yorel::yomm2::policy::fast_perfect_hash
defined in <yorel/yomm2/policy.hpp>, also provided by <yorel/yomm2/core.hpp>, <yorel/yomm2/keywords.hpp>
template<class Policy> struct fast_perfect_hash;
template<class Policy> struct checked_perfect_hash;
fast_perfect_hash is an implementation of type_hash. It uses the
following hash function:
H(x) = (x * M) >> S
The M and S coefficients are determined during initialization so that, for
the given set of type ids, the hash function is perfect, i.e. collision-free.
It is not minimal: there may be gaps in the output interval.
hash_type_id must not be called with ids not present in the input set passed
to hash_initialize. This happens when the class of a virtual argument was not
registered via use_classes or register_classes. ref_perfect_hash will
silently return an index in the output range, to be used to fetch the v-table
for the class from a vector. From there, two things can happen. In the lucky
scenario, the index will correspond to a gap, a wasted entry in the v-table
vector, which will contain a null pointer. The program will segfault. Such
mistake is fairly easy to troubleshoot. In the unlucky scenario, there will be a
collision - the hash function is perfect only for the input ids - and the wrong
method will be called. The program will carry on, crashing far from the root
cause of the error, and, in the worst case, not at all.
To help detect such registration errors, it is highly recommended to run debug
builds of the program with checked_perfect_hash, which detects
missing class registrations.
default_policy uses fast_perfect_hash in release builds, and
checked_perfect_hash in debug builds.
Policy - the policy containing the facet.
| hash_initialize | finds a hash function |
| hash_type_id | returns the hashed type_id |
| hash_length | length of the result interval |
template <typename ForwardIterator> static size_t
hash_initialize(ForwardIterator first, ForwardIterator last);
Finds the M and S parameters of the hash function. ForwardIterator is an
iterator that satisfies the requirements of forward iterators; dereferencing it
yields a type_id. Sets type_hash_length to the maximum hash value, plus one.
first, last - a range of type_ids
None.
static type_id hash_type_id(type_id type);
Returns the hashed value of type, which must be one of the values passed to
hash_initialize.
type - a type_id
None.
None.
static std::size_t hash_length;
The length of the zero-based interval that contains the hashed values for all the input values. In other words, the maximum value returned for legal inputs, plus one.
error_handler - to report error conditions.error_output - for diagnostics.trace_output - for trace.