Java superpermutation finder

Main.java

package com.helospark;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Set;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ConcurrentSkipListSet;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

class CustomStringBuilder {
    private char[] string;
    private int endIndex;

    public CustomStringBuilder(int length) {
        string = new char[length];
        endIndex = 0;
    }

    public CustomStringBuilder(CustomStringBuilder currentString) {
        this.string = new char[currentString.string.length];
        this.endIndex = currentString.endIndex;
        for (int i = 0; i < currentString.string.length; ++i) {
            this.string[i] = currentString.string[i];
        }
    }

    public char charAt(int index) {
        return string[index];
    }

    public void trimTo(int index) {
        endIndex = index;
    }

    @Override
    public String toString() {
        String result = "";
        for (int i = 0; i < endIndex; ++i) {
            result += string[i];
        }
        return result;
    }

    public int length() {
        return endIndex;
    }

    public void append(char charAt) {
        string[endIndex] = charAt;
        ++endIndex;
    }
}

class CustomList {
    private String[] elements;

    public CustomList(int size) {
        elements = new String[size];
    }

    public CustomList(List<String> permutations) {
        elements = new String[permutations.size()];
        for (int i = 0; i < permutations.size(); ++i) {
            elements[i] = permutations.get(i);
        }
    }

    public CustomList(CustomList permutations) {
        int previousSize = permutations.elements.length;
        this.elements = new String[previousSize];
        for (int i = 0; i < previousSize; ++i) {
            this.elements[i] = permutations.elements[i];
        }
    }

    public void swap(int i, int j) {
        String tmp = elements[i];
        elements[i] = elements[j];
        elements[j] = tmp;
    }

    public String get(int i) {
        return elements[i];
    }
}

class TaskHolder {
    private CustomStringBuilder string;
    private CustomList list;
    private int listSize;

    private TaskHolder(Builder builder) {
        this.string = builder.string;
        this.list = builder.list;
        this.listSize = builder.listSize;
    }

    public CustomStringBuilder getString() {
        return string;
    }

    public CustomList getList() {
        return list;
    }

    public int getListSize() {
        return listSize;
    }

    public static Builder builder() {
        return new Builder();
    }

    public static final class Builder {
        private CustomStringBuilder string;
        private CustomList list;
        private int listSize;

        private Builder() {
        }

        public Builder withString(CustomStringBuilder string) {
            this.string = string;
            return this;
        }

        public Builder withList(CustomList list) {
            this.list = list;
            return this;
        }

        public Builder withListSize(int listSize) {
            this.listSize = listSize;
            return this;
        }

        public TaskHolder build() {
            return new TaskHolder(this);
        }
    }
}

class SuperPermutationTask {
    public static final int MAX_PRODUCER_DEPTH = 9;
    private int number;
    BlockingQueue<TaskHolder> queue;
    volatile boolean hasMoreElement = true;
    volatile AtomicInteger maxSize;
    volatile AtomicInteger numberOfThreadsRunning = new AtomicInteger(0);
    volatile Set<String> output;

    private SuperPermutationTask(Builder builder) {
        this.number = builder.number;
        this.queue = builder.queue;
        this.hasMoreElement = builder.hasMoreElement;
        this.maxSize = builder.maxSize;
        this.output = builder.output;
    }

    public static Builder builder() {
        return new Builder();
    }

    public static final class Builder {
        private int number;
        private BlockingQueue<TaskHolder> queue;
        private boolean hasMoreElement;
        private AtomicInteger maxSize;
        private Set<String> output = Collections.emptySet();

        private Builder() {
        }

        public Builder withNumber(int number) {
            this.number = number;
            return this;
        }

        public Builder withQueue(BlockingQueue<TaskHolder> queue) {
            this.queue = queue;
            return this;
        }

        public Builder withHasMoreElement(boolean hasMoreElement) {
            this.hasMoreElement = hasMoreElement;
            return this;
        }

        public Builder withMaxSize(AtomicInteger maxSize) {
            this.maxSize = maxSize;
            return this;
        }

        public Builder withOutput(Set<String> output) {
            this.output = output;
            return this;
        }

        public SuperPermutationTask build() {
            return new SuperPermutationTask(this);
        }
    }

}

public class Main {

    public static void main(String[] args) {
        for (int i = 4; i < 7; ++i) {
            String input = createString(i);
            SuperPermutationTask processResult = process(input);
            waitUntilTaskFinished(processResult);
            findShortest(input, processResult.output);
        }
    }

    private static void waitUntilTaskFinished(SuperPermutationTask processResult) {
        while (processResult.hasMoreElement || processResult.numberOfThreadsRunning.get() > 0) {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

    }

    private static void findShortest(String input, Set<String> processResult) {
        String shortest = "";
        for (String a : processResult) {
            if (shortest.isEmpty() || a.length() < shortest.length()) {
                shortest = a;
            }
        }
        System.err.println("######### " + input + " " + shortest + " " + shortest.length());
    }

    private static String createString(int end) {
        String result = "";
        for (int i = 1; i <= end; ++i) {
            result += i;
        }
        return result;
    }

    static final int NUM_TASKS = Runtime.getRuntime().availableProcessors();

    static ExecutorService executorService = Executors.newFixedThreadPool(NUM_TASKS + 1);

    static SuperPermutationTask process(String chars) {
        List<String> permutations = new ArrayList<>();
        permutation("", chars, permutations);

        int maxSize = permutations.size() * chars.length();

        SuperPermutationTask task = SuperPermutationTask.builder()
                .withHasMoreElement(true)
                .withMaxSize(new AtomicInteger(maxSize))
                .withQueue(new LinkedBlockingQueue<>(10))
                .withOutput(new ConcurrentSkipListSet<>())
                .build();

        executorService
                .submit(() -> superPermutationProducer(new CustomList(permutations), permutations.size(), new CustomStringBuilder(maxSize), 0, task));

        for (int i = 0; i < NUM_TASKS; ++i) {
            task.numberOfThreadsRunning.incrementAndGet();
            executorService.submit(() -> consumer(task));
        }

        return task;
    }

    static void superPermutationProducer(CustomList permutations, int listSize, CustomStringBuilder currentString, int currentDepth,
            SuperPermutationTask task) {
        if (currentDepth == task.MAX_PRODUCER_DEPTH || listSize == 0 || currentString.length() >= task.maxSize.get()) {
            TaskHolder taskHolder = TaskHolder.builder()
                    .withList(new CustomList(permutations))
                    .withListSize(listSize)
                    .withString(new CustomStringBuilder(currentString))
                    .build();
            try {
                task.queue.put(taskHolder);
            } catch (Exception e) {
                System.out.println(e);
            }
            return;
        }
        for (int i = 0; i < listSize; ++i) {
            int originalSize = currentString.length();
            CustomStringBuilder modifiedString = mergeCurrentString(currentString, permutations.get(i));
            permutations.swap(i, listSize - 1);
            superPermutationProducer(permutations, listSize - 1, modifiedString, currentDepth + 1, task);
            modifiedString.trimTo(originalSize);
            permutations.swap(i, listSize - 1);
        }
        if (currentDepth == 0) {
            task.hasMoreElement = false;
        }
    }

    static void consumer(SuperPermutationTask task) {
        try {
            while (task.hasMoreElement) {
                TaskHolder taskHolder = task.queue.poll(10, TimeUnit.SECONDS);
                if (taskHolder != null) {
                    System.out.println("Consuming new task");
                    int currentMaxSize = task.maxSize.get();
                    int newMaxSize = superPermutationConsumer(taskHolder.getList(), taskHolder.getListSize(), taskHolder.getString(), currentMaxSize,
                            task.output);
                    synchronized (task.maxSize) {
                        int previous = task.maxSize.get();
                        if (newMaxSize < previous) {
                            task.maxSize.set(newMaxSize);
                        }
                    }
                    System.out.println("Current min: " + task.maxSize.get());
                }
            }
        } catch (Exception e) {
            System.out.println(e);
        }
        task.numberOfThreadsRunning.decrementAndGet();
    }

    static int superPermutationConsumer(CustomList permutations, int listSize, CustomStringBuilder currentString, int maxSize, Set<String> output) {
        if (currentString.length() >= maxSize) {
            return maxSize;
        }
        if (listSize == 0) {
            System.out.println("Found: '" + currentString + "' length: " + currentString.length() + "      " + System.currentTimeMillis());
            output.add(currentString.toString());
            return currentString.length();
        }
        int newMaxSize = maxSize;
        for (int i = 0; i < listSize; ++i) {
            int originalSize = currentString.length();
            CustomStringBuilder modifiedString = mergeCurrentString(currentString, permutations.get(i));
            permutations.swap(i, listSize - 1);
            int foundSize = superPermutationConsumer(permutations, listSize - 1, modifiedString, newMaxSize, output);
            if (foundSize < newMaxSize) {
                newMaxSize = foundSize;
            }
            modifiedString.trimTo(originalSize);
            permutations.swap(i, listSize - 1);
        }
        return newMaxSize;
    }

    static CustomStringBuilder mergeCurrentString(CustomStringBuilder currentString, String toMergeWith) {
        int i = Math.min(toMergeWith.length(), currentString.length());
        for (; i >= 0; --i) {
            boolean doesMatch = true;
            for (int j = 0; j < i; ++j) {
                char a = currentString.charAt(currentString.length() - i + j);
                char b = toMergeWith.charAt(j);
                if (a != b) {
                    doesMatch = false;
                    break;
                }
            }
            if (doesMatch) {
                break;
            }
        }

        for (int j = i; j < toMergeWith.length(); ++j) {
            currentString.append(toMergeWith.charAt(j));
        }
        return currentString;
    }

    private static void permutation(String prefix, String str, List<String> result) {
        int n = str.length();
        if (n == 0) {
            result.add(prefix);
        } else {
            for (int i = 0; i < n; i++) {
                permutation(prefix + str.charAt(i), str.substring(0, i) + str.substring(i + 1, n), result);
            }
        }
    }

}