public final class PartwiseMinimumBase extends Object
compute(ImmutableGraph, NodeColouringStrategy, ArcColouringStrategy)starts from a graph (possibly with a colouring on its nodes and/or on its arcs) and returns an array, say
a, with exactly as many elements as there are nodes in the graph, and with the following properties:
yare in the same fibre of the minimal (coloured, if the graph is such) fibrations;
- the values contained in
arange from 0 to
kis the number of nodes in the minimum base;
- the values in
aare assigned canonically, that is, if
bis the array returned by the method on an isomorphic graph (with the same colours, if the graph is coloured) and if
frepresents the isomorphism, then
a[x]=b[f(x)]for every node
The algorithm is a partwise variant of that implemented by
MinimumBase. The algorithm keeps a list of touched
parts, as opposed to a list of touched nodes, resulting in less memory
consumption and, in practice, in faster execution and reduced memory usage, even if it is not
possible for this implementation to prove the good bounds given for
MinimumBase. The canonical labels created by the two
algorithms, however, are identical.
In the following, by partition of a set we mean a subdivision of the set into a number of nonempty disjoint subsets, called parts.
The algorithm execution happens in rounds; at the end of each round, a certain partition of the nodes is established. The starting partition is the one determined by node colours, if any, or it is simply the trivial partition with just one part. At every round, the old partition is refined (i.e., some of the parts are further subdivided into subparts). The algorithm stops as soon as no part is actually subdivided at the end of a round: the final partition is the desired one (i.e., the nodes are partitioned according to the fibres of the minimum fibration).
Basic data structures
Current partition: The current partition is stored into two arrays: the first,
part simply contains a permutation of the nodes with the property that
nodes belonging to the same part appear consecutively; the second, called
contains, for each node, the index of
part where its class belongs to. More
formally, suppose that
part[end-1] is one of the parts; then, if
x=part[j] for some
end (i.e., if
x is one of
the nodes in the part) we have
start[x]=begin. In the following, unless otherwise
specified, we shall identify a part with its starting index in the array
Active parts: At the beginning of each round, there is a certain set of active
parts; their number is stored in
numActiveParts, and they are stored in the
startActivePart array, in arbitrary order.
Touched parts: During each round, some of the parts are deemed as touched; their
number (at the end of the round) is stored in
touchedListLength, and they are stored
touchedList array; additionally, there is an array of boolean values,
touched such that
touched[i] is true iff
i is the
starting index of a touched part.
First phase: assigning labels to nodes
The final aim of the first phase is to assign to each node
x a label that is the
list of all nodes
y that have an arc towards
x and appear in some
active parts; such labels (whose length cannot be larger than the indegree of
will be contained in the array
inFrom[x], its length being stored in
To obtain this result, the algorithm scans all the nodes in all the active parts, and for each
y considers all outgoing arcs, writing
y in all the labels of
the target nodes of such arcs.
In this phase, we mark all parts containing at least one target node as touched.
Second phase: refining touched parts
We consider all touched parts, in the (arbitrary) order in which we find them in the touched
list. In this phase (some of) these parts will be partitioned into subparts: this amounts in
permuting the portion of the array
part where the part is stored, and changing
some of the entries of the array
start (those that are relative to nodes in the
part that is being subpartitioned).
Note that at a certain point of this phase we have some parts that have already been subpartitioned (we call them completed), a part that is being considered (we call it current) and some other touched parts that will be considered later on.
Suppose that the current part starts at index
begin and ends at index
end (exclusive). First of all, for each node
x in the part, the label
inFrom[x][0..inFill[x]-1] is sorted according to the following lexicographic order:
- if the colour of the arc (
x) is smaller than the colour of (
ymust appear before
- if the colour of the arc (
x) is the same as the colour of (
x), but the part of
yis smaller than the part of
ymust appear before
After sorting, the current part is subdivided according to the equivalence relation induced by
the previously described lexicographic order. Some care must be taken, though: when comparing the
y' we are considering the new partitioning for the
completed parts, but we use the old partitioning for the current part (i.e., nodes in the current
part are considered to have partition number
Modifier and Type Method Description
ImmutableGraph g, NodeColouringStrategy nodeColouring, ArcColouringStrategy arcColouring)(Returns a labelling of an immutable graph such that two nodes have the same label iff they are in the same fibre of minimal fibrations.
computepublic static int compute(ImmutableGraph g, NodeColouringStrategy nodeColouring, ArcColouringStrategy arcColouring)Returns a labelling of an immutable graph such that two nodes have the same label iff they are in the same fibre of minimal fibrations.
Note that the labelling is surjective—if a node has label k, there are nodes with label j, for every 0≤j≤k.
g- an immutable graph.
nodeColouring- a colouring for the nodes, or
arcColouring- a colouring for the arcs, or
- an array of integers labelling the graph so that two nodes have the same label iff they are in the same fibre of minimal fibrations.