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hashlib: document merged hash_top_ops with hash_ops

This commit is contained in:
Emil J. Tywoniak 2025-01-20 16:25:52 +01:00
parent aa01ef3312
commit a2c26a00f2
1 changed files with 15 additions and 4 deletions
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19
docs/source/yosys_internals/hashing.rst
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@ -97,8 +97,8 @@ Making a type hashable
Let's first take a look at the external interface on a simplified level.
Generally, to get the hash for ``T obj``, you would call the utility function
``run_hash<T>(const T& obj)``, corresponding to ``hash_top_ops<T>::hash(obj)``,
the default implementation of which is ``hash_ops<T>::hash_into(Hasher(), obj)``.
``run_hash<T>(const T& obj)``, corresponding to ``hash_ops<T>::hash(obj)``,
the default implementation of which uses ``hash_ops<T>::hash_into(Hasher(), obj)``.
``Hasher`` is the class actually implementing the hash function, hiding its
initialized internal state, and passing it out on ``hash_t yield()`` with
perhaps some finalization steps.
@ -121,8 +121,14 @@ size containers like ``std::vector<U>`` the size of the container is hashed
first. That is also how implementing hashing for a custom record data type
should be - unless there is strong reason to do otherwise, call ``h.eat(m)`` on
the ``Hasher h`` you have received for each member in sequence and ``return
h;``. If you do have a strong reason to do so, look at how
``hash_top_ops<RTLIL::SigBit>`` is implemented in ``kernel/rtlil.h``.
h;``.
The ``hash_ops<T>::hash(obj)`` method is not indended to be called when
context of implementing the hashing for a record or other compound type.
When writing it, you should connect it to ``hash_ops<T>::hash_into(Hasher h)``
as shown below. If you have a strong reason to do so, and you have
to create a special implementation for top-level hashing, look at how
``hash_ops<RTLIL::SigBit>::hash(...)`` is implemented in ``kernel/rtlil.h``.
Porting plugins from the legacy interface
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -148,6 +154,11 @@ based on the existance and value of `YS_HASHING_VERSION`.
h.eat(b);
return h;
}
Hasher T::hash() const {
Hasher h;
h.eat(*this);
return h;
}
#else
#error "Unsupported hashing interface"
#endif