StringGenerator.java

/******************************************************************************
 * Copyright (C) 2014 Yevgeny Krasik                                          *
 *                                                                            *
 * 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.github.ykrasik.jaci.util.trie;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

/**
 * @author Yevgeny Krasik
 */
public class StringGenerator {
    private static final char DEFAULT_START_CHAR = 'a';

    private final char startChar;
    private final int numChars;

    public StringGenerator(int numChars) {
        this(DEFAULT_START_CHAR, numChars);
    }

    public StringGenerator(char startChar, int numChars) {
        this.startChar = startChar;
        this.numChars = numChars;
    }

    public List<String> generateAllFixedLengthStringPermutations(int length) {
        if (length == 0) {
            return Collections.singletonList("");
        }

        final List<String> permutations = generateAllFixedLengthStringPermutations(length - 1);
        final List<String> strings = new ArrayList<>(permutations.size() * numChars);
        for (int i = 0; i < numChars; i++) {
            final char c = charPlus(startChar, i);
            for (String permutation : permutations) {
                strings.add(c + permutation);
            }
        }
        return strings;
    }

    private char charPlus(char c, int i) {
        return (char) (c + i);
    }
}