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/*
* Copyright (c) 2013-2018 Cinchapi Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.cinchapi.common.collect;
import java.util.AbstractList;
import java.util.List;
import java.util.function.Function;
import java.util.function.Supplier;
import com.google.common.collect.Lists;
/**
* A {@link Continuation} is a {@link List} whose elements are generated on
* the fly using a {@link Function} or {@link Supplier}. Therefore, the size of
* the list is "infinite" (e.g. {@link Integer#MAX_VALUE}. Previously generated
* elements are remembered such that subsequent access to an index is always
* idempotent.
*
* A {@link Continuation} cannot be directly mutated with {@link #add(Object)}
* or {@link #remove(int)} methods.
*
*
* @author Jeff Nelson
*/
public class Continuation extends AbstractList {
/**
* Return a {@link Continuation} that contains elements supplied by the
* {@code generator}.
*
* The provided {@code generator} will be passed the index of an element
* that is being accessed in the list, which may be useful in generating the
* element that should be contained at the index.
*
*
* @param generator
* @return a new {@link Continuation}
*/
public static Continuation of(Function generator) {
return new Continuation<>(generator);
}
/**
* Return a {@link Continuation} that contains elements supplied by the
* {@code generator}.
*
* @param generator
* @return a new {@link Continuation}
*/
public static Continuation of(Supplier generator) {
return of(ignore -> generator.get());
}
/**
* The list of previously generated elements.
*/
private final List generated;
/**
* The generating function. The function should return an element, given the
* element's index.
*/
private final Function generator;
/**
* Construct a new instance.
*
* @param generator
*/
public Continuation(Function generator) {
this.generator = generator;
this.generated = Lists.newArrayList();
}
@Override
public T get(int index) {
for (int i = generated.size(); i <= index; ++i) {
// The requested index is larger than the current list. So, backfill
// the items between the end of the list and the requested index
// with null.
generated.add(null);
}
T element = generated.get(index);
if(element == null) {
element = generator.apply(index);
generated.set(index, element);
}
return element;
}
@Override
public int size() {
return Integer.MAX_VALUE;
}
}