001// --------------------------------------------------------------------------------
002// Copyright 2002-2024 Echo Three, LLC
003//
004// Licensed under the Apache License, Version 2.0 (the "License");
005// you may not use this file except in compliance with the License.
006// You may obtain a copy of the License at
007//
008//     http://www.apache.org/licenses/LICENSE-2.0
009//
010// Unless required by applicable law or agreed to in writing, software
011// distributed under the License is distributed on an "AS IS" BASIS,
012// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013// See the License for the specific language governing permissions and
014// limitations under the License.
015// --------------------------------------------------------------------------------
016
017package com.echothree.model.control.sequence.server.logic.checksum;
018
019import com.echothree.model.control.sequence.server.logic.SequenceGeneratorLogic;
020import com.google.common.base.Charsets;
021
022public class Mod36SequenceChecksum
023        implements SequenceChecksum {
024
025    private Mod36SequenceChecksum() {
026        super();
027    }
028
029    private static class SequenceChecksumHolder {
030        static SequenceChecksum instance = new Mod36SequenceChecksum();
031    }
032
033    public static SequenceChecksum getInstance() {
034        return SequenceChecksumHolder.instance;
035    }
036
037    @Override
038    public String calculate(String value) {
039        var bytes = value.getBytes(Charsets.UTF_8);
040        int sum = 0;
041
042        for(int i = 0 ; i < bytes.length; i++) {
043            sum += (bytes [i] * (i + 1));
044        }
045
046        char[] modCharacter = { SequenceGeneratorLogic.ALPHANUMERIC_VALUES.charAt(sum % SequenceGeneratorLogic.ALPHANUMERIC_MAX_INDEX) };
047        return new String(modCharacter);
048    }
049
050    @Override
051    public String regexp() {
052        return "[\\p{Upper}\\p{Digit}]";
053    }
054
055    @Override
056    public boolean verify(String value) {
057        var codePoints = value.codePoints().toArray();
058        var length = codePoints.length;
059        var valueWithoutChecksum = new String(codePoints, 0, length - 1);
060        var checksumFromValue = new String(codePoints, length - 1, 1);
061        var checksum = calculate(valueWithoutChecksum);
062
063        return checksumFromValue.equals(checksum);
064    }
065
066}