swarm: network rewrite merge

swarm/pot/doc.go

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