template<typename GR, typename V, typename C, typename TR>
class lemon::CostScaling< GR, V, C, TR >
CostScaling implements a cost scaling algorithm that performs push/augment and relabel operations for finding a minimum cost flow [1], [17], [18], [2]. It is a highly efficient primal-dual solution method, which can be viewed as the generalization of the preflow push-relabel algorithm for the maximum flow problem. It is a polynomial algorithm, its running time complexity is , where K denotes the maximum arc cost.
In general, NetworkSimplex and CostScaling are the fastest implementations available in LEMON for solving this problem. (For more information, see the module page.)
Most of the parameters of the problem (except for the digraph) can be given using separate functions, and the algorithm can be executed using the run() function. If some parameters are not specified, then default values will be used.
- Template Parameters
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GR | The digraph type the algorithm runs on. |
V | The number type used for flow amounts, capacity bounds and supply values in the algorithm. By default, it is int . |
C | The number type used for costs and potentials in the algorithm. By default, it is the same as V . |
TR | The traits class that defines various types used by the algorithm. By default, it is CostScalingDefaultTraits<GR, V, C>. In most cases, this parameter should not be set directly, consider to use the named template parameters instead. |
- Warning
- Both
V
and C
must be signed number types.
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All input data (capacities, supply values, and costs) must be integer.
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This algorithm does not support negative costs for arcs having infinite upper bound.
- Note
- CostScaling provides three different internal methods, from which the most efficient one is used by default. For more information, see Method.
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| CostScaling (const GR &graph) |
| Constructor. More...
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The parameters of the algorithm can be specified using these functions.
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template<typename LowerMap > |
CostScaling & | lowerMap (const LowerMap &map) |
| Set the lower bounds on the arcs. More...
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template<typename UpperMap > |
CostScaling & | upperMap (const UpperMap &map) |
| Set the upper bounds (capacities) on the arcs. More...
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template<typename CostMap > |
CostScaling & | costMap (const CostMap &map) |
| Set the costs of the arcs. More...
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template<typename SupplyMap > |
CostScaling & | supplyMap (const SupplyMap &map) |
| Set the supply values of the nodes. More...
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CostScaling & | stSupply (const Node &s, const Node &t, Value k) |
| Set single source and target nodes and a supply value. More...
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The algorithm can be executed using run().
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ProblemType | run (Method method=PARTIAL_AUGMENT, int factor=16) |
| Run the algorithm. More...
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CostScaling & | resetParams () |
| Reset all the parameters that have been given before. More...
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CostScaling & | reset () |
| Reset the internal data structures and all the parameters that have been given before. More...
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The results of the algorithm can be obtained using these functions.
The run() function must be called before using them.
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template<typename Number > |
Number | totalCost () const |
| Return the total cost of the found flow. More...
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Value | flow (const Arc &a) const |
| Return the flow on the given arc. More...
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template<typename FlowMap > |
void | flowMap (FlowMap &map) const |
| Copy the flow values (the primal solution) into the given map. More...
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Cost | potential (const Node &n) const |
| Return the potential (dual value) of the given node. More...
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template<typename PotentialMap > |
void | potentialMap (PotentialMap &map) const |
| Copy the potential values (the dual solution) into the given map. More...
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Enum type containing constants for selecting the internal method for the run() function.
CostScaling provides three internal methods that differ mainly in their base operations, which are used in conjunction with the relabel operation. By default, the so called Partial Augment-Relabel method is used, which turned out to be the most efficient and the most robust on various test inputs. However, the other methods can be selected using the run() function with the proper parameter.
Enumerator |
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PUSH |
Local push operations are used, i.e. flow is moved only on one admissible arc at once.
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AUGMENT |
Augment operations are used, i.e. flow is moved on admissible paths from a node with excess to a node with deficit.
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PARTIAL_AUGMENT |
Partial augment operations are used, i.e. flow is moved on admissible paths started from a node with excess, but the lengths of these paths are limited. This method can be viewed as a combined version of the previous two operations.
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This function runs the algorithm. The paramters can be specified using functions lowerMap(), upperMap(), costMap(), supplyMap(), stSupply(). For example,
* CostScaling<ListDigraph> cs(graph);
* cs.lowerMap(lower).upperMap(upper).costMap(cost)
* .supplyMap(sup).run();
*
This function can be called more than once. All the given parameters are kept for the next call, unless resetParams() or reset() is used, thus only the modified parameters have to be set again. If the underlying digraph was also modified after the construction of the class (or the last reset() call), then the reset() function must be called.
- Parameters
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method | The internal method that will be used in the algorithm. For more information, see Method. |
factor | The cost scaling factor. It must be at least two. |
- Returns
INFEASIBLE
if no feasible flow exists,
OPTIMAL
if the problem has optimal solution (i.e. it is feasible and bounded), and the algorithm has found optimal flow and node potentials (primal and dual solutions),
UNBOUNDED
if the digraph contains an arc of negative cost and infinite upper bound. It means that the objective function is unbounded on that arc, however, note that it could actually be bounded over the feasible flows, but this algroithm cannot handle these cases.
- See Also
- ProblemType, Method
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resetParams(), reset()
This function resets all the paramaters that have been given before using functions lowerMap(), upperMap(), costMap(), supplyMap(), stSupply().
It is useful for multiple run() calls. Basically, all the given parameters are kept for the next run() call, unless resetParams() or reset() is used. If the underlying digraph was also modified after the construction of the class or the last reset() call, then the reset() function must be used, otherwise resetParams() is sufficient.
For example,
* CostScaling<ListDigraph> cs(graph);
*
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* cs.lowerMap(lower).upperMap(upper).costMap(cost)
* .supplyMap(sup).run();
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* cost[e] += 100;
* cs.costMap(cost).run();
*
*
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* cs.resetParams();
* cs.upperMap(capacity).costMap(cost)
* .supplyMap(sup).run();
*
- Returns
(*this)
- See Also
- reset(), run()