NetBSD/lib/libc/string/bm.3

.\" $NetBSD: bm.3,v 1.5 2002/02/07 07:00:31 ross Exp $
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.\" Copyright (c) 1994
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.\" This code is derived from software contributed to Berkeley by
.\" Andrew Hume of AT&T Bell Laboratories.
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.\"	from: @(#)bm.3	8.4 (Berkeley) 6/21/94
.\"
.Dd April 8, 2001
.Dt BM 3
.Os
.Sh NAME
.Nm bm_comp ,
.Nm bm_exec ,
.Nm bm_free
.Nd Boyer-Moore string search
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.Fd #include \*[Lt]sys/types.h\*[Gt]
.Fd #include \*[Lt]bm.h\*[Gt]
.Ft bm_pat *
.Fn bm_comp "u_char *pattern" "size_t patlen" "u_char freq[256]"
.Ft u_char *
.Fn bm_exec "bm_pat *pdesc" "u_char *text" "size_t len"
.Ft void
.Fn bm_free "bm_pat *pdesc"
.Sh DESCRIPTION
These routines implement an efficient mechanism to find an
occurrence of a byte string within another byte string.
.Pp
.Fn bm_comp
evaluates the
.Fa patlen
bytes starting at
.Fa pattern ,
and returns a pointer to a structure describing them.
The bytes referenced by
.Fa pattern
may be of any value.
.Pp
The search takes advantage of the frequency distribution of the
bytes in the text to be searched.
If specified,
.Fa freq
should be an array of 256 values,
with higher values indicating that the corresponding character occurs
more frequently.
(A less than optimal frequency distribution can only result in less
than optimal performance, not incorrect results.)
If
.Fa freq
is NULL,
a system default table is used.
.Pp
.Fn bm_exec
returns a pointer to the leftmost occurrence of the string given to
.Fn bm_comp
within
.Fa text ,
or NULL if none occurs.
The number of bytes in
.Fa text
must be specified by
.Fa len .
.Pp
Space allocated for the returned description is discarded
by calling
.Fn bm_free
with the returned description as an argument.
.Pp
The asymptotic speed of
.Fn bm_exec
is O(len/patlen).
.Sh SEE ALSO
.Xr regexp 3 ,
.Xr strstr 3
.Rs
.%A Hume and Sunday
.%D November 1991
.%J "Software Practice and Experience"
.%P pp. 1221-48
.%T "Fast String Searching"
.%V Vol. 21, 11
.Re