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diff --git a/swh/model/merkle.py b/swh/model/merkle.py
new file mode 100644
--- /dev/null
+++ b/swh/model/merkle.py
@@ -0,0 +1,286 @@
+# Copyright (C) 2017 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
+
+"""Merkle tree data structure"""
+
+import abc
+import collections
+
+
+def deep_update(left, right):
+ """Recursively update the left mapping with deeply nested values from the right
+ mapping.
+
+ This function is useful to merge the results of several calls to
+ :func:`MerkleNode.collect`.
+
+ Arguments:
+ left: a mapping (modified by the update operation)
+ right: a mapping
+
+ Returns:
+ the left mapping, updated with nested values from the right mapping
+
+ Example:
+ >>> a = {
+ ... 'key1': {
+ ... 'key2': {
+ ... 'key3': 'value1/2/3',
+ ... },
+ ... },
+ ... }
+ >>> deep_update(a, {
+ ... 'key1': {
+ ... 'key2': {
+ ... 'key4': 'value1/2/4',
+ ... },
+ ... },
+ ... }) == {
+ ... 'key1': {
+ ... 'key2': {
+ ... 'key3': 'value1/2/3',
+ ... 'key4': 'value1/2/4',
+ ... },
+ ... },
+ ... }
+ True
+ >>> deep_update(a, {
+ ... 'key1': {
+ ... 'key2': {
+ ... 'key3': 'newvalue1/2/3',
+ ... },
+ ... },
+ ... }) == {
+ ... 'key1': {
+ ... 'key2': {
+ ... 'key3': 'newvalue1/2/3',
+ ... 'key4': 'value1/2/4',
+ ... },
+ ... },
+ ... }
+ True
+
+ """
+ for key, rvalue in right.items():
+ if isinstance(rvalue, collections.Mapping):
+ new_lvalue = deep_update(left.get(key, {}), rvalue)
+ left[key] = new_lvalue
+ else:
+ left[key] = rvalue
+ return left
+
+
+class MerkleNode(dict, metaclass=abc.ABCMeta):
+ """Representation of a node in a Merkle Tree.
+
+ A (generalized) `Merkle Tree`_ is a tree in which every node is labeled
+ with a hash of its own data and the hash of its children.
+
+ .. _Merkle Tree: https://en.wikipedia.org/wiki/Merkle_tree
+
+ In pseudocode::
+
+ node.hash = hash(node.data
+ + sum(child.hash for child in node.children))
+
+ This class efficiently implements the Merkle Tree data structure on top of
+ a Python :class:`dict`, minimizing hash computations and new data
+ collections when updating nodes.
+
+ Node data is stored in the :attr:`data` attribute, while (named) children
+ are stored as items of the underlying dictionary.
+
+ Addition, update and removal of objects are instrumented to automatically
+ invalidate the hashes of the current node as well as its registered
+ parents; It also resets the collection status of the objects so the updated
+ objects can be collected.
+
+ The collection of updated data from the tree is implemented through the
+ :func:`collect` function and associated helpers.
+
+ Attributes:
+ data (dict): data associated to the current node
+ parents (list): known parents of the current node
+ collected (bool): whether the current node has been collected
+
+ """
+ __slots__ = ['parents', 'data', '__hash', 'collected']
+
+ type = None
+ """Type of the current node (used as a classifier for :func:`collect`)"""
+
+ def __init__(self, data=None):
+ super().__init__()
+ self.parents = []
+ self.data = data
+ self.__hash = None
+ self.collected = False
+
+ def invalidate_hash(self):
+ """Invalidate the cached hash of the current node."""
+ if not self.__hash:
+ return
+
+ self.__hash = None
+ self.collected = False
+ for parent in self.parents:
+ parent.invalidate_hash()
+
+ def update_hash(self, *, force=False):
+ """Recursively compute the hash of the current node.
+
+ Args:
+ force (bool): invalidate the cache and force the computation for
+ this node and all children.
+ """
+ if self.__hash and not force:
+ return self.__hash
+
+ if force:
+ self.invalidate_hash()
+
+ for child in self.values():
+ child.update_hash(force=force)
+
+ self.__hash = self.compute_hash()
+ return self.__hash
+
+ @property
+ def hash(self):
+ """The hash of the current node, as calculated by
+ :func:`compute_hash`.
+ """
+ return self.update_hash()
+
+ @abc.abstractmethod
+ def compute_hash(self):
+ """Compute the hash of the current node.
+
+ The hash should depend on the data of the node, as well as on hashes
+ of the children nodes.
+ """
+ raise NotImplementedError('Must implement compute_hash method')
+
+ def __setitem__(self, name, new_child):
+ """Add a child, invalidating the current hash"""
+ self.invalidate_hash()
+
+ super().__setitem__(name, new_child)
+
+ new_child.parents.append(self)
+
+ def __delitem__(self, name):
+ """Remove a child, invalidating the current hash"""
+ if name in self:
+ self.invalidate_hash()
+ self[name].parents.remove(self)
+ super().__delitem__(name)
+ else:
+ raise KeyError(name)
+
+ def update(self, new_children):
+ """Add several named children from a dictionary"""
+ if not new_children:
+ return
+
+ self.invalidate_hash()
+
+ for name, new_child in new_children.items():
+ new_child.parents.append(self)
+ if name in self:
+ self[name].parents.remove(self)
+
+ super().update(new_children)
+
+ def get_data(self, **kwargs):
+ """Retrieve and format the collected data for the current node, for use by
+ :func:`collect`.
+
+ Can be overridden, for instance when you want the collected data to
+ contain information about the child nodes.
+
+ Arguments:
+ kwargs: allow subclasses to alter behaviour depending on how
+ :func:`collect` is called.
+
+ Returns:
+ data formatted for :func:`collect`
+ """
+ return self.data
+
+ def collect_node(self, **kwargs):
+ """Collect the data for the current node, for use by :func:`collect`.
+
+ Arguments:
+ kwargs: passed as-is to :func:`get_data`.
+
+ Returns:
+ A :class:`dict` compatible with :func:`collect`.
+ """
+ if not self.collected:
+ self.collected = True
+ return {self.type: {self.hash: self.get_data(**kwargs)}}
+ else:
+ return {}
+
+ def collect(self, **kwargs):
+ """Collect the data for all nodes in the subtree rooted at `self`.
+
+ The data is deduplicated by type and by hash.
+
+ Arguments:
+ kwargs: passed as-is to :func:`get_data`.
+
+ Returns:
+ A :class:`dict` with the following structure::
+
+ {
+ 'typeA': {
+ node1.hash: node1.get_data(),
+ node2.hash: node2.get_data(),
+ },
+ 'typeB': {
+ node3.hash: node3.get_data(),
+ ...
+ },
+ ...
+ }
+ """
+ ret = self.collect_node(**kwargs)
+ for child in self.values():
+ deep_update(ret, child.collect(**kwargs))
+
+ return ret
+
+ def reset_collect(self):
+ """Recursively unmark collected nodes in the subtree rooted at `self`.
+
+ This lets the caller use :func:`collect` again.
+ """
+ self.collected = False
+
+ for child in self.values():
+ child.reset_collect()
+
+
+class MerkleLeaf(MerkleNode):
+ """A leaf to a Merkle tree.
+
+ A Merkle leaf is simply a Merkle node with children disabled.
+ """
+ __slots__ = []
+
+ def __setitem__(self, name, child):
+ raise ValueError('%s is a leaf' % self.__class__.__name__)
+
+ def __getitem__(self, name):
+ raise ValueError('%s is a leaf' % self.__class__.__name__)
+
+ def __delitem__(self, name):
+ raise ValueError('%s is a leaf' % self.__class__.__name__)
+
+ def update(self, new_children):
+ """Children update operation. Disabled for leaves."""
+ raise ValueError('%s is a leaf' % self.__class__.__name__)
diff --git a/swh/model/tests/test_merkle.py b/swh/model/tests/test_merkle.py
new file mode 100644
--- /dev/null
+++ b/swh/model/tests/test_merkle.py
@@ -0,0 +1,229 @@
+# Copyright (C) 2017 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
+
+import unittest
+
+from swh.model import merkle
+
+
+class TestedMerkleNode(merkle.MerkleNode):
+ type = 'tested_merkle_node_type'
+
+ def __init__(self, data):
+ super().__init__(data)
+ self.compute_hash_called = 0
+
+ def compute_hash(self):
+ self.compute_hash_called += 1
+ child_data = [
+ child + b'=' + self[child].hash
+ for child in sorted(self)
+ ]
+
+ return (
+ b'hash('
+ + b', '.join([self.data['value']] + child_data)
+ + b')'
+ )
+
+
+class TestedMerkleLeaf(merkle.MerkleLeaf):
+ type = 'tested_merkle_leaf_type'
+
+ def __init__(self, data):
+ super().__init__(data)
+ self.compute_hash_called = 0
+
+ def compute_hash(self):
+ self.compute_hash_called += 1
+ return b'hash(' + self.data['value'] + b')'
+
+
+class TestMerkleLeaf(unittest.TestCase):
+ def setUp(self):
+ self.data = {'value': b'value'}
+ self.instance = TestedMerkleLeaf(self.data)
+
+ def test_hash(self):
+ self.assertEqual(self.instance.compute_hash_called, 0)
+ instance_hash = self.instance.hash
+ self.assertEqual(self.instance.compute_hash_called, 1)
+ instance_hash2 = self.instance.hash
+ self.assertEqual(self.instance.compute_hash_called, 1)
+ self.assertEqual(instance_hash, instance_hash2)
+
+ def test_data(self):
+ self.assertEqual(self.instance.get_data(), self.data)
+
+ def test_collect(self):
+ collected = self.instance.collect()
+ self.assertEqual(
+ collected, {
+ self.instance.type: {
+ self.instance.hash: self.instance.get_data(),
+ },
+ },
+ )
+ collected2 = self.instance.collect()
+ self.assertEqual(collected2, {})
+ self.instance.reset_collect()
+ collected3 = self.instance.collect()
+ self.assertEqual(collected, collected3)
+
+ def test_leaf(self):
+ with self.assertRaisesRegex(ValueError, 'is a leaf'):
+ self.instance[b'key1'] = 'Test'
+
+ with self.assertRaisesRegex(ValueError, 'is a leaf'):
+ del self.instance[b'key1']
+
+ with self.assertRaisesRegex(ValueError, 'is a leaf'):
+ self.instance[b'key1']
+
+ with self.assertRaisesRegex(ValueError, 'is a leaf'):
+ self.instance.update(self.data)
+
+
+class TestMerkleNode(unittest.TestCase):
+ maxDiff = None
+
+ def setUp(self):
+ self.root = TestedMerkleNode({'value': b'root'})
+ self.nodes = {b'root': self.root}
+ for i in (b'a', b'b', b'c'):
+ value = b'root/' + i
+ node = TestedMerkleNode({
+ 'value': value,
+ })
+ self.root[i] = node
+ self.nodes[value] = node
+ for j in (b'a', b'b', b'c'):
+ value2 = value + b'/' + j
+ node2 = TestedMerkleNode({
+ 'value': value2,
+ })
+ node[j] = node2
+ self.nodes[value2] = node2
+ for k in (b'a', b'b', b'c'):
+ value3 = value2 + b'/' + j
+ node3 = TestedMerkleNode({
+ 'value': value3,
+ })
+ node2[j] = node3
+ self.nodes[value3] = node3
+
+ def test_hash(self):
+ for node in self.nodes.values():
+ self.assertEqual(node.compute_hash_called, 0)
+
+ # Root hash will compute hash for all the nodes
+ hash = self.root.hash
+ for node in self.nodes.values():
+ self.assertEqual(node.compute_hash_called, 1)
+ self.assertIn(node.data['value'], hash)
+
+ # Should use the cached value
+ hash2 = self.root.hash
+ self.assertEqual(hash, hash2)
+ for node in self.nodes.values():
+ self.assertEqual(node.compute_hash_called, 1)
+
+ # Should still use the cached value
+ hash3 = self.root.update_hash(force=False)
+ self.assertEqual(hash, hash3)
+ for node in self.nodes.values():
+ self.assertEqual(node.compute_hash_called, 1)
+
+ # Force update of the cached value for a deeply nested node
+ self.root[b'a'][b'b'].update_hash(force=True)
+ for key, node in self.nodes.items():
+ # update_hash rehashes all children
+ if key.startswith(b'root/a/b'):
+ self.assertEqual(node.compute_hash_called, 2)
+ else:
+ self.assertEqual(node.compute_hash_called, 1)
+
+ hash4 = self.root.hash
+ self.assertEqual(hash, hash4)
+ for key, node in self.nodes.items():
+ # update_hash also invalidates all parents
+ if key in (b'root', b'root/a') or key.startswith(b'root/a/b'):
+ self.assertEqual(node.compute_hash_called, 2)
+ else:
+ self.assertEqual(node.compute_hash_called, 1)
+
+ def test_collect(self):
+ collected = self.root.collect()
+ self.assertEqual(len(collected[self.root.type]), len(self.nodes))
+ for node in self.nodes.values():
+ self.assertTrue(node.collected)
+ collected2 = self.root.collect()
+ self.assertEqual(collected2, {})
+
+ def test_get(self):
+ for key in (b'a', b'b', b'c'):
+ self.assertEqual(self.root[key], self.nodes[b'root/' + key])
+
+ with self.assertRaisesRegex(KeyError, "b'nonexistent'"):
+ self.root[b'nonexistent']
+
+ def test_del(self):
+ hash_root = self.root.hash
+ hash_a = self.nodes[b'root/a'].hash
+ del self.root[b'a'][b'c']
+ hash_root2 = self.root.hash
+ hash_a2 = self.nodes[b'root/a'].hash
+
+ self.assertNotEqual(hash_root, hash_root2)
+ self.assertNotEqual(hash_a, hash_a2)
+
+ self.assertEqual(self.nodes[b'root/a/c'].parents, [])
+
+ with self.assertRaisesRegex(KeyError, "b'nonexistent'"):
+ del self.root[b'nonexistent']
+
+ def test_update(self):
+ hash_root = self.root.hash
+ hash_b = self.root[b'b'].hash
+ new_children = {
+ b'c': TestedMerkleNode({'value': b'root/b/new_c'}),
+ b'd': TestedMerkleNode({'value': b'root/b/d'}),
+ }
+
+ # collect all nodes
+ self.root.collect()
+
+ self.root[b'b'].update(new_children)
+
+ # Ensure everyone got reparented
+ self.assertEqual(new_children[b'c'].parents, [self.root[b'b']])
+ self.assertEqual(new_children[b'd'].parents, [self.root[b'b']])
+ self.assertEqual(self.nodes[b'root/b/c'].parents, [])
+
+ hash_root2 = self.root.hash
+ self.assertNotEqual(hash_root, hash_root2)
+ self.assertIn(b'root/b/new_c', hash_root2)
+ self.assertIn(b'root/b/d', hash_root2)
+
+ hash_b2 = self.root[b'b'].hash
+ self.assertNotEqual(hash_b, hash_b2)
+
+ for key, node in self.nodes.items():
+ if key in (b'root', b'root/b'):
+ self.assertEqual(node.compute_hash_called, 2)
+ else:
+ self.assertEqual(node.compute_hash_called, 1)
+
+ # Ensure we collected root, root/b, and both new children
+ collected_after_update = self.root.collect()
+ self.assertCountEqual(
+ collected_after_update[TestedMerkleNode.type],
+ [self.nodes[b'root'].hash, self.nodes[b'root/b'].hash,
+ new_children[b'c'].hash, new_children[b'd'].hash],
+ )
+
+ # test that noop updates doesn't invalidate anything
+ self.root[b'a'][b'b'].update({})
+ self.assertEqual(self.root.collect(), {})

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