Add AbstractFilteringList as a super for such lists

1 files changed, 80 insertions, 0 deletions

diff --git a/vespajlib/src/main/java/com/yahoo/collections/AbstractFilteringList.java b/vespajlib/src/main/java/com/yahoo/collections/AbstractFilteringList.java
new file mode 100644
index 00000000000..efcdeeaebfc
--- /dev/null
+++ b/vespajlib/src/main/java/com/yahoo/collections/AbstractFilteringList.java
@@ -0,0 +1,80 @@
+package com.yahoo.collections;
+
+import java.util.Collection;
+import java.util.Comparator;
+import java.util.HashSet;
+import java.util.List;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.BiFunction;
+import java.util.function.Function;
+import java.util.function.Predicate;
+import java.util.stream.Stream;
+
+import static java.util.stream.Collectors.reducing;
+import static java.util.stream.Collectors.toUnmodifiableList;
+
+/**
+ * Abstract, immutable list for subclassing with concrete types, and domain specific filters.
+ *
+ * @author jonmv
+ */
+public abstract class AbstractFilteringList<Type, ListType extends AbstractFilteringList<Type, ListType>> {
+
+    private final List<Type> items;
+    private final boolean negate;
+    private final BiFunction<List<Type>, Boolean, ListType> constructor;
+
+    // Subclass constructor:
+    // private SomeFilteringList(Collection<? extends SomeType> items, boolean negate) {
+    //     super(items, negate, SomeFilteringList::new);
+    // }
+    protected AbstractFilteringList(Collection<? extends Type> items, boolean negate, BiFunction<List<Type>, Boolean, ListType> constructor) {
+        this.items = List.copyOf(items);
+        this.negate = negate;
+        this.constructor = constructor;
+    }
+
+    /** Negates the next filter operation. All other operations which return a new list reset this modifier. */
+    public final ListType not() {
+        return constructor.apply(items, ! negate);
+    }
+
+    /** Returns a new list which is the result of filtering with the -- possibly negated -- condition. */
+    public final ListType matching(Predicate<Type> condition) {
+        return constructor.apply(items.stream().filter(negate ? condition.negate() : condition).collect(toUnmodifiableList()), false);
+    }
+
+    /** Returns the first n items in this list, or everything except those if negated. */
+    public ListType first(int n) {
+        n = Math.min(n, items.size());
+        return constructor.apply(items.subList(negate ? n : 0, negate ? items.size() : n), false);
+    }
+
+    /** Returns the subset of items in this which are (not) present in the other list. */
+    public ListType in(ListType others) {
+        return matching(new HashSet<>(others.asList())::contains);
+    }
+
+    /** Returns the union of the two lists. */
+    public ListType and(ListType others) {
+        return constructor.apply(Stream.concat(items.stream(), others.asList().stream()).collect(toUnmodifiableList()), false);
+    }
+
+    /** Returns the items in this as an immutable list. */
+    public final List<Type> asList() { return items; }
+
+    /** Returns the items in this as an immutable list after mapping with the given function. */
+    public final <OtherType> List<OtherType> mapToList(Function<Type, OtherType> mapper) {
+        return items.stream().map(mapper).collect(toUnmodifiableList());
+    }
+
+    /** Returns the items sorted by the given comparator. */
+    public final ListType sortedBy(Comparator<? super Type> comparator) {
+        return constructor.apply(items.stream().sorted(comparator).collect(toUnmodifiableList()), false);
+    }
+
+    public final boolean isEmpty() { return items.isEmpty(); }
+
+    public final int size() { return items.size(); }
+
+}