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swh/perfecthash/__init__.py

# Copyright (C) 2021 The Software Heritage developers
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+ # Copyright (C) 2021 The Software Heritage developers
+ # See the AUTHORS file at the top-level directory of this distribution
+ # License: GNU General Public License version 3, or any later version
+ # See top-level LICENSE file for more information
+
from _hash_cffi import lib
from cffi import FFI

copyright headers to match other swh modules?

ardumont: copyright headers to match other swh modules?
dacharyAuthorUnsubmitted
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Good point, done!

dachary: Good point, done!
# See the AUTHORS file at the top-level directory of this distribution
# License: GNU General Public License version 3, or any later version
# See top-level LICENSE file for more information
from typing import NewType
from _hash_cffi import lib
from cffi import FFI
Key = NewType("Key", bytes)
HashObject = NewType("HashObject", bytes)
class Shard:
"""Low level management for files indexed with a perfect hash table.
This class allows creating a Read Shard by adding key/object pairs
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and looking up the content of an object when given the key.
"""
- def __init__(self, path):
+ def __init__(self, path: str):
"""Initialize with an existing Read Shard.
Args:
- path (str): path to an existing Read Shard file or device
+ path: path to an existing Read Shard file or device
"""
self.ffi = FFI()

Let's add some types.

ardumont: Let's add some types.
and looking up the content of an object when given the key.
"""
def __init__(self, path: str):
"""Initialize with an existing Read Shard.
Args:
path: path to an existing Read Shard file or device
"""
self.ffi = FFI()
self.shard = lib.shard_init(path.encode("utf-8"))
def __del__(self):
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lib.shard_destroy(self.shard)
- def create(self, objects_count):
+ def create(self, objects_count: int) -> "Shard":
"""Wipe out the content of the Read Shard. It must be followed by
**object_count** calls to the **write** method otherwise the content
of the Read Shard will be inconsistent. When all objects are inserted,
the Read Shard must be made persistent by calling the **save** method.
Args:
- objects_count (int): number of objects in the Read Shard.
+ objects_count: number of objects in the Read Shard.
Returns:

fyi, we now are documenting directly the types within the code and no longer in the docstring.

ardumont: fyi, we now are documenting directly the types within the code and no longer in the docstring.
lib.shard_destroy(self.shard)
def create(self, objects_count: int) -> "Shard":
"""Wipe out the content of the Read Shard. It must be followed by
**object_count** calls to the **write** method otherwise the content
of the Read Shard will be inconsistent. When all objects are inserted,
the Read Shard must be made persistent by calling the **save** method.
Args:
objects_count: number of objects in the Read Shard.
Returns:
self.
"""
assert lib.shard_create(self.shard, objects_count) != -1
return self
def load(self):
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return self
- def load(self):
+ def load(self) -> "Shard":
"""Open the Read Shard file in read-only mode.

From the docstring.

ardumont: From the docstring.
"""Open the Read Shard file in read-only mode.
Returns:
self.
"""
assert lib.shard_load(self.shard) != -1
return self
def save(self):
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return self
- def save(self):
+ def save(self) -> int:
"""Create the perfect hash table the **lookup** method

From the docstring ;)

ardumont: From the docstring ;)
"""Create the perfect hash table the **lookup** method
relies on to find the content of the objects.
It must be called after **create** an **write** otherwise the
content of the Read Shard will be inconsistent.
Returns:
0 on success, -1 on error.
"""
return lib.shard_save(self.shard)
def lookup(self, key: Key) -> HashObject:
"""Fetch the object matching the key in the Read Shard.
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indent is off by 2 chars almost everywhere.
We normalize on a 4 chars indent below "Args:" and "Returns:" (and "Raises:" if any).

ardumont: indent is off by 2 chars almost everywhere. We normalize on a 4 chars indent below "Args:" and…
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return lib.shard_save(self.shard)
- def lookup(self, key):
+ def lookup(self, key: Key) -> HashObject:
"""Fetch the object matching the key in the Read Shard.
Fetching an object is O(1): one lookup in the index to obtain
the offset of the object in the Read Shard and one read to get
the payload.
Args:
- key (32 bytes): the key associated with the object to retrieve.
+ key: the key associated with the object to retrieve.
Returns:
- the object as bytes.
+ the object as bytes.
"""
object_pointer = self.ffi.new("char**")

Let's add some types around this, then define at the top-level module something like:

from typing import NewType

Key = NewType("Key", bytes)
HashObjects = NewType("HashObject", bytes)

I believe that improves the reading significantly.

ardumont: Let's add some types around this, then define at the top-level module something like: ``` from…
Fetching an object is O(1): one lookup in the index to obtain
the offset of the object in the Read Shard and one read to get
the payload.
Args:
key: the key associated with the object to retrieve.
Returns:
the object as bytes.
"""
object_size_pointer = self.ffi.new("size_t*")
lib.shard_lookup_object_size(self.shard, key, object_size_pointer)
object_size = object_size_pointer[0]
object_pointer = self.ffi.new("char[]", object_size)
lib.shard_lookup_object(self.shard, object_pointer, object_size)
return self.ffi.buffer(object_pointer, object_size)
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Something hit me, can't the lookup fail?

What would be the return if that was the case None?
(if so, the type must be changed to Optional[HashObject]).

ardumont: Something hit me, can't the lookup fail? What would be the return if that was the case None?
dacharyAuthorUnsubmitted
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I overlooked this question, sorry. The lookup cannot fail: if given a key that does not exist it will return a random object. Which is ok because the global index ensures the Read Shard is only queried with objects that are known to exist in the hash map.

dachary: I overlooked this question, sorry. The lookup cannot fail: if given a key that does not exist…
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you did not overlook it, it's a new one!

If anything, that's me who overlooked that code last time ;)

Great! "Can't happen!"

ardumont: you did not overlook it, it's a new one! If anything, that's me who overlooked that code last…
def write(self, key: Key, object: HashObject) -> int:
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return self.ffi.buffer(object_pointer[0], object_size_pointer[0])
- def write(self, key, object):
+ def write(self, key: Key, object: HashObject) -> int:
"""Add the key/object pair to the Read Shard.
The **create** method must have been called prior to calling
the **write** method.
Args:
- key (32 bytes): the unique key associated with the object.
- object (bytes): the object
+ key: the unique key associated with the object.
+ object: the object
Returns:
- 0 on success, -1 on error.
+ 0 on success, -1 on error.
"""
return lib.shard_object_write(self.shard, key, object, len(object))
ardumont:
"""Add the key/object pair to the Read Shard.
The **create** method must have been called prior to calling
the **write** method.
Args:
key: the unique key associated with the object.
object: the object
Returns:
0 on success, -1 on error.
"""
return lib.shard_object_write(self.shard, key, object, len(object))
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