144 lines
4.2 KiB
Groff
144 lines
4.2 KiB
Groff
.\" $NetBSD: nbperf.1,v 1.7 2017/06/20 15:50:04 abhinav Exp $
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.\"
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.\" Copyright (c) 2009 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Joerg Sonnenberger.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd September 25, 2012
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.Dt NBPERF 1
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.Os
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.Sh NAME
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.Nm nbperf
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.Nd compute a perfect hash function
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.Sh SYNOPSIS
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.Nm
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.Op Fl ps
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.Op Fl a Ar algorithm
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.Op Fl c Ar utilisation
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.Op Fl h Ar hash
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.Op Fl i Ar iterations
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.Op Fl m Ar map-file
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.Op Fl n Ar name
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.Op Fl o Ar output
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.Op Ar input
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.Sh DESCRIPTION
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.Nm
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reads a number of keys one per line from standard input or
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.Ar input .
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It computes a minimal perfect hash function and writes it to stdout or
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.Ar output .
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The default algorithm is
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.Qq Sy chm .
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.Pp
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The
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.Fl m
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argument instructs
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.Nm
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to write the resulting key mapping to
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.Ar map-file .
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Each line gives the result of the hash function for the corresponding input
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key.
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.Pp
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The parameter
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.Ar utilisation
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determines the space efficiency.
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.Pp
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Supported arguments for
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.Fl a :
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.Bl -tag -width "chm"
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.It Sy chm
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This results in an order preserving minimal perfect hash function.
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The
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.Ar utilisation
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must be at least 2, the default.
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The number of iterations needed grows if the utilisation is very near to 2.
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.It Sy chm3
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Similar to
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.Ar chm .
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The resulting hash function needs three instead of two table lookups when
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compared to
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.Ar chm .
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The
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.Ar utilisation
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must be at least 1.24, the default.
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This makes the output for
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.Ar chm3
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noticeably smaller than the output for
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.Ar chm .
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.It Sy bpz
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This results in a non-order preserving minimal perfect hash function.
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Output size is approximately 2.79 bit per key for the default value of
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.Ar utilisation ,
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1.24.
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This is also the smallest supported value.
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.El
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.Pp
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Supported arguments for
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.Fl h :
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.Bl -tag -width "mi_vector_hash"
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.It Sy mi_vector_hash
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Platform-independent version of Jenkins parallel hash.
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See
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.Xr mi_vector_hash 3 .
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.El
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.Pp
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The number of iterations can be limited with
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.Fl i .
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.Nm
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outputs a function matching
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.Ft uint32_t
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.Fn hash "const void * restrict" "size_t"
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to stdout.
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The function expects the key length as second argument, for strings not
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including the terminating NUL.
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It is the responsibility of the caller to pass in only valid keys or compare
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the resulting index to the key.
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The function name can be changed using
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.Fl n Ar name .
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If the
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.Fl s
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flag is specified, it will be static.
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.Pp
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After each failing iteration, a dot is written to stderr.
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.Pp
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.Nm
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checks for duplicate keys on the first iteration that passed
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basic hash distribution tests.
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In that case, an error message is printed and the program terminates.
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.Pp
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If the
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.Fl p
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flag is specified, the hash function is seeded in a stable way.
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This may take longer than the normal random seed, but ensures
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that the output is the same for repeated invocations as long as
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the input is constant.
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.Sh EXIT STATUS
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.Ex -std
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.Sh SEE ALSO
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.Xr mi_vector_hash 3
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.Sh AUTHORS
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.An J\(:org Sonnenberger
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