Blocking flow by Dinic: Difference between revisions

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'''Abstract view:'''
'''Abstract view:'''
# Run a modified [[Depth-first search|DFS]] from <math>s</math> that [[Graph traversal#Remarks|terminates early]] if <math>t</math> is seen.
# Run a [[Depth-first search|DFS]] from <math>s</math> that may [[Graph traversal#Remarks|terminate early]], namely if <math>t</math> is seen.
# If <math>t</math> is not seen, the break condition applies, and the algorithm is terminated.
# If <math>t</math> is not seen, the break condition applies, and the algorithm is terminated.
# Otherwise:
# Otherwise:

Revision as of 04:11, 20 October 2014

General information

Algorithmic problem: Blocking flow.

Type of algorithm: loop.

Abstract view

Invariant: The current flow is feasible.

Variant: The number of arcs decreases.

Break condition: There is no more flow-augmenting ordinary [math]\displaystyle{ (s,t) }[/math]-path in [math]\displaystyle{ G }[/math] (that is, all arcs on the path are forward arcs).

Induction basis

Abstract view: Initialize [math]\displaystyle{ f }[/math] to be a feasible flow, for example, the zero flow.

Implementation: Obvious.

Proof: Obvious.

Induction step

Abstract view:

  1. Run a DFS from [math]\displaystyle{ s }[/math] that may terminate early, namely if [math]\displaystyle{ t }[/math] is seen.
  2. If [math]\displaystyle{ t }[/math] is not seen, the break condition applies, and the algorithm is terminated.
  3. Otherwise:
    1. Let [math]\displaystyle{ p }[/math] be the [math]\displaystyle{ (s,t) }[/math]-path found in step 1.
    2. Let [math]\displaystyle{ \Delta }[/math] be the minimum of the values [math]\displaystyle{ u(a) }[/math] of all arcs [math]\displaystyle{ a }[/math] on </math>.
    3. For each arc [math]\displaystyle{ a }[/math] on [math]\displaystyle{ p }[/math]:
      1. Increase [math]\displaystyle{ f(a) }[/math] by [math]\displaystyle{ \Delta }[/math] and decrease [math]\displaystyle{ u(a) }[/math] by [math]\displaystyle{ \Delta }[/math].
      2. If [math]\displaystyle{ u(a)=0 }[/math], remove [math]\displaystyle{ a }[/math] from [math]\displaystyle{ G }[/math].
      3. If the tail of [math]\displaystyle{ a }[/math] has no outgoing arcs anymore, <math> it is removed as well.