www.delorie.com/gnu/docs/gperf/gperf_18.html

search

	Buy GNU books!

Perfect Hash Function Generator

4.4 Options for changing the Algorithms employed by gperf

`-k keys'

`--key-positions=keys'

Allows selection of the character key positions used in the keywords' hash function. The allowable choices range between 1-126, inclusive. The positions are separated by commas, e.g., `-k 9,4,13,14'; ranges may be used, e.g., `-k 2-7'; and positions may occur in any order. Furthermore, the meta-character '*' causes the generated hash function to consider all character positions in each key, whereas '$' instructs the hash function to use the "final character" of a key (this is the only way to use a character position greater than 126, incidentally).

For instance, the option `-k 1,2,4,6-10,'$'' generates a hash function that considers positions 1,2,4,6,7,8,9,10, plus the last character in each key (which may differ for each key, obviously). Keys with length less than the indicated key positions work properly, since selected key positions exceeding the key length are simply not referenced in the hash function.

`-l'

`--compare-strlen'

Compare key lengths before trying a string comparison. This might cut down on the number of string comparisons made during the lookup, since keys with different lengths are never compared via strcmp. However, using `-l' might greatly increase the size of the generated C code if the lookup table range is large (which implies that the switch option `-S' is not enabled), since the length table contains as many elements as there are entries in the lookup table. This option is mandatory for binary comparisons (see section 3.3 Use of NUL characters).

`-D'

`--duplicates'

Handle keywords whose key position sets hash to duplicate values. Duplicate hash values occur for two reasons:

• Since gperf does not backtrack it is possible for it to process all your input keywords without finding a unique mapping for each word. However, frequently only a very small number of duplicates occur, and the majority of keys still require one probe into the table.

• Sometimes a set of keys may have the same names, but possess different attributes. With the -D option gperf treats all these keys as part of an equivalence class and generates a perfect hash function with multiple comparisons for duplicate keys. It is up to you to completely disambiguate the keywords by modifying the generated C code. However, gperf helps you out by organizing the output.

Option `-D' is extremely useful for certain large or highly redundant keyword sets, e.g., assembler instruction opcodes. Using this option usually means that the generated hash function is no longer perfect. On the other hand, it permits gperf to work on keyword sets that it otherwise could not handle.

`-f iteration-amount'

`--fast=iteration-amount'

Generate the perfect hash function "fast". This decreases gperf's running time at the cost of minimizing generated table-size. The iteration amount represents the number of times to iterate when resolving a collision. `0' means iterate by the number of keywords. This option is probably most useful when used in conjunction with options `-D' and/or `-S' for large keyword sets.

`-i initial-value'

`--initial-asso=initial-value'

Provides an initial value for the associate values array. Default is 0. Increasing the initial value helps inflate the final table size, possibly leading to more time efficient keyword lookups. Note that this option is not particularly useful when `-S' is used. Also, `-i' is overridden when the `-r' option is used.

`-j jump-value'

`--jump=jump-value'

Affects the "jump value", i.e., how far to advance the associated character value upon collisions. Jump-value is rounded up to an odd number, the default is 5. If the jump-value is 0 gperf jumps by random amounts.

`-n'

`--no-strlen'

Instructs the generator not to include the length of a keyword when computing its hash value. This may save a few assembly instructions in the generated lookup table.

`-o'

`--occurrence-sort'

Reorders the keywords by sorting the keywords so that frequently occuring key position set components appear first. A second reordering pass follows so that keys with "already determined values" are placed towards the front of the keylist. This may decrease the time required to generate a perfect hash function for many keyword sets, and also produce more minimal perfect hash functions. The reason for this is that the reordering helps prune the search time by handling inevitable collisions early in the search process. On the other hand, if the number of keywords is very large using `-o' may increase gperf's execution time, since collisions will begin earlier and continue throughout the remainder of keyword processing. See Cichelli's paper from the January 1980 Communications of the ACM for details.

`-r'

`--random'

Utilizes randomness to initialize the associated values table. This frequently generates solutions faster than using deterministic initialization (which starts all associated values at 0). Furthermore, using the randomization option generally increases the size of the table. If gperf has difficultly with a certain keyword set try using `-r' or `-D'.

`-s size-multiple'

`--size-multiple=size-multiple'

Affects the size of the generated hash table. The numeric argument for this option indicates "how many times larger or smaller" the maximum associated value range should be, in relationship to the number of keys. If the size-multiple is negative the maximum associated value is calculated by dividing it into the total number of keys. For example, a value of 3 means "allow the maximum associated value to be about 3 times larger than the number of input keys".

Conversely, a value of -3 means "allow the maximum associated value to be about 3 times smaller than the number of input keys". Negative values are useful for limiting the overall size of the generated hash table, though this usually increases the number of duplicate hash values.

If `generate switch' option `-S' is not enabled, the maximum associated value influences the static array table size, and a larger table should decrease the time required for an unsuccessful search, at the expense of extra table space.

The default value is 1, thus the default maximum associated value about the same size as the number of keys (for efficiency, the maximum associated value is always rounded up to a power of 2). The actual table size may vary somewhat, since this technique is essentially a heuristic. In particular, setting this value too high slows down gperf's runtime, since it must search through a much larger range of values. Judicious use of the `-f' option helps alleviate this overhead, however.