111 lines
3.0 KiB
Java
111 lines
3.0 KiB
Java
/*
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* The MIT License
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* Copyright © 2014-2021 Ilkka Seppälä
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package com.iluwatar.trampoline;
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import java.util.stream.Stream;
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/**
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* Trampoline pattern allows to define recursive algorithms by iterative loop.
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*
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* <p>When get is called on the returned Trampoline, internally it will iterate calling ‘jump’
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* on the returned Trampoline as long as the concrete instance returned is {@link
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* #more(Trampoline)}, stopping once the returned instance is {@link #done(Object)}.
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*
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* <p>Essential we convert looping via recursion into iteration,
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* the key enabling mechanism is the fact that {@link #more(Trampoline)} is a lazy operation.
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*
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* @param <T> is type for returning result.
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*/
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public interface Trampoline<T> {
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T get();
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/**
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* Jump to next stage.
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*
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* @return next stage
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*/
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default Trampoline<T> jump() {
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return this;
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}
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default T result() {
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return get();
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}
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/**
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* Checks if complete.
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*
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* @return true if complete
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*/
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default boolean complete() {
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return true;
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}
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/**
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* Created a completed Trampoline.
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*
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* @param result Completed result
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* @return Completed Trampoline
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*/
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static <T> Trampoline<T> done(final T result) {
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return () -> result;
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}
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/**
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* Create a Trampoline that has more work to do.
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*
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* @param trampoline Next stage in Trampoline
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* @return Trampoline with more work
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*/
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static <T> Trampoline<T> more(final Trampoline<Trampoline<T>> trampoline) {
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return new Trampoline<T>() {
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@Override
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public boolean complete() {
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return false;
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}
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@Override
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public Trampoline<T> jump() {
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return trampoline.result();
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}
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@Override
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public T get() {
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return trampoline(this);
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}
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T trampoline(final Trampoline<T> trampoline) {
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return Stream.iterate(trampoline, Trampoline::jump)
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.filter(Trampoline::complete)
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.findFirst()
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.map(Trampoline::result)
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.get();
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}
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};
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}
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}
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