001/*
002 * Copyright 2010-2013 JetBrains s.r.o.
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 org.jetbrains.jet.lang.types;
018
019public enum Variance {
020    INVARIANT("", true, true, 0),
021    IN_VARIANCE("in", true, false, -1),
022    OUT_VARIANCE("out", false, true, +1);
023
024    private final String label;
025    private final boolean allowsInPosition;
026    private final boolean allowsOutPosition;
027    private final int superpositionFactor;
028
029    Variance(String label, boolean allowsInPosition, boolean allowsOutPosition, int superpositionFactor) {
030        this.label = label;
031        this.allowsInPosition = allowsInPosition;
032        this.allowsOutPosition = allowsOutPosition;
033        this.superpositionFactor = superpositionFactor;
034    }
035
036    public boolean allowsInPosition() {
037        return allowsInPosition;
038    }
039
040    public boolean allowsOutPosition() {
041        return allowsOutPosition;
042    }
043
044    public Variance superpose(Variance other) {
045        int r = this.superpositionFactor * other.superpositionFactor;
046        switch (r) {
047            case  0: return INVARIANT;
048            case -1: return IN_VARIANCE;
049            case +1: return OUT_VARIANCE;
050        }
051        throw new IllegalStateException();
052    }
053
054    public Variance opposite() {
055        switch (this) {
056            case INVARIANT:
057                return INVARIANT;
058            case IN_VARIANCE:
059                return OUT_VARIANCE;
060            case OUT_VARIANCE:
061                return IN_VARIANCE;
062        }
063        throw new IllegalStateException("Impossible variance: " + this);
064    }
065
066    @Override
067    public String toString() {
068        return label;
069    }
070}