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NBPERF(1) BSD General Commands Manual NBPERF(1)
nbperf -- compute a perfect hash function
nbperf [-ps] [-a algorithm] [-c utilisation] [-h hash] [-i iterations] [-m map-file]
[-n name] [-o output] [input]
nbperf reads a number of keys one per line from standard input or input. It computes a min-
imal perfect hash function and writes it to stdout or output. The default algorithm is
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 corresponding 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 utilisa-
tion is very near to 2.
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 function. 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
The number of iterations can be limited with -i. nbperf outputs a function 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 constant.
The nbperf utility exits 0 on success, and >0 if an error occurs.
BSD September 25, 2012 BSD
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