UnpackedObjectCache.java

/*
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package org.eclipse.jgit.internal.storage.file;

import java.util.concurrent.atomic.AtomicReferenceArray;

import org.eclipse.jgit.lib.AnyObjectId;
import org.eclipse.jgit.lib.ObjectId;

/** Remembers objects that are currently unpacked. */
class UnpackedObjectCache {
    private static final int INITIAL_BITS = 5; // size = 32

    private static final int MAX_BITS = 11; // size = 2048

    private volatile Table table;

    UnpackedObjectCache() {
        table = new Table(INITIAL_BITS);
    }

    boolean isUnpacked(AnyObjectId objectId) {
        return table.contains(objectId);
    }

    void add(AnyObjectId objectId) {
        Table t = table;
        if (t.add(objectId)) {
            // The object either already exists in the table, or was
            // successfully added. Either way leave the table alone.
            //
        } else {
            // The object won't fit into the table. Implement a crude
            // cache removal by just dropping the table away, but double
            // it in size for the next incarnation.
            //
            Table n = new Table(Math.min(t.bits + 1, MAX_BITS));
            n.add(objectId);
            table = n;
        }
    }

    void remove(AnyObjectId objectId) {
        if (isUnpacked(objectId))
            clear();
    }

    void clear() {
        table = new Table(INITIAL_BITS);
    }

    private static class Table {
        private static final int MAX_CHAIN = 8;

        private final AtomicReferenceArray<ObjectId> ids;

        private final int shift;

        final int bits;

        Table(int bits) {
            this.ids = new AtomicReferenceArray<>(1 << bits);
            this.shift = 32 - bits;
            this.bits = bits;
        }

        boolean contains(AnyObjectId toFind) {
            int i = index(toFind);
            for (int n = 0; n < MAX_CHAIN; n++) {
                ObjectId obj = ids.get(i);
                if (obj == null)
                    break;

                if (AnyObjectId.equals(obj, toFind))
                    return true;

                if (++i == ids.length())
                    i = 0;
            }
            return false;
        }

        boolean add(AnyObjectId toAdd) {
            int i = index(toAdd);
            for (int n = 0; n < MAX_CHAIN;) {
                ObjectId obj = ids.get(i);
                if (obj == null) {
                    if (ids.compareAndSet(i, null, toAdd.copy()))
                        return true;
                    else
                        continue;
                }

                if (AnyObjectId.equals(obj, toAdd))
                    return true;

                if (++i == ids.length())
                    i = 0;
                n++;
            }
            return false;
        }

        private int index(AnyObjectId id) {
            return id.hashCode() >>> shift;
        }
    }
}