NBPERF(1) BSD General Commands Manual NBPERF(1)NAMEnbperf — compute a perfect hash function
SYNOPSISnbperf [-ps] [-a algorithm] [-c utilisation] [-h hash] [-i iterations]
[-m map-file] [-n name] [-o output] [input]
DESCRIPTIONnbperf 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.
bdz 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 ALSOmi_vector_hash(3)AUTHORS
Jörg Sonnenberger
BSD September 25, 2012 BSD
