conversion is trivial: it just changes the type of the node from
"BTAST_NUMBER" to "BTAST_STRING". "Number" values are always
stored as a string of digits, just as they appear in the file.)
This would be done with the call
bt_postprocess_value
(value, BTO_CONVERT│BTO_EXPAND│BTO_COLLAPSE,TRUE);
which would change the list to
(string,"Bob Jones")
(string,"and")
(string,"Jim Smith")
Note that whitespace is collapsed here before any concatenation can
be done; this is probably a bad idea. But you can do it if you
wish. (If you get any ideas about cooking up your own value post-
processing scheme by doing it in little steps like this, take a
look at the source to "bt_postprocess_value()"; it should dissuade
you from such a venture.)
bt_postprocess_field ()
char * bt_postprocess_field (AST * field,
ushort options,
boolean replace);
This is little more than a front-end to "bt_postprocess_value()";
the only difference is that you pass it a "field" AST node (eg. the
"(field,"AuThOr")" in the above example), and that it transforms
the field name in addition to its value. In particular, the field
name is forced to lowercase; this behaviour is (currently) not
optional.
Returns the string returned by "bt_postprocess_value()".
bt_postprocess_entry ()
void bt_postprocess_entry (AST * entry,
ushort options);
Post-processes all values in an entry. If "entry" points to the
AST for a "regular" or "macro definition" entry, then the values
are just what you’d expect: everything on the right-hand side of a
field or macro "assignment." You can also post-process comment and
preamble entries, though. Comment entries are essentially one big
string, so only whitespace collapsing makes sense on them.
Preambles may have multiple strings pasted together, so all the
string-processing options apply to them. (And there’s nothing to
prevent you from using macros in a preamble.)
SEE ALSO
btparse, bt_input, bt_traversal
AUTHOR
Greg Ward <gward@python.net>