1 files changed, 83 insertions, 0 deletions

diff --git a/subprojects/viatra-runtime-matchers/src/main/java/tools/refinery/viatra/runtime/matchers/algorithms/OrderedIterableMerge.java b/subprojects/viatra-runtime-matchers/src/main/java/tools/refinery/viatra/runtime/matchers/algorithms/OrderedIterableMerge.java
new file mode 100644
index 00000000..1917e909
--- /dev/null
+++ b/subprojects/viatra-runtime-matchers/src/main/java/tools/refinery/viatra/runtime/matchers/algorithms/OrderedIterableMerge.java
@@ -0,0 +1,83 @@
+/*******************************************************************************
+ * Copyright (c) 2010-2018, Gabor Bergmann, IncQuery Labs Ltd.
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-v20.html.
+ * 
+ * SPDX-License-Identifier: EPL-2.0
+ *******************************************************************************/
+package tools.refinery.viatra.runtime.matchers.algorithms;
+
+import java.util.Comparator;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+
+/**
+ * @author Gabor Bergmann
+ * @since 2.1
+ *
+ */
+public class OrderedIterableMerge {
+    
+    private OrderedIterableMerge() {
+        // Hidden utility class constructor
+    }
+    
+    /**
+     * Lazily merges two iterables, each ordered according to a given comparator. 
+     * Retains order in the result, and also eliminates any duplicates that appear in both arguments.
+     */
+    public static <T> Iterable<T> mergeUniques(Iterable<T> first, Iterable<T> second, Comparator<T> comparator) {
+        return () -> new Iterator<T>() {
+            Iterator<T> firstIterator = first.iterator();
+            Iterator<T> secondIterator = second.iterator();
+            T firstItem;
+            T secondItem;
+            
+            {
+                fetchFirst();
+                fetchSecond();
+            }
+            
+            private T fetchFirst() {
+                T previous = firstItem;
+                if (firstIterator.hasNext()) 
+                    firstItem = firstIterator.next();
+                else 
+                    firstItem = null;
+                return previous;
+            }
+            private T fetchSecond() {
+               T previous = secondItem;
+               if (secondIterator.hasNext()) 
+                    secondItem = secondIterator.next();
+                else 
+                    secondItem = null;
+               return previous;
+            }
+            
+            @Override
+            public boolean hasNext() {
+                return firstItem != null || secondItem != null;
+            }
+            @Override
+            public T next() {
+                if (!hasNext()) throw new NoSuchElementException();
+                if (firstItem != null && secondItem != null) {
+                    if (secondItem == firstItem) { // duplicates
+                        fetchFirst();
+                        return fetchSecond();
+                    } else if (comparator.compare(firstItem, secondItem) < 0) {
+                        return fetchFirst();
+                    } else {
+                        return fetchSecond();
+                    }
+                } else if (firstItem != null) {
+                    return fetchFirst();
+                } else { // secondItem must be non-null
+                    return fetchSecond();
+                }
+            }
+        };
+    }
+}