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96 lines
2.7 KiB
96 lines
2.7 KiB
/*
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* Fast QR Code generator library
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*
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* Copyright (c) Project Nayuki. (MIT License)
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* https://www.nayuki.io/page/fast-qr-code-generator-library
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy of
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* this software and associated documentation files (the "Software"), to deal in
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* the Software without restriction, including without limitation the rights to
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* use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
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* the Software, and to permit persons to whom the Software is furnished to do so,
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* subject to the following conditions:
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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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* - 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 the
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* Software.
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*/
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package io.nayuki.fastqrcodegen;
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import java.lang.ref.SoftReference;
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import java.util.HashSet;
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import java.util.Map;
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import java.util.Set;
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import java.util.concurrent.ConcurrentHashMap;
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import java.util.function.Function;
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// A thread-safe cache based on soft references.
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final class Memoizer<T,R> {
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private final Function<T,R> function;
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Map<T,SoftReference<R>> cache = new ConcurrentHashMap<>();
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private Set<T> pending = new HashSet<>();
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// Creates a memoizer based on the given function that takes one input to compute an output.
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public Memoizer(Function<T,R> func) {
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function = func;
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}
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// Computes function.apply(arg) or returns a cached copy of a previous call.
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public R get(T arg) {
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// Non-blocking fast path
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{
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SoftReference<R> ref = cache.get(arg);
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if (ref != null) {
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R result = ref.get();
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if (result != null)
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return result;
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}
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}
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// Sequential slow path
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while (true) {
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synchronized(this) {
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SoftReference<R> ref = cache.get(arg);
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if (ref != null) {
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R result = ref.get();
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if (result != null)
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return result;
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cache.remove(arg);
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}
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assert !cache.containsKey(arg);
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if (pending.add(arg))
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break;
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try {
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this.wait();
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} catch (InterruptedException e) {
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throw new RuntimeException(e);
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}
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}
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}
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try {
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R result = function.apply(arg);
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cache.put(arg, new SoftReference<>(result));
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return result;
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} finally {
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synchronized(this) {
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pending.remove(arg);
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this.notifyAll();
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}
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}
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}
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}
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