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[Docs] Include explanation of how indexed dynamic-length event args are encoded

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Steve Waldman 2017-12-19 01:44:33 -08:00
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docs/abi-spec.rst
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@ -288,6 +288,8 @@ In effect, a log entry using this ABI is described as:
- ``topics[n]``: ``EVENT_INDEXED_ARGS[n - 1]`` (``EVENT_INDEXED_ARGS`` is the series of ``EVENT_ARGS`` that are indexed);
- ``data``: ``abi_serialise(EVENT_NON_INDEXED_ARGS)`` (``EVENT_NON_INDEXED_ARGS`` is the series of ``EVENT_ARGS`` that are not indexed, ``abi_serialise`` is the ABI serialisation function used for returning a series of typed values from a function, as described above).
For all fixed-length Solidity types, the ``EVENT_INDEXED_ARGS`` array contains the 32-byte encoded value directly. However, for *types of dynamic length*, which include ``string``, ``bytes``, and arrays, ``EVENT_INDEXED_ARGS`` will contain the *Keccak hash* of the encoded value, rather than the encoded value directly. This allows applications to efficiently query for values of dynamic-length types (by setting the hash of the encoded value as the topic), but leaves applications unable to decode indexed values they have not queried for. For dynamic-length types, application developers face a trade-off between fast search for predetermined values (if the argument is indexed) and legibility of arbitrary values (which requires that the arguments not be indexed).
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