## type_hash

### Synopsis

Defined in <boost/openmethod/policies/basic_policy.hpp>.

```c++
namespace boost::openmethod::policies {

struct type_hash : policy {};

} // boost::openmethod::policies
```

### Description

`type_hash` is a policy that provides a hash function for a fixed set of
`type_id`{empty}s.

### Requirements

### hash_type_id

```c++
static auto hash_type_id(type_id type) -> type_id;
```

Returns the hash of `type`.

#### hash_initialize

```c++
template<typename ForwardIterator>
static auto hash_initialize(ForwardIterator first, ForwardIterator last)
  -> Report;
```

Finds a hash function for the `type_id`{empty}s in the range `[first, last)`.
`ForwardIterator` is the same as in `vptr_vector::register_vptrs`.

`hash_initialize` returns a `Report` object which is required to have two
members, `first` and `last`, which define the range `[first, last)` of the
possible output values of the hash function.

## fast_perfect_hash

### Synopsis

Defined in <boost/openmethod/policies/fast_perfect_hash.hpp>.

```c++
class fast_perfect_hash : type_hash
{
  public:
    static auto hash_type_id(type_id type) -> type_id;
    template<typename ForwardIterator>
    static auto hash_initialize(ForwardIterator first, ForwardIterator last) -> Report;
};
```

### Description

`fast_perfect_hash` implements a very fast, perfect (but not minimal) hash
function for `type_id`{empty}s.

### Members

Find two factors

#### hash_type_id

```c++
static auto hash_type_id(type_id type) -> type_id;
```

Returns `(type * M) >> S`, where `M` and `S` are factors found by
`hash_initialize`.

If the policy has a `runtime_checks` policy, `hash_type_id` checks that `type`
corresponds to a registered class. If not, it reports a `unknown_class_error`
using the policy's error_handler policy, if present, then calls `abort`.

#### hash_initialize

```c++
template<typename ForwardIterator>
auto hash_initialize(ForwardIterator first, ForwardIterator last) -> Report;
```

Finds factors `M` and `S` such that `hash_type_id` is a collision-free hash
function.

If no such factors cannot be found, `hash_initialize` reports a
`hash_search_error` using the policy's error_handler policy, if present, the
calls `abort`.

If the policy has a `trace` policy, `hash_initialize` uses it to write a
summary of the search.