84 lines
3.5 KiB
Go
84 lines
3.5 KiB
Go
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// Copyright 2017 The go-ethereum Authors
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// This file is part of the go-ethereum library.
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//
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// The go-ethereum library is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Lesser General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// The go-ethereum library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU Lesser General Public License for more details.
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//
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// You should have received a copy of the GNU Lesser General Public License
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// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
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/*
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Package pot (proximity order tree) implements a container similar to a binary tree.
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The elements are generic Val interface types.
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Each fork in the trie is itself a value. Values of the subtree contained under
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a node all share the same order when compared to other elements in the tree.
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Example of proximity order is the length of the common prefix over bitvectors.
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(which is equivalent to the reverse rank of order of magnitude of the MSB first X
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OR distance over finite set of integers).
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Methods take a comparison operator (pof, proximity order function) to compare two
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value types. The default pof assumes Val to be or project to a byte slice using
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the reverse rank on the MSB first XOR logarithmic disctance.
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If the address space if limited, equality is defined as the maximum proximity order.
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The container offers applicative (funcional) style methods on PO trees:
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* adding/removing en element
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* swap (value based add/remove)
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* merging two PO trees (union)
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as well as iterator accessors that respect proximity order
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When synchronicity of membership if not 100% requirement (e.g. used as a database
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of network connections), applicative structures have the advantage that nodes
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are immutable therefore manipulation does not need locking allowing for
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concurrent retrievals.
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For the use case where the entire container is supposed to allow changes by
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concurrent routines,
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Pot
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* retrieval, insertion and deletion by key involves log(n) pointer lookups
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* for any item retrieval (defined as common prefix on the binary key)
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* provide synchronous iterators respecting proximity ordering wrt any item
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* provide asynchronous iterator (for parallel execution of operations) over n items
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* allows cheap iteration over ranges
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* asymmetric concurrent merge (union)
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Note:
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* as is, union only makes sense for set representations since which of two values
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with equal keys survives is random
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* intersection is not implemented
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* simple get accessor is not implemented (but derivable from EachNeighbour)
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Pinned value on the node implies no need to copy keys of the item type.
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Note that
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* the same set of values allows for a large number of alternative
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POT representations.
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* values on the top are accessed faster than lower ones and the steps needed to
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retrieve items has a logarithmic distribution.
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As a consequence one can organise the tree so that items that need faster access
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are torwards the top. In particular for any subset where popularity has a power
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distriution that is independent of proximity order (content addressed storage of
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chunks), it is in principle possible to create a pot where the steps needed to
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access an item is inversely proportional to its popularity.
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Such organisation is not implemented as yet.
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TODO:
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* overwrite-style merge
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* intersection
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* access frequency based optimisations
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*/
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package pot
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