add DatasetAttributes.builder

src/main/java/org/janelia/saalfeldlab/n5/DatasetAttributes.java

@@ -341,6 +341,88 @@ public HashMap<String, Object> asMap() {
 		return map;
 	}
 
+	public static Builder builder(final long[] dimensions, final DataType dataType) {
+
+		return new Builder(dimensions, dataType);
+	}
+
+	public static Builder builder(final DatasetAttributes attributes) {
+
+		return new Builder(attributes);
+	}
+
+	private static final int[] DEFAULT_1D_BLOCK_SIZE = new int[]{65536};
+	private static final int[] DEFAULT_2D_BLOCK_SIZE = new int[]{512,512};
+	private static final int[] DEFAULT_3D_BLOCK_SIZE = new int[]{128,128,128};
+	private static final int DEFAULT_ND_DIM_LEN = 64;
+
+	protected static int[] defaultBlockSize(final long[] dimensions) {
+
+		final int[] blockSize;
+		if (dimensions.length == 1)
+			blockSize = DEFAULT_1D_BLOCK_SIZE.clone();
+		else if (dimensions.length == 2)
+			blockSize = DEFAULT_2D_BLOCK_SIZE.clone();
+		else if (dimensions.length == 3)
+			blockSize = DEFAULT_3D_BLOCK_SIZE.clone();
+		else {
+			blockSize = new int[dimensions.length];
+			Arrays.fill(blockSize, DEFAULT_ND_DIM_LEN);
+		}
+		for (int i = 0; i < blockSize.length; i++)
+			blockSize[i] = (int)Math.min(blockSize[i], dimensions[i]);
+		return blockSize;
+	}
+
+	public static class Builder {
+
+		private final long[] dimensions;
+		private final DataType dataType;
+
+		private int[] blockSize;
+		private DataCodecInfo[] dataCodecInfos = new DataCodecInfo[0];
+
+		public Builder(final long[] dimensions, final DataType dataType) {
+
+			this.dimensions = dimensions.clone();
+			this.dataType = dataType;
+			this.blockSize = defaultBlockSize(dimensions);
+		}
+
+		public Builder(final DatasetAttributes attributes) {
+
+			this.dimensions = attributes.getDimensions();
+			this.dataType = attributes.getDataType();
+			this.blockSize = attributes.getBlockSize();
+			this.dataCodecInfos = attributes.getDataCodecInfos();
+		}
+
+		public Builder blockSize(final int[] blockSize) {
+
+			this.blockSize = blockSize.clone();
+			return this;
+		}
+
+		/**
+		 * Sets the compression codec. Has no effect if {@code compression} is
+		 * null or {@link RawCompression}.
+		 *
+		 * @param compression the compression to use
+		 * @return this builder
+		 */
+		public Builder compression(final Compression compression) {
+
+			if (compression != null && !(compression instanceof RawCompression))
+				this.dataCodecInfos = new DataCodecInfo[]{compression};
+			return this;
+		}
+
+		public DatasetAttributes build() {
+			final int[] resolvedBlockSize = blockSize != null ? blockSize : defaultBlockSize(dimensions);
+			return new DatasetAttributes(dimensions, resolvedBlockSize, dataType, new N5BlockCodecInfo(), null, dataCodecInfos);
+		}
+	}
+
 	private static DatasetAttributesAdapter adapter = null;
 
 	public static DatasetAttributesAdapter getJsonAdapter() {

src/test/java/org/janelia/saalfeldlab/n5/DatasetAttributesTest.java

@@ -1,6 +1,8 @@
 package org.janelia.saalfeldlab.n5;
 
+import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 
@@ -82,6 +84,39 @@ private static DatasetAttributes shardDatasetAttributes(
 		return new DatasetAttributes(dimensions, shardSize, dataType, blockCodecInfo);
 	}
 
+	@Test
+	public void builderTests() {
+
+		final long[] dims = new long[]{100, 200, 300};
+		final int[] blk = new int[]{32, 32, 32};
+
+		// default blockSize uses defaultBlockSize, not full dimensions
+		final DatasetAttributes defaultBlk = DatasetAttributes.builder(dims, DataType.FLOAT32).build();
+		assertArrayEquals(DatasetAttributes.defaultBlockSize(dims), defaultBlk.getBlockSize());
+
+		// blockSize is reflected in output
+		final DatasetAttributes withBlk = DatasetAttributes.builder(dims, DataType.FLOAT32)
+				.blockSize(blk).build();
+		assertArrayEquals(blk, withBlk.getBlockSize());
+		assertFalse(withBlk.isSharded());
+
+		// compression sets a data codec; RawCompression is a no-op
+		final DatasetAttributes withGzip = DatasetAttributes.builder(dims, DataType.FLOAT32)
+				.blockSize(blk).compression(new GzipCompression()).build();
+		assertEquals(1, withGzip.getDataCodecInfos().length);
+
+		final DatasetAttributes withRaw = DatasetAttributes.builder(dims, DataType.FLOAT32)
+				.blockSize(blk).compression(new RawCompression()).build();
+		assertEquals(0, withRaw.getDataCodecInfos().length);
+
+		// round-trip through Builder(DatasetAttributes)
+		final DatasetAttributes roundTrip = DatasetAttributes.builder(withGzip).build();
+		assertArrayEquals(withGzip.getDimensions(), roundTrip.getDimensions());
+		assertArrayEquals(withGzip.getBlockSize(), roundTrip.getBlockSize());
+		assertEquals(withGzip.getDataType(), roundTrip.getDataType());
+		assertEquals(withGzip.getDataCodecInfos().length, roundTrip.getDataCodecInfos().length);
+	}
+
 	/**
 	 * Test that validateBlockShardSizes method rejects invalid shard and block size combinations.
 	 */
@@ -91,7 +126,7 @@ public void testValidateBlockShardSizesInvalid() {
 		final long[] dimensions = new long[]{100, 200, 300};
 		final DataType dataType = DataType.UINT8;
 
-		// Block size too small
+		// Block size too smallcompression != null && !(compression instanceof RawCompression)
 		IllegalArgumentException ex0 = assertThrows(
 				IllegalArgumentException.class,
 				() -> shardDatasetAttributes(dimensions, new int[]{1, 1, 1}, new int[]{1, 0, -1}, dataType));