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-rw-r--r--src/algorithms/sorting/other/TopologicalSort.java243
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diff --git a/src/algorithms/sorting/other/TopologicalSort.java b/src/algorithms/sorting/other/TopologicalSort.java
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+/**
+ * Copyright © 2014 Mattias Andrée (maandree@member.fsf.org)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program 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 Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+package algorithms.sorting.other;
+
+import java.util.*;
+
+
+/**
+ * Topological sorting class. There are a few ways to perform
+ * a topological sort, this class implements an topological
+ * sort with advisory dependencies and strict dependencies.
+ * Dependencies are broken when there are cyclic dependencies,
+ * but if the dependency is strict rather than advisory, the
+ * algorithm fails. Additionally, this this, implementation
+ * uses two input parameters, a map from items to sets of
+ * dependencies, and a map from items, to a set of strict
+ * dependencies. The strict dependencies must appear in both
+ * of the sets, whilst the advisory dependencies must only
+ * appear in the first set of dependencies. This implemention
+ * tries to minimise the number of transversially broken
+ * dependencies. It is advisable to use both hash based maps
+ * and sets.
+ *
+ * @param <T> The data type that has been sorted
+ */
+public class TopologicalSort<T>
+{
+ /**
+ * Constructor
+ */
+ private TopologicalSort()
+ {
+ /* Do nothing */
+ }
+
+
+
+ /**
+ * List of missing dependencies. There may be duplicates,
+ * see {@link #missingDependent}.
+ */
+ public ArrayList<T> missingDependency = new ArrayList<T>();
+
+ /**
+ * List of items with missing dependencies. For any give index
+ * i, the element in {@link #missingDependency} with index i
+ * is required by the element with index i in this list.
+ */
+ public ArrayList<T> missingDependent = new ArrayList<T>();
+
+ /**
+ * The items in topological order
+ */
+ public ArrayList<T> order = new ArrayList<T>();
+
+ /**
+ * If an items exists as a key in this map it has been used
+ * to break a cyclic dependency. The associated value of a
+ * key is a set of all direct dependencies that will appear
+ * later in the topologically ordered list {@link #order}.
+ */
+ public HashMap<T, HashSet<T>> breaks = new HashMap<T, HashSet<T>>();
+
+ /**
+ * Whether the topological sort was successful, i.e., not
+ * absolute dependencies needed to be broken. If this value
+ * is {@code false}, the topologically ordered list and
+ * the map of cyclic dependency breaks may no be complete.
+ */
+ public boolean successful = false;
+
+
+
+ /**
+ * Topolocally sort data.
+ *
+ * @param dependencies Map for items to their advisory dependencies, will be modified
+ * @param absolutes Map for items to their absolute dependencies
+ * @return A data structure with the result of the topological sort
+ */
+ @SuppressWarnings("unchecked")
+ public static <T> TopologicalSort<T> tsort(Map<T, Set<T>> dependencies, Map<T, Set<T>> absolutes)
+ {
+ TopologicalSort<T> rc = new TopologicalSort<T>();
+ if (absolutes == null) /* sugar */
+ absolutes = new HashMap<T, Set<T>>();
+
+ /* Find, list and remove missing dependencies */
+ {
+ HashMap<T, ArrayList<T>> removed = new HashMap<T, ArrayList<T>>();
+ for (Map.Entry<T, Set<T>> req_deps : dependencies.entrySet())
+ {
+ T req = req_deps.getKey();
+ for (T dep : req_deps.getValue())
+ if (dependencies.containsKey(dep) == false)
+ {
+ rc.missingDependency.add(dep);
+ rc.missingDependent.add(req);
+ if (removed.containsKey(dep) == false)
+ removed.put(dep, new ArrayList<T>());
+ removed.get(dep).add(req);
+ }
+ }
+ for (Map.Entry<T, ArrayList<T>> remove_reqs : removed.entrySet())
+ {
+ T remove = remove_reqs.getKey();
+ for (T req : remove_reqs.getValue())
+ dependencies.get(req).remove(remove);
+ }
+ }
+
+ /* Sort cyclic graph topologically */
+ {
+ ArrayList<T> removed = new ArrayList<T>();
+ removed.add(null);
+ while (removed.size() > 0)
+ {
+ /* Sort acyclic graph topologically */
+ while (removed.size() > 0)
+ {
+ removed.clear();
+ /* Report and remove items with no unresolved dependency */
+ T[] dependencies_ = (T[])(new Object[dependencies.size()]); /* needed to avoid concurrent modification */
+ int i = 0;
+ for (T key : dependencies.keySet())
+ dependencies_[i++] = key;
+ for (T item : dependencies_)
+ if (dependencies.get(item).size() == 0)
+ {
+ rc.order.add(item);
+ removed.add(item);
+ dependencies.remove(item);
+ }
+ /* Remove reported items from dependency lists */
+ for (Map.Entry<T, Set<T>> req_deps : dependencies.entrySet())
+ {
+ T req = req_deps.getKey();
+ Set<T> deps = req_deps.getValue();
+ for (T old : removed)
+ if (deps.contains(old))
+ deps.remove(old);
+ }
+ }
+
+ /* Break one cycle with as few transversial dependencies as possible */
+ if (dependencies.size() > 0)
+ {
+ T best = null;
+ HashSet<T> bestBreaks = null;
+ /* Find best cycle break */
+ for (Map.Entry<T, Set<T>> item_deps : dependencies.entrySet())
+ {
+ T item = item_deps.getKey();
+ Set<T> queue_ = item_deps.getValue();
+ Set<T> abs_ = absolutes.get(item);
+
+ HashSet<T> abs = new HashSet<T>();
+ if (abs_ != null)
+ for (T elem : abs_)
+ abs.add(elem);
+
+ HashSet<T> deps = new HashSet<T>();
+ T[] queue = (T[])(new Object[queue_.size()]);
+ int queue_h = 0, queue_t = 0;
+ for (T elem : queue_)
+ queue[queue_t++] = elem;
+ deps.add(item);
+ boolean bad = false;
+
+ while (queue_h != queue_t)
+ {
+ T dep = queue[queue_h++];
+ if (abs.contains(dep))
+ {
+ bad = true;
+ break;
+ }
+ if (deps.contains(dep))
+ continue;
+ deps.add(dep);
+
+ Set<T> extra = dependencies.get(dep);
+ queue_t -= queue_h;
+ System.arraycopy(queue, queue_h, queue = (T[])(new Object[queue_t + extra.size()]), 0, queue_t);
+ queue_h = 0;
+ for (T elem : extra)
+ queue[queue_t++] = elem;
+
+ if ((extra = absolutes.get(dep)) != null)
+ for (T elem : extra)
+ abs.add(elem);
+ }
+
+ if (bad)
+ continue;
+ deps.remove(item);
+ if ((best == null) || (bestBreaks.size() > deps.size()))
+ {
+ best = item;
+ bestBreaks = deps;
+ }
+ }
+
+ /* List cycle break and remove item */
+ if (best == null)
+ return rc;
+ rc.order.add(best);
+ rc.breaks.put(best, bestBreaks);
+ removed.add(best);
+ dependencies.remove(best);
+
+ /* Remove item from dependency lists */
+ for (Map.Entry<T, Set<T>> item_deps : dependencies.entrySet())
+ {
+ T item = item_deps.getKey();
+ Set<T> deps = item_deps.getValue();
+ if (deps.contains(item) == false)
+ deps.remove(best);
+ }
+ }
+ }
+ }
+
+ rc.successful = true;
+ return rc;
+ }
+
+}
+
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