Add flat-slice dataset access (getDataFlat with offset)

jhdf/src/main/java/io/jhdf/WritableDatasetImpl.java

@@ -124,6 +124,11 @@ public Object getDataFlat() {
 		return flatten(data);
 	}
 
+	@Override
+	public Object getDataFlat(long[] sliceOffset, int[] sliceDimensions) {
+		throw new HdfWritingException("Slicing a writable dataset not supported");
+	}
+
 	@Override
 	public Object getData(long[] sliceOffset, int[] sliceDimensions) {
 		throw new HdfWritingException("Slicing a writable dataset not supported");

jhdf/src/main/java/io/jhdf/api/Dataset.java

@@ -130,6 +130,22 @@ public interface Dataset extends Node {
 	 */
 	Object getDataFlat();
 
+	/**
+	 * Gets a slice of data from the HDF5 dataset and converts it to a Java object, it differs from
+	 * {@link #getData(long[], int[])} as the data will be flat (i.e. 1D). Can offer improved performance over
+	 * {@link #getData(long[], int[])} for multiple dimension arrays.
+	 * <p>
+	 * The type of the returned array will be that returned by {@link #getJavaType()}. The size of the array returned
+	 * will be the product of the slice dimensions.
+	 *
+	 * @param sliceOffset     the position in the dataset the slice starts. Must have length equal to number of dimensions
+	 * @param sliceDimensions the dimensions of the slice to return. Must have length equal to number of dimensions
+	 * @return the slice of data from the dataset as a 1D array
+	 * @throws InvalidSliceHdfException if the <code>sliceOffset</code> or <code>sliceDimensions</code> are invalid
+	 * @throws HdfException             if the dataset can't be sliced
+	 */
+	Object getDataFlat(long[] sliceOffset, int[] sliceDimensions);
+
 	/**
 	 * Gets a slice of data from the HDF5 dataset and converts it to a Java object.
 	 * <p>

jhdf/src/main/java/io/jhdf/dataset/DatasetBase.java

@@ -157,6 +157,28 @@ public Object getDataFlat() {
 		return data;
 	}
 
+	@Override
+	public Object getDataFlat(long[] sliceOffset, int[] sliceDimensions) {
+		if (isEmpty()) {
+			throw new HdfException("Cannot slice empty dataset");
+		}
+		if (isScalar()) {
+			throw new HdfException("Cannot slice scalar dataset");
+		}
+
+		validateSliceRequest(sliceOffset, sliceDimensions);
+
+		logger.debug("Getting flat data slice offset={} dimensions={} for [{}]'...", sliceOffset, sliceDimensions, getPath());
+		ByteBuffer sliceDataBuffer = getSliceDataBuffer(sliceOffset, sliceDimensions);
+
+		long elements = 1;
+		for (int dimension : sliceDimensions) {
+			elements = Math.multiplyExact(elements, dimension);
+		}
+
+		return DatasetReader.readDataset(getDataType(), sliceDataBuffer, Math.toIntExact(elements), hdfBackingStorage);
+	}
+
 	@Override
 	public Object getData(long[] sliceOffset, int[] sliceDimensions) {
 		if (isEmpty()) {

jhdf/src/test/java/io/jhdf/dataset/SlicingTest.java

@@ -16,12 +16,14 @@
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
 
+import static io.jhdf.Utils.flatten;
 import static io.jhdf.Utils.getDimensions;
 import static org.apache.commons.lang3.ArrayUtils.subarray;
 import static org.apache.commons.lang3.ArrayUtils.toObject;
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.arrayContaining;
 import static org.hamcrest.Matchers.is;
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 class SlicingTest {
@@ -66,6 +68,20 @@ void test3DSliceIntDataset() {
 		assertThat(toObject(slicedData[1][4]), is(subarray(fullData[1][4], 0, 10)));
 	}
 
+	@Test
+	void test3DSliceIntDatasetFlat() {
+		Dataset dataset = hdfFile.getDatasetByPath("/nD_Datasets/3D_int32");
+
+		long[] sliceOffset = new long[]{0, 2, 30};
+		int[] sliceDimensions = new int[]{1, 3, 15};
+
+		Object slicedData = dataset.getData(sliceOffset, sliceDimensions);
+		Object[] expectedFlat = flatten(slicedData);
+
+		int[] slicedFlat = (int[]) dataset.getDataFlat(sliceOffset, sliceDimensions);
+		assertArrayEquals(expectedFlat, toObject(slicedFlat));
+	}
+
 	@Test
 	void test3DSliceFloatDataset() {
 		Dataset dataset = hdfFile.getDatasetByPath("/nD_Datasets/3D_float32");
@@ -126,11 +142,13 @@ void testInvalidSliceRequests() {
 	void testEmptyDatasetThrows() {
 		Dataset emptyDataset = scalarEmptyDatasetsHdfFile.getDatasetByPath("empty_uint_32");
 		assertThrows(HdfException.class, () -> emptyDataset.getData(new long[]{3}, new int[]{3}));
+		assertThrows(HdfException.class, () -> emptyDataset.getDataFlat(new long[]{3}, new int[]{3}));
 	}
 
 	@Test
 	void testScalarDatasetThrows() {
 		Dataset emptyDataset = scalarEmptyDatasetsHdfFile.getDatasetByPath("scalar_float_64");
 		assertThrows(HdfException.class, () -> emptyDataset.getData(new long[]{3}, new int[]{3}));
+		assertThrows(HdfException.class, () -> emptyDataset.getDataFlat(new long[]{3}, new int[]{3}));
 	}
 }