nbperf(1) - NetBSD Manual Pages

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NBPERF(1)               NetBSD General Commands Manual               NBPERF(1)


NAME

     nbperf -- compute a perfect hash function


SYNOPSIS

     nbperf [-ps] [-a algorithm] [-c utilisation] [-h hash] [-i iterations]
            [-m map-file] [-n name] [-o output] [input]


DESCRIPTION

     nbperf reads a number of keys one per line from standard input or input.
     It computes a minimal perfect hash function and writes it to stdout or
     output.  The default algorithm is "chm".

     The -m argument instructs nbperf to write the resulting key mapping to
     map-file.  Each line gives the result of the hash function for the corre-
     sponding input key.

     The parameter utilisation determines the space efficiency.

     Supported arguments for -a:

     chm  This results in an order preserving minimal perfect hash function.
          The utilisation must be at least 2, the default.  The number of
          iterations needed grows if the utilisation is very near to 2.

     chm3
          Similar to chm.  The resulting hash function needs three instead of
          two table lookups when compared to chm.  The utilisation must be at
          least 1.24, the default.  This makes the output for chm3 noticable
          smaller than the output for chm.

     bpz  This results in a non-order preserving minimal perfect hash func-
          tion.  Output size is approximately 2.79 bit per key for the default
          value of utilisation, 1.24.  This is also the smallest supported
          value.

     Supported arguments for -h:

     mi_vector_hash  Platform-independent version of Jenkins parallel hash.
                     See mi_vector_hash(3).

     The number of iterations can be limited with -i.  nbperf outputs a func-
     tion matching uint32_t hash(const void * restrict, size_t) to stdout.
     The function expects the key length as second argument, for strings not
     including the terminating NUL.  It is the responsibility of the caller to
     pass in only valid keys or compare the resulting index to the key.  The
     function name can be changed using -n name.  If the -s flag is specified,
     it will be static.

     After each failing iteration, a dot is written to stderr.

     nbperf checks for duplicate keys on the first iteration that passed basic
     hash distribution tests.  In that case, an error message is printed and
     the program terminates.

     If the -p flag is specified, the hash function is seeded in a stable way.
     This may take longer than the normal random seed, but ensures that the
     output is the same for repeated invocations as long as the input is con-
     stant.


EXIT STATUS

     The nbperf utility exits 0 on success, and >0 if an error occurs.


SEE ALSO

     mi_vector_hash(3)


AUTHORS

     Jörg Sonnenberger

NetBSD 6.1.2                  September 25, 2012                  NetBSD 6.1.2