A hash function in datastructures is a function H which maps a value X to a small integer (where a small integer is an integer in the range [-2^28..2^28-1] on a 32-bit system, and [-2^60..2^60-1] on a 64-bit system), under the requirement that if X=Y, then H(X)=H(Y).
A variety of hash functions is provided by datastructures, with different behaviours. A bad choice of hash function can lead to serious performance problems.
datastructures does not guarantee consistency of hash values across release or GAP sessions.
‣ HashBasic( obj... ) | ( function ) |
Returns: a small integer
Hashes any values built inductively from
built-in types, namely integers, booleans, permutations, transformations, partial permutations, and
constructors for lists and records.
This function is variadic, treating more than one argument as equivalent to a list containing the arguments, that is HashBasic(x,y,z) = HashBasic([x,y,z]).
datastructures provides two hash functions for permutation groups; Hash_PermGroup_Fast (6.3-1) is the faster one, with higher likelihood of collisions and Hash_PermGroup_Complete (6.3-2) is slower but provides a lower likelihood of collisions.
‣ Hash_PermGroup_Fast( group ) | ( function ) |
Returns: a small integer
Hash_PermGroup_Fast is faster than Hash_PermGroup_Complete (6.3-2), but will return the same value for groups with the same size, orbits and degree of transitivity.
‣ Hash_PermGroup_Complete( group ) | ( function ) |
Returns: a small integer
Hash_PermGroup_Complete is slower than Hash_PermGroup_Fast (6.3-1), but is extremely unlikely to return the same hash for two different groups.
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