GVPR(1) User Commands GVPR(1)

NAME

gvpr - graph pattern scanning and processing language

SYNOPSIS

gvpr [-icnqV?] [ -o outfile ] [ -a args ] [ 'prog' | -f progfile ] [
files ]

DESCRIPTION

gvpr (previously known as gpr) is a graph stream editor inspired by
awk. It copies input graphs to its output, possibly transforming
their structure and attributes, creating new graphs, or printing
arbitrary information. The graph model is that provided by
libcgraph(3). In particular, gvpr reads and writes graphs using the
dot language.

Basically, gvpr traverses each input graph, denoted by $G, visiting
each node and edge, matching it with the predicate-action rules
supplied in the input program. The rules are evaluated in order.
For each predicate evaluating to true, the corresponding action is
performed. During the traversal, the current node or edge being
visited is denoted by $.

For each input graph, there is a target subgraph, denoted by $T,
initially empty and used to accumulate chosen entities, and an output
graph, $O, used for final processing and then written to output. By
default, the output graph is the target graph. The output graph can
be set in the program or, in a limited sense, on the command line.

OPTIONS

The following options are supported:

-a args
The string args is split into whitespace-separated tokens,
with the individual tokens available as strings in the gvpr
program as ARGV[0],...,ARGV[ARGC-1]. Whitespace characters
within single or double quoted substrings, or preceded by a
backslash, are ignored as separators. In general, a backslash
character turns off any special meaning of the following
character. Note that the tokens derived from multiple -a
flags are concatenated.

-c Use the source graph as the output graph.

-i Derive the node-induced subgraph extension of the output graph
in the context of its root graph.

-o outfile
Causes the output stream to be written to the specified file;
by default, output is written to stdout.

-f progfile
Use the contents of the specified file as the program to
execute on the input. If progfile contains a slash character,
the name is taken as the pathname of the file. Otherwise, gvpr
will use the directories specified in the environment variable
GVPRPATH to look for the file. If -f is not given, gvpr will
use the first non-option argument as the program.

-q Turns off warning messages.

-n Turns off graph read-ahead. By default, the variable $NG is
set to the next graph to be processed. This requires a read of
the next graph before processing the current graph, which may
block if the next graph is only generated in response to some
action pertaining to the processing of the current graph.

-V Causes the program to print version information and exit.

-? Causes the program to print usage information and exit.

OPERANDS

The following operand is supported:

files Names of files containing 1 or more graphs in the dot
language. If no -f option is given, the first name is
removed from the list and used as the input program. If the
list of files is empty, stdin will be used.

PROGRAMS

A gvpr program consists of a list of predicate-action clauses, having
one of the forms:

BEGIN { action }

BEG_G { action }

N [ predicate ] { action }

E [ predicate ] { action }

END_G { action }

END { action }

A program can contain at most one of each of the BEGIN, END_G and END
clauses. There can be any number of BEG_G, N and E statements, the
first applied to graphs, the second to nodes, the third to edges.
These are separated into blocks, a block consisting of an optional
BEG_G statement and all N and E statements up to the next BEG_G
statement, if any. The top-level semantics of a gvpr program are:

Evaluate the BEGIN clause, if any.
For each input graph G {
For each block {
Set G as the current graph and current object.
Evaluate the BEG_G clause, if any.
For each node and edge in G {
Set the node or edge as the current object.
Evaluate the N or E clauses, as appropriate.
}
}
Set G as the current object.
Evaluate the END_G clause, if any.
}
Evaluate the END clause, if any.

The actions of the BEGIN, BEG_G, END_G and END clauses are performed
when the clauses are evaluated. For N or E clauses, either the
predicate or action may be omitted. If there is no predicate with an
action, the action is performed on every node or edge, as
appropriate. If there is no action and the predicate evaluates to
true, the associated node or edge is added to the target graph.

The blocks are evaluated in the order in which they occur. Within a
block, the N clauses (E clauses, respectively) are evaluated in the
order in which the occur. Note, though, that within a block, N or E
clauses may be interlaced, depending on the traversal order.

Predicates and actions are sequences of statements in the C dialect
supported by the expr(3) library. The only difference between
predicates and actions is that the former must have a type that may
interpreted as either true or false. Here the usual C convention is
followed, in which a non-zero value is considered true. This would
include non-empty strings and non-empty references to nodes, edges,
etc. However, if a string can be converted to an integer, this value
is used.

In addition to the usual C base types (void, int, char, float, long,
unsigned and double), gvpr provides string as a synonym for char*,
and the graph-based types node_t, edge_t, graph_t and obj_t. The
obj_t type can be viewed as a supertype of the other 3 concrete
types; the correct base type is maintained dynamically. Besides
these base types, the only other supported type expressions are
(associative) arrays.

Constants follow C syntax, but strings may be quoted with either
"..." or '...'. gvpr accepts C++ comments as well as cpp-type
comments. For the latter, if a line begins with a '#' character, the
rest of the line is ignored.

A statement can be a declaration of a function, a variable or an
array, or an executable statement. For declarations, there is a
single scope. Array declarations have the form:

type array [ type0 ]

where type0 is optional. If it is supplied, the parser will enforce
that all array subscripts have the specified type. If it is not
supplied, objects of all types can be used as subscripts. As in C,
variables and arrays must be declared. In particular, an undeclared
variable will be interpreted as the name of an attribute of a node,
edge or graph, depending on the context.

Executable statements can be one of the following:

{ [ statement ... ] }
expression // commonly var = expression
if( expression ) statement [ else statement ]
for( expression ; expression ; expression ) statement
for( array [ var ]) statement
forr( array [ var ]) statement
while( expression ) statement
switch( expression ) case statements
break [ expression ]
continue [ expression ]
return [ expression ]
Items in brackets are optional.

In the second form of the for statement and the forr statement, the
variable var is set to each value used as an index in the specified
array and then the associated statement is evaluated. For numeric and
string indices, the indices are returned in increasing (decreasing)
numeric or lexicographic order for for (forr, respectively). This can
be used for sorting.

Function definitions can only appear in the BEGIN clause.

Expressions include the usual C expressions. String comparisons
using == and != treat the right hand operand as a pattern for the
purpose of regular expression matching. Patterns use ksh(1) file
match pattern syntax. (For simple string equality, use the strcmp
function.

gvpr will attempt to use an expression as a string or numeric value
as appropriate. Both C-like casts and function templates will cause
conversions to be performed, if possible.

Expressions of graphical type (i.e., graph_t, node_t, edge_t, obj_t)
may be followed by a field reference in the form of .name. The
resulting value is the value of the attribute named name of the given
object. In addition, in certain contexts an undeclared, unmodified
identifier is taken to be an attribute name. Specifically, such
identifiers denote attributes of the current node or edge,
respectively, in N and E clauses, and the current graph in BEG_G and
END_G clauses.

As usual in the libcgraph(3) model, attributes are string-valued. In
addition, gvpr supports certain pseudo-attributes of graph objects,
not necessarily string-valued. These reflect intrinsic properties of
the graph objects and cannot be set by the user.

head : node_t
the head of an edge.

tail : node_t
the tail of an edge.

name : string
the name of an edge, node or graph. The name of an edge has
the form "<tail-name><edge-op><head-name>[<key>]", where
<edge-op> is "->" or "--" depending on whether the graph is
directed or not. The bracket part [<key>] only appears if the
edge has a non-trivial key.

indegree : int
the indegree of a node.

outdegree : int
the outdegree of a node.

degree : int
the degree of a node.

X : double
the X coordinate of a node. (Assumes the node has a pos
attribute.)

Y : double
the Y coordinate of a node. (Assumes the node has a pos
attribute.)

root : graph_t
the root graph of an object. The root of a root graph is
itself.

parent : graph_t
the parent graph of a subgraph. The parent of a root graph is
NULL

n_edges : int
the number of edges in the graph

n_nodes : int
the number of nodes in the graph

directed : int
true (non-zero) if the graph is directed

strict : int
true (non-zero) if the graph is strict

BUILT-IN FUNCTIONS
The following functions are built into gvpr. Those functions
returning references to graph objects return NULL in case of failure.

Graphs and subgraph

graph(s : string, t : string) : graph_t
creates a graph whose name is s and whose type is specified by
the string t. Ignoring case, the characters U, D, S, N have
the interpretation undirected, directed, strict, and
non-strict, respectively. If t is empty, a directed,
non-strict graph is generated.

subg(g : graph_t, s : string) : graph_t
creates a subgraph in graph g with name s. If the subgraph
already exists, it is returned.

isSubg(g : graph_t, s : string) : graph_t
returns the subgraph in graph g with name s, if it exists, or
NULL otherwise.

fstsubg(g : graph_t) : graph_t
returns the first subgraph in graph g, or NULL if none exists.

nxtsubg(sg : graph_t) : graph_t
returns the next subgraph after sg, or NULL.

isDirect(g : graph_t) : int
returns true if and only if g is directed.

isStrict(g : graph_t) : int
returns true if and only if g is strict.

nNodes(g : graph_t) : int
returns the number of nodes in g.

nEdges(g : graph_t) : int
returns the number of edges in g.

Nodes

node(sg : graph_t, s : string) : node_t
creates a node in graph g of name s. If such a node already
exists, it is returned.

subnode(sg : graph_t, n : node_t) : node_t
inserts the node n into the subgraph g. Returns the node.

fstnode(g : graph_t) : node_t
returns the first node in graph g, or NULL if none exists.

nxtnode(n : node_t) : node_t
returns the next node after n in the root graph, or NULL.

nxtnode_sg(sg : graph_t, n : node_t) : node_t
returns the next node after n in sg, or NULL.

isNode(sg : graph_t, s : string) : node_t
looks for a node in (sub)graph sg of name s. If such a node
exists, it is returned. Otherwise, NULL is returned.

isSubnode(sg : graph_t, n : node_t) : int
returns non-zero if node n is in (sub)graph sg, or zero
otherwise.

indegreeOf(sg : graph_t, n : node_t) : int
returns the indegree of node n in (sub)graph sg.

outdegreeOf(sg : graph_t, n : node_t) : int
returns the outdegree of node n in (sub)graph sg.

degreeOf(sg : graph_t, n : node_t) : int
returns the degree of node n in (sub)graph sg.

Edges

edge(t : node_t, h : node_t, s : string) : edge_t
creates an edge with tail node t, head node h and name s in
the root graph. If the graph is undirected, the distinction
between head and tail nodes is unimportant. If such an edge
already exists, it is returned.

edge_sg(sg : graph_t, t : node_t, h : node_t, s : string) : edge_t
creates an edge with tail node t, head node h and name s in
(sub)graph sg (and all parent graphs). If the graph is
undirected, the distinction between head and tail nodes is
unimportant. If such an edge already exists, it is returned.

subedge(g : graph_t, e : edge_t) : edge_t
inserts the edge e into the subgraph g. Returns the edge.

isEdge(t : node_t, h : node_t, s : string) : edge_t
looks for an edge with tail node t, head node h and name s. If
the graph is undirected, the distinction between head and tail
nodes is unimportant. If such an edge exists, it is returned.
Otherwise, NULL is returned.

isEdge_sg(sg : graph_t, t : node_t, h : node_t, s : string) : edge_t
looks for an edge with tail node t, head node h and name s in
(sub)graph sg. If the graph is undirected, the distinction
between head and tail nodes is unimportant. If such an edge
exists, it is returned. Otherwise, NULL is returned.

isSubedge(g : graph_t, e : edge_t) : int
returns non-zero if edge e is in (sub)graph sg, or zero
otherwise.

fstout(n : node_t) : edge_t
returns the first outedge of node n in the root graph.

fstout_sg(sg : graph_t, n : node_t) : edge_t
returns the first outedge of node n in (sub)graph sg.

nxtout(e : edge_t) : edge_t
returns the next outedge after e in the root graph.

nxtout_sg(sg : graph_t, e : edge_t) : edge_t
returns the next outedge after e in graph sg.

fstin(n : node_t) : edge_t
returns the first inedge of node n in the root graph.

fstin_sg(sg : graph_t, n : node_t) : edge_t
returns the first inedge of node n in graph sg.

nxtin(e : edge_t) : edge_t
returns the next inedge after e in the root graph.

nxtin_sg(sg : graph_t, e : edge_t) : edge_t
returns the next inedge after e in graph sg.

fstedge(n : node_t) : edge_t
returns the first edge of node n in the root graph.

fstedge_sg(sg : graph_t, n : node_t) : edge_t
returns the first edge of node n in graph sg.

nxtedge(e : edge_t, node_t) : edge_t
returns the next edge after e in the root graph.

nxtedge_sg(sg : graph_t, e : edge_t, node_t) : edge_t
returns the next edge after e in the graph sg.

opp(e : edge_t, node_t) : node_t
returns the node on the edge e not equal to n. Returns NULL
if n is not a node of e. This can be useful when using
fstedge and nxtedge to enumerate the neighbors of n.

Graph I/O
write(g : graph_t) : void
prints g in dot format onto the output stream.

writeG(g : graph_t, fname : string) : void
prints g in dot format into the file fname.

fwriteG(g : graph_t, fd : int) : void
prints g in dot format onto the open stream denoted by the
integer fd.

readG(fname : string) : graph_t
returns a graph read from the file fname. The graph should be
in dot format. If no graph can be read, NULL is returned.

freadG(fd : int) : graph_t
returns the next graph read from the open stream fd. Returns
NULL at end of file.

Graph miscellany

delete(g : graph_t, x : obj_t) : void
deletes object x from graph g. If g is NULL, the function
uses the root graph of x. If x is a graph or subgraph, it is
closed unless x is locked.

isIn(g : graph_t, x : obj_t) : int
returns true if x is in subgraph g.

cloneG(g : graph_t, s : string) : graph_t
creates a clone of graph g with name of s. If s is "", the
created graph has the same name as g.

clone(g : graph_t, x : obj_t) : obj_t
creates a clone of object x in graph g. In particular, the
new object has the same name/value attributes and structure as
the original object. If an object with the same key as x
already exists, its attributes are overlaid by those of x and
the object is returned. If an edge is cloned, both endpoints
are implicitly cloned. If a graph is cloned, all nodes, edges
and subgraphs are implicitly cloned. If x is a graph, g may
be NULL, in which case the cloned object will be a new root
graph. In this case, the call is equivalent to cloneG(x,"").

copy(g : graph_t, x : obj_t) : obj_t
creates a copy of object x in graph g, where the new object
has the same name/value attributes as the original object. If
an object with the same key as x already exists, its
attributes are overlaid by those of x and the object is
returned. Note that this is a shallow copy. If x is a graph,
none of its nodes, edges or subgraphs are copied into the new
graph. If x is an edge, the endpoints are created if
necessary, but they are not cloned. If x is a graph, g may be
NULL, in which case the cloned object will be a new root
graph.

copyA(src : obj_t, tgt : obj_t) : int
copies the attributes of object src to object tgt, overwriting
any attribute values tgt may initially have.

induce(g : graph_t) : void
extends g to its node-induced subgraph extension in its root
graph.

hasAttr(src : obj_t, name : string) : int
returns non-zero if object src has an attribute whose name is
name. It returns 0 otherwise.

isAttr(g : graph_t, kind : string, name : string) : int
returns non-zero if an attribute name has been defined in g
for objects of the given kind. For nodes, edges, and graphs,
kind should be "N", "E", and "G", respectively. It returns 0
otherwise.

aget(src : obj_t, name : string) : string
returns the value of attribute name in object src. This is
useful for those cases when name conflicts with one of the
keywords such as "head" or "root". If the attribute has not
been declared in the graph, the function will initialize it
with a default value of "". To avoid this, one should use the
hasAttr or isAttr function to check that the attribute exists.

aset(src : obj_t, name : string, value : string) : int
sets the value of attribute name in object src to value.
Returns 0 on success, non-zero on failure. See aget above.

getDflt(g : graph_t, kind : string, name : string) : string
returns the default value of attribute name in objects in g of
the given kind. For nodes, edges, and graphs, kind should be
"N", "E", and "G", respectively. If the attribute has not
been declared in the graph, the function will initialize it
with a default value of "". To avoid this, one should use the
isAttr function to check that the attribute exists.

setDflt(g : graph_t, kind : string, name : string, value : string) :
int sets the default value of attribute name to value in objects
in g of the given kind. For nodes, edges, and graphs, kind
should be "N", "E", and "G", respectively. Returns 0 on
success, non-zero on failure. See getDflt above.

fstAttr(g : graph_t, kind : string) : string
returns the name of the first attribute of objects in g of the
given kind. For nodes, edges, and graphs, kind should be "N",
"E", and "G", respectively. If there are no attributes, the
string "" is returned.

nxtAttr(g : graph_t, kind : string, name : string) : string
returns the name of the next attribute of objects in g of the
given kind after the attribute name. The argument name must
be the name of an existing attribute; it will typically be the
return value of an previous call to fstAttr or nxtAttr. For
nodes, edges, and graphs, kind should be "N", "E", and "G",
respectively. If there are no attributes left, the string ""
is returned.

compOf(g : graph_t, n : node_t) : graph_t
returns the connected component of the graph g containing node
n, as a subgraph of g. The subgraph only contains the nodes.
One can use induce to add the edges. The function fails and
returns NULL if n is not in g. Connectivity is based on the
underlying undirected graph of g.

kindOf(obj : obj_t) : string
returns an indication of the type of obj. For nodes, edges,
and graphs, it returns "N", "E", and "G", respectively.

lock(g : graph_t, v : int) : int
implements graph locking on root graphs. If the integer v is
positive, the graph is set so that future calls to delete have
no immediate effect. If v is zero, the graph is unlocked. If
there has been a call to delete the graph while it was locked,
the graph is closed. If v is negative, nothing is done. In
all cases, the previous lock value is returned.

Strings

sprintf(fmt : string, ...) : string
returns the string resulting from formatting the values of the
expressions occurring after fmt according to the printf(3)
format fmt

gsub(str : string, pat : string) : string

gsub(str : string, pat : string, repl : string) : string
returns str with all substrings matching pat deleted or
replaced by repl, respectively.

sub(str : string, pat : string) : string

sub(str : string, pat : string, repl : string) : string
returns str with the leftmost substring matching pat deleted
or replaced by repl, respectively. The characters '^' and '$'
may be used at the beginning and end, respectively, of pat to
anchor the pattern to the beginning or end of str.

substr(str : string, idx : int) : string

substr(str : string, idx : int, len : int) : string
returns the substring of str starting at position idx to the
end of the string or of length len, respectively. Indexing
starts at 0. If idx is negative or idx is greater than the
length of str, a fatal error occurs. Similarly, in the second
case, if len is negative or idx + len is greater than the
length of str, a fatal error occurs.

strcmp(s1 : string, s2 : string) : int
provides the standard C function strcmp(3).

length(s : string) : int
returns the length of string s.

index(s : string, t : string) : int

rindex(s : string, t : string) : int
returns the index of the character in string s where the
leftmost (rightmost) copy of string t can be found, or -1 if t
is not a substring of s.

match(s : string, p : string) : int
returns the index of the character in string s where the
leftmost match of pattern p can be found, or -1 if no
substring of s matches p.

toupper(s : string) : string
returns a version of s with the alphabetic characters
converted to upper-case.

tolower(s : string) : string
returns a version of s with the alphabetic characters
converted to lower-case.

canon(s : string) : string
returns a version of s appropriate to be used as an identifier
in a dot file.

html(g : graph_t, s : string) : string
returns a ``magic'' version of s as an HTML string. This will
typically be used to attach an HTML-like label to a graph
object. Note that the returned string lives in g. In
particular, it will be freed when g is closed, and to act as
an HTML string, it has to be used with an object of g. In
addition, note that the angle bracket quotes should not be
part of s. These will be added if g is written in concrete DOT
format.

ishtml(s : string) : int
returns non-zero if and only if s is an HTML string.

xOf(s : string) : string
returns the string "x" if s has the form "x,y", where both x
and y are numeric.

yOf(s : string) : string
returns the string "y" if s has the form "x,y", where both x
and y are numeric.

llOf(s : string) : string
returns the string "llx,lly" if s has the form
"llx,lly,urx,ury", where all of llx, lly, urx, and ury are
numeric.

urOf(s)
urOf(s : string) : string returns the string "urx,ury" if s
has the form "llx,lly,urx,ury", where all of llx, lly, urx,
and ury are numeric.

sscanf(s : string, fmt : string, ...) : int
scans the string s, extracting values according to the
sscanf(3) format fmt. The values are stored in the addresses
following fmt, addresses having the form &v, where v is some
declared variable of the correct type. Returns the number of
items successfully scanned.

split(s : string, arr : array, seps : string) : int

split(s : string, arr : array) : int

tokens(s : string, arr : array, seps : string) : int

tokens(s : string, arr : array) : int
The split function breaks the string s into fields, while the
tokens function breaks the string into tokens. A field
consists of all non-separator characters between two separator
characters or the beginning or end of the string. Thus, a
field may be the empty string. A token is a maximal, non-empty
substring not containing a separator character. The separator
characters are those given in the seps argument. If seps is
not provided, the default value is " \t\n". The functions
return the number of fields or tokens.

The fields and tokens are stored in the argument array. The
array must be string-valued and have int as its index type.
The entries are indexed by consecutive integers, starting at
0. Any values already stored in the array will be either
overwritten, or still be present after the function returns.

I/O
print(...) : void
print( expr, ... ) prints a string representation of each
argument in turn onto stdout, followed by a newline.

printf(fmt : string, ...) : int

printf(fd : int, fmt : string, ...) : int
prints the string resulting from formatting the values of the
expressions following fmt according to the printf(3) format
fmt. Returns 0 on success. By default, it prints on stdout.
If the optional integer fd is given, output is written on the
open stream associated with fd.

scanf(fmt : string, ...) : int

scanf(fd : int, fmt : string, ...) : int
scans in values from an input stream according to the scanf(3)
format fmt. The values are stored in the addresses following
fmt, addresses having the form &v, where v is some declared
variable of the correct type. By default, it reads from
stdin. If the optional integer fd is given, input is read
from the open stream associated with fd. Returns the number
of items successfully scanned.

openF(s : string, t : string) : int
opens the file s as an I/O stream. The string argument t
specifies how the file is opened. The arguments are the same
as for the C function fopen(3). It returns an integer
denoting the stream, or -1 on error.

As usual, streams 0, 1 and 2 are already open as stdin,
stdout, and stderr, respectively. Since gvpr may use stdin to
read the input graphs, the user should avoid using this
stream.

closeF(fd : int) : int
closes the open stream denoted by the integer fd. Streams 0,
1 and 2 cannot be closed. Returns 0 on success.

readL(fd : int) : string
returns the next line read from the input stream fd. It
returns the empty string "" on end of file. Note that the
newline character is left in the returned string.

Math

exp(d : double) : double
returns e to the dth power.

log(d : double) : double
returns the natural log of d.

sqrt(d : double) : double
returns the square root of the double d.

pow(d : double, x : double) : double
returns d raised to the xth power.

cos(d : double) : double
returns the cosine of d.

sin(d : double) : double
returns the sine of d.

atan2(y : double, x : double) : double
returns the arctangent of y/x in the range -pi to pi.

MIN(y : double, x : double) : double
returns the minimum of y and x.

MAX(y : double, x : double) : double
returns the maximum of y and x.

Associative Arrays

# arr : int
returns the number of elements in the array arr.

idx in arr : int
returns 1 if a value has been set for index idx in the array
arr. It returns 0 otherwise.

unset(v : array, idx) : int
removes the item indexed by idx. It returns 1 if the item
existed, 0 otherwise.

unset(v : array) : void
re-initializes the array.

Miscellaneous

exit(v : int) : void
causes gvpr to exit with the exit code v.

system(cmd : string) : int
provides the standard C function system(3). It executes cmd
in the user's shell environment, and returns the exit status
of the shell.

rand() : double
returns a pseudo-random double between 0 and 1.

srand() : int

srand(v : int) : int
sets a seed for the random number generator. The optional
argument gives the seed; if it is omitted, the current time is
used. The previous seed value is returned. srand should be
called before any calls to rand.

colorx(color : string, fmt : string) : string
translates a color from one format to another. The color
argument should be a color in one of the recognized string
representations. The fmt value should be one of "RGB", "RGBA",
"HSV", or "HSVA". An empty string is returned on error.

BUILT-IN VARIABLES
gvpr provides certain special, built-in variables, whose values are
set automatically by gvpr depending on the context. Except as noted,
the user cannot modify their values.

$ : obj_t
denotes the current object (node, edge, graph) depending on
the context. It is not available in BEGIN or END clauses.

$F : string
is the name of the current input file.

$G : graph_t
denotes the current graph being processed. It is not available
in BEGIN or END clauses.

$NG : graph_t
denotes the next graph to be processed. If $NG is NULL, the
current graph $G is the last graph. Note that if the input
comes from stdin, the last graph cannot be determined until
the input pipe is closed. It is not available in BEGIN or END
clauses, or if the -n flag is used.

$O : graph_t
denotes the output graph. Before graph traversal, it is
initialized to the target graph. After traversal and any END_G
actions, if it refers to a non-empty graph, that graph is
printed onto the output stream. It is only valid in N, E and
END_G clauses. The output graph may be set by the user.

$T : graph_t
denotes the current target graph. It is a subgraph of $G and
is available only in N, E and END_G clauses.

$tgtname : string
denotes the name of the target graph. By default, it is set
to "gvpr_result". If used multiple times during the execution
of gvpr, the name will be appended with an integer. This
variable may be set by the user.

$tvroot : node_t
indicates the starting node for a (directed or undirected)
depth-first or breadth-first traversal of the graph (cf.
$tvtype below). The default value is NULL for each input
graph. After the traversal at the given root, if the value of
$tvroot has changed, a new traversal will begin with the new
value of $tvroot. Also, set $tvnext below.

$tvnext : node_t
indicates the next starting node for a (directed or
undirected) depth-first or breadth-first traversal of the
graph (cf. $tvtype below). If a traversal finishes and the
$tvroot has not been reset but the $tvnext has been set but
not used, this node will be used as the next choice for
$tvroot. The default value is NULL for each input graph.

$tvedge : edge_t
For BFS and DFS traversals, this is set to the edge used to
arrive at the current node or edge. At the beginning of a
traversal, or for other traversal types, the value is NULL.

$tvtype : tvtype_t
indicates how gvpr traverses a graph. It can only take one of
the constant values with the prefix "TV_" described below.
TV_flat is the default.

In the underlying graph library cgraph(3), edges in undirected
graphs are given an arbitrary direction. This is used for
traversals, such as TV_fwd, requiring directed edges.

ARGC : int
denotes the number of arguments specified by the -a args
command-line argument.

ARGV : string array
denotes the array of arguments specified by the -a args
command-line argument. The ith argument is given by ARGV[i].

BUILT-IN CONSTANTS
There are several symbolic constants defined by gvpr.

NULL : obj_t
a null object reference, equivalent to 0.

TV_flat : tvtype_t
a simple, flat traversal, with graph objects visited in
seemingly arbitrary order.

TV_ne : tvtype_t
a traversal which first visits all of the nodes, then all of
the edges.

TV_en : tvtype_t
a traversal which first visits all of the edges, then all of
the nodes.

TV_dfs : tvtype_t
TV_postdfs : tvtype_t
TV_prepostdfs : tvtype_t
a traversal of the graph using a depth-first search on the
underlying undirected graph. To do the traversal, gvpr will
check the value of $tvroot. If this has the same value that it
had previously (at the start, the previous value is
initialized to NULL.), gvpr will simply look for some
unvisited node and traverse its connected component. On the
other hand, if $tvroot has changed, its connected component
will be toured, assuming it has not been previously visited
or, if $tvroot is NULL, the traversal will stop. Note that
using TV_dfs and $tvroot, it is possible to create an infinite
loop.

By default, the traversal is done in pre-order. That is, a
node is visited before all of its unvisited edges. For
TV_postdfs, all of a node's unvisited edges are visited before
the node. For TV_prepostdfs, a node is visited twice, before
and after all of its unvisited edges.

TV_fwd : tvtype_t
TV_postfwd : tvtype_t
TV_prepostfwd : tvtype_t
A traversal of the graph using a depth-first search on the
graph following only forward arcs. The choice of roots for
the traversal is the same as described for TV_dfs above. The
different order of visitation specified by TV_fwd, TV_postfwd
and TV_prepostfwd are the same as those specified by the
analogous traversals TV_dfs, TV_postdfs and TV_prepostdfs.

TV_rev : tvtype_t
TV_postrev : tvtype_t
TV_prepostrev : tvtype_t
A traversal of the graph using a depth-first search on the
graph following only reverse arcs. The choice of roots for
the traversal is the same as described for TV_dfs above. The
different order of visitation specified by TV_rev, TV_postrev
and TV_prepostrev are the same as those specified by the
analogous traversals TV_dfs, TV_postdfs and TV_prepostdfs.

TV_bfs : tvtype_t
A traversal of the graph using a breadth-first search on the
graph ignoring edge directions. See the item on TV_dfs above
for the role of $tvroot.

EXAMPLES

gvpr -i 'N[color=="blue"]' file.gv

Generate the node-induced subgraph of all nodes with color blue.

gvpr -c 'N[color=="blue"]{color = "red"}' file.gv

Make all blue nodes red.

BEGIN { int n, e; int tot_n = 0; int tot_e = 0; }
BEG_G {
n = nNodes($G);
e = nEdges($G);
printf ("%d nodes %d edges %s\n", n, e, $G.name);
tot_n += n;
tot_e += e;
}
END { printf ("%d nodes %d edges total\n", tot_n, tot_e) }

Version of the program gc.

gvpr -c ""

Equivalent to nop.

BEG_G { graph_t g = graph ("merge", "S"); }
E {
node_t h = clone(g,$.head);
node_t t = clone(g,$.tail);
edge_t e = edge(t,h,"");
e.weight = e.weight + 1;
}
END_G { $O = g; }

Produces a strict version of the input graph, where the weight
attribute of an edge indicates how many edges from the input graph
the edge represents.

BEGIN {node_t n; int deg[]}
E{deg[head]++; deg[tail]++; }
END_G {
for (deg[n]) {
printf ("deg[%s] = %d\n", n.name, deg[n]);
}
}

Computes the degrees of nodes with edges.

BEGIN {
int i, indent;
int seen[string];
void prInd (int cnt) {
for (i = 0; i < cnt; i++) printf (" ");
}
}
BEG_G {

$tvtype = TV_prepostfwd;
$tvroot = node($,ARGV[0]);
}
N {
if (seen[$.name]) indent--;
else {
prInd(indent);
print ($.name);
seen[$.name] = 1;
indent++;
}
}

Prints the depth-first traversal of the graph, starting with the node
whose name is ARGV[0], as an indented list.

ENVIRONMENT

GVPRPATH
Colon-separated list of directories to be searched to find the
file specified by the -f option. gvpr has a default list built
in. If GVPRPATH is not defined, the default list is used. If
GVPRPATH starts with colon, the list is formed by appending
GVPRPATH to the default list. If GVPRPATH ends with colon, the
list is formed by appending the default list to GVPRPATH.
Otherwise, GVPRPATH is used for the list.

On Windows systems, replace ``colon'' with ``semicolon'' in the
previous paragraph.

BUGS AND WARNINGS

Scripts should be careful deleting nodes during N{} and E{} blocks
using BFS and DFS traversals as these rely on stacks and queues of
nodes.

When the program is given as a command line argument, the usual shell
interpretation takes place, which may affect some of the special
names in gvpr. To avoid this, it is best to wrap the program in
single quotes.

If string constants contain pattern metacharacters that you want to
escape to avoid pattern matching, two backslashes will probably be
necessary, as a single backslash will be lost when the string is
originally scanned. Usually, it is simpler to use strcmp to avoid
pattern matching.

As of 24 April 2008, gvpr switched to using a new, underlying graph
library, which uses the simpler model that there is only one copy of
a node, not one copy for each subgraph logically containing it. This
means that iterators such as nxtnode cannot traverse a subgraph using
just a node argument. For this reason, subgraph traversal requires
new functions ending in "_sg", which also take a subgraph argument.
The versions without that suffix will always traverse the root graph.

There is a single global scope, except for formal function
parameters, and even these can interfere with the type system. Also,
the extent of all variables is the entire life of the program. It
might be preferable for scope to reflect the natural nesting of the
clauses, or for the program to at least reset locally declared
variables. For now, it is advisable to use distinct names for all
variables.

If a function ends with a complex statement, such as an IF statement,
with each branch doing a return, type checking may fail. Functions
should use a return at the end.

The expr library does not support string values of (char*)0. This
means we can't distinguish between "" and (char*)0 edge keys. For
the purposes of looking up and creating edges, we translate "" to be
(char*)0, since this latter value is necessary in order to look up
any edge with a matching head and tail.

Related to this, strings converted to integers act like char
pointers, getting the value 0 or 1 depending on whether the string
consists solely of zeroes or not. Thus, the ((int)"2") evaluates to
1.

The language inherits the usual C problems such as dangling
references and the confusion between '=' and '=='.

AUTHOR

Emden R. Gansner <erg@research.att.com>

SEE ALSO

awk(1), gc(1), dot(1), nop(1), expr(3), cgraph(3)

29 August 2013 GVPR(1)