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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.dataflow.constantpropagation;

import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.dataflow.analysis.Store;
import org.checkerframework.dataflow.cfg.node.IntegerLiteralNode;
import org.checkerframework.dataflow.cfg.node.LocalVariableNode;
import org.checkerframework.dataflow.cfg.node.Node;
import org.checkerframework.dataflow.cfg.visualize.CFGVisualizer;
import org.checkerframework.dataflow.expression.JavaExpression;
import org.plumelib.util.ArrayMap;
import org.plumelib.util.CollectionsPlume;

import java.util.LinkedHashMap;
import java.util.Map;

/** A store that records information about constant values. */
public class ConstantPropagationStore implements Store {

    /** Information about variables gathered so far. */
    private final Map contents;

    /** Creates a new ConstantPropagationStore. */
    public ConstantPropagationStore() {
        contents = new LinkedHashMap<>();
    }

    protected ConstantPropagationStore(Map contents) {
        this.contents = contents;
    }

    public Constant getInformation(Node n) {
        if (contents.containsKey(n)) {
            return contents.get(n);
        }
        return new Constant(Constant.Type.TOP);
    }

    public void mergeInformation(Node n, Constant val) {
        Constant value;
        if (contents.containsKey(n)) {
            value = val.leastUpperBound(contents.get(n));
        } else {
            value = val;
        }
        // TODO: remove (only two nodes supported atm)
        assert n instanceof IntegerLiteralNode || n instanceof LocalVariableNode;
        contents.put(n, value);
    }

    public void setInformation(Node n, Constant val) {
        // TODO: remove (only two nodes supported atm)
        assert n instanceof IntegerLiteralNode || n instanceof LocalVariableNode;
        contents.put(n, val);
    }

    @Override
    public ConstantPropagationStore copy() {
        return new ConstantPropagationStore(new LinkedHashMap<>(contents));
    }

    @Override
    public ConstantPropagationStore leastUpperBound(ConstantPropagationStore other) {
        Map newContents =
                ArrayMap.newArrayMapOrLinkedHashMap(contents.size() + other.contents.size());

        // go through all of the information of the other class
        for (Map.Entry e : other.contents.entrySet()) {
            Node n = e.getKey();
            Constant otherVal = e.getValue();
            if (contents.containsKey(n)) {
                // merge if both contain information about a variable
                newContents.put(n, otherVal.leastUpperBound(contents.get(n)));
            } else {
                // add new information
                newContents.put(n, otherVal);
            }
        }

        for (Map.Entry e : contents.entrySet()) {
            Node n = e.getKey();
            Constant thisVal = e.getValue();
            if (!other.contents.containsKey(n)) {
                // add new information
                newContents.put(n, thisVal);
            }
        }

        return new ConstantPropagationStore(newContents);
    }

    @Override
    public ConstantPropagationStore widenedUpperBound(ConstantPropagationStore previous) {
        return leastUpperBound(previous);
    }

    @Override
    public boolean equals(@Nullable Object o) {
        if (o == null) {
            return false;
        }
        if (!(o instanceof ConstantPropagationStore)) {
            return false;
        }
        ConstantPropagationStore other = (ConstantPropagationStore) o;
        // go through all of the information of the other object
        for (Map.Entry e : other.contents.entrySet()) {
            Node n = e.getKey();
            Constant otherVal = e.getValue();
            if (otherVal.isBottom()) {
                continue; // no information
            }
            if (contents.containsKey(n)) {
                if (!otherVal.equals(contents.get(n))) {
                    return false;
                }
            } else {
                return false;
            }
        }
        // go through all of the information of the this object
        for (Map.Entry e : contents.entrySet()) {
            Node n = e.getKey();
            Constant thisVal = e.getValue();
            if (thisVal.isBottom()) {
                continue; // no information
            }
            if (other.contents.containsKey(n)) {
                continue;
            } else {
                return false;
            }
        }
        return true;
    }

    @Override
    public int hashCode() {
        int s = 0;
        for (Map.Entry e : contents.entrySet()) {
            if (!e.getValue().isBottom()) {
                s += e.hashCode();
            }
        }
        return s;
    }

    @Override
    public String toString() {
        // Only output local variable information.
        // This output is very terse, so a CFG containing it fits well in the manual.
        Map contentsLocalVars =
                new LinkedHashMap<>(CollectionsPlume.mapCapacity(contents));
        for (Map.Entry e : contents.entrySet()) {
            if (e.getKey() instanceof LocalVariableNode) {
                contentsLocalVars.put(e.getKey(), e.getValue());
            }
        }
        return contentsLocalVars.toString();
    }

    @Override
    public boolean canAlias(JavaExpression a, JavaExpression b) {
        return true;
    }

    @Override
    public String visualize(CFGVisualizer viz) {
        return viz.visualizeStoreKeyVal("constant propagation", toString());
    }
}