.\"	$OpenBSD: tsort.1,v 1.19 2007/05/31 19:20:18 jmc Exp $
.\"	$NetBSD: tsort.1,v 1.6 1996/01/17 20:37:49 mycroft Exp $
.\"
.\" Copyright (c) 1990, 1993, 1994
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.\" This manual is derived from one contributed to Berkeley by
.\" Michael Rendell of Memorial University of Newfoundland.
.\"
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.\" modification, are permitted provided that the following conditions
.\" are met:
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.\"     @(#)tsort.1	8.3 (Berkeley) 4/1/94
.\"
.Dd $Mdocdate: May 31 2007 $
.Dt TSORT 1
.Os
.Sh NAME
.Nm tsort
.Nd topological sort of a directed graph
.Sh SYNOPSIS
.Nm tsort
.Op Fl flqrvw
.Op Fl h Ar file
.Op Ar file
.Sh DESCRIPTION
.Nm tsort
takes a list of pairs of node names representing directed arcs in
a graph and prints the nodes in topological order on standard output.
That is, the input describes a partial ordering relation, from which
.Nm
computes a total order compatible with this partial ordering.
.Pp
Input is taken from the named
.Ar file ,
or from standard input if no file
is given.
.Pp
Node names in the input are separated by white space and there must
be an even number of node pairs.
.Pp
Presence of a node in a graph can be represented by an arc from the node
to itself.
This is useful when a node is not connected to any other nodes.
.Pp
If the graph contains a cycle (and therefore cannot be properly sorted),
one of the arcs in the cycle is ignored and the sort continues.
Cycles are reported on standard error.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl f
Resolve ambiguities by selecting nodes based on the order of appearance
of the first component of the pairs.
.It Fl h Ar file
Use
.Ar file ,
which holds an ordered list of nodes, to resolve ambiguities.
In case of duplicates, the first entry is chosen.
.It Fl l
Search for and display the longest cycle.
Can take a very long time, as it may need to solve an NP-complete problem.
.It Fl q
Do not display informational messages about cycles.
This is primarily intended for building libraries, where optimal ordering
is not critical, and cycles occur often.
.It Fl r
Reverse the ordering relation.
.It Fl v
Inform on the exact number of edges broken while breaking cycles.
If a hints file was used, inform on seen nodes absent from that file.
.It Fl w
Exit with exit code the number of cycles
.Nm
had to break.
.El
.Sh EXAMPLES
Faced with the input:
.Bd -literal -offset indent
a b
b c
b d
d f
c e
.Ed
.Pp
.Nm
outputs:
.Bd -literal -offset indent
a
b
c
e
d
f
.Ed
.Pp
which is one total ordering compatible with the individual relations.
There is no unicity, another compatible total ordering would be:
.Bd -literal -offset indent
a
b
c
d
e
f
.Ed
.Pp
.Nm
is commonly used to analyze dependencies and find a correct build order
in a static way, whereas
.Xr make 1
accomplishes the same task in a dynamic way.
.Sh SEE ALSO
.Xr ar 1 ,
.Xr lorder 1 ,
.Xr make 1
.Rs
.%A Donald E. Knuth
.%B The Art of Computer Programming
.%V Vol. 1
.%P pp 258-268
.%D 1973
.Re
.Sh STANDARDS
The
.Nm
utility is compliant with the
.St -xpg4
specification.
.Pp
The flags
.Op Fl fhlqrvw
are extensions to that specification.
.Sh HISTORY
A
.Nm
command appeared in
.At v7 .
This
.Nm tsort
command was completely rewritten by Marc Espie for
.Ox ,
to finally use the well-known optimal algorithms for topological sorting.