Map of small maps

[Clive Cox]

Hi,

 My use case is to use ChronicleMap for Map<Long,Map<String,Float>> or Map<Long,Map<Integer,Float>> where each of the value maps is usually quite small, e.g. 1-10 entries.
 I was thinking maybe I could create a BytesMarshalable class with int size, and two arrays for the variable length sets of data. Is this the best way/sensible way to handle this use case efficiently?
 If so, I'm not sure how to tell ChronicleMap the entry set size for variable sized data. Or do I not need to?

 Clive

[Roman Leventov]

Yes, your are right.

You can use this marshaller:

public final class IntFloatMapMarshaller
        implements BytesReader<Map<Integer, Float>>, BytesWriter<Map<Integer, Float>> {

    public static final IntFloatMapMarshaller INSTANCE = new IntFloatMapMarshaller();

    private IntFloatMapMarshaller() {}

    @NotNull
    @Override
    public Map<Integer, Float> read(@NotNull Bytes bytes, long sizeInBytes) {
        return read(bytes, sizeInBytes, null);
    }

    @NotNull
    @Override
    public Map<Integer, Float> read(@NotNull Bytes bytes, long sizeInBytes,
                                    @Nullable Map<Integer, Float> toReuse) {
        int mapSize = (int) (sizeInBytes / 8);
        if (toReuse == null) {
            // Might want to create a more efficient, specialized Map here, instead of HashMap

            // * 4 / 3 because HashMap's default load factor is 0.75
            // This is java.util.HashMap-specific, not needed for a different Map impl
            int hashMapCapacity = mapSize * 4 / 3 + 1;
            toReuse = new HashMap<>(hashMapCapacity);
        } else {
            toReuse.clear();
        }
        for (int i = 0; i < mapSize; i++) {
            toReuse.put(bytes.readInt(), bytes.readFloat());
        }
        return toReuse;
    }

    @Override
    public long size(@NotNull Map<Integer, Float> map) {
        return map.size() * 8L;
    }

    @Override
    public void write(@NotNull Bytes bytes, @NotNull Map<Integer, Float> map) {
        map.forEach((k, v) -> {
            bytes.writeInt(k);
            bytes.writeFloat(v);
        });
    }
}

Then specify:

ChronicleMapBuilder.of(...)..

   .valueMarshallers(IntFloatMapMarshaller.INSTANCE,  IntFloatMapMarshaller.INSTANCE)

   .averageValueSize(MY_AVERAGE_MAP_SIZE * 8) /* multiplying by 8 designates one entry takes 8 bytes in memory */

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