perf: eliminate byte[] alloc in decode path

Replace toArray()/new byte[]+copy patterns with direct
ByteBuffer/MemorySegment operations: parseFrom(ByteBuffer),
asByteBuffer().put(), and MemorySegment-based unpack in
DeltaCodec. Removes heap round-trips in all five codecs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dfa1

core/src/main/java/io/github/dfa1/vortex/encoding/ConstantCodec.java

@@ -61,11 +61,9 @@ public EncodeResult encode(DType dtype, Object data) {
 	@Override
 	public Array decode(DecodeContext ctx) {
 		MemorySegment scalarBuf = ctx.buffer(0);
-		byte[] scalarBytes = scalarBuf.toArray(ValueLayout.JAVA_BYTE);
-
 		ScalarProtos.ScalarValue scalar;
 		try {
-			scalar = ScalarProtos.ScalarValue.parseFrom(scalarBytes);
+			scalar = ScalarProtos.ScalarValue.parseFrom(scalarBuf.asByteBuffer());
 		} catch (InvalidProtocolBufferException e) {
 			throw new VortexException(CodecId.VORTEX_CONSTANT, "invalid scalar value", e);
 		}
@@ -104,7 +102,7 @@ private static Array decodeString(DecodeContext ctx, ScalarProtos.ScalarValue sc
 		// Store the string bytes once; all n offsets point into the same [0, strLen] range.
 		// Offsets layout: [0, strLen, 0, strLen, ...] — each pair encodes the same slice.
 		MemorySegment bytesSeg = ctx.arena().allocate(strLen);
-		MemorySegment.copy(MemorySegment.ofArray(strBytes), 0, bytesSeg, 0, strLen);
+		bytesSeg.asByteBuffer().put(strBytes);
 
 		// Offsets: n+1 I32 values, alternating start/end of the single string.
 		MemorySegment offsetsSeg = ctx.arena().allocate((n + 1) * 4L, 4);

core/src/main/java/io/github/dfa1/vortex/encoding/DeltaCodec.java

@@ -38,15 +38,16 @@ private static ByteBuffer pack(long[] values, long frameOfRef, int bitWidth) {
 		return ByteBuffer.wrap(buf);
 	}
 
-	private static long[] unpack(byte[] packed, int count, int bitWidth, long frameOfRef) {
+	private static long[] unpack(MemorySegment packed, int count, int bitWidth, long frameOfRef) {
 		long[] out = new long[count];
+		long size = packed.byteSize();
 		for (int i = 0; i < count; i++) {
 			long val = 0L;
 			int bitPos = i * bitWidth;
 			for (int b = 0; b < bitWidth; b++) {
 				int byteIdx = (bitPos + b) >>> 3;
 				int bitIdx = (bitPos + b) & 7;
-				if (byteIdx < packed.length && ((packed[byteIdx] >>> bitIdx) & 1) == 1) {
+				if (byteIdx < size && ((packed.get(ValueLayout.JAVA_BYTE, byteIdx) >>> bitIdx) & 1) == 1) {
 					val |= 1L << b;
 				}
 			}
@@ -238,13 +239,11 @@ public Array decode(DecodeContext ctx) {
 		long rowCount = ctx.rowCount();
 
 		MemorySegment packed = ctx.buffer(0);
-		byte[] packedBytes = new byte[(int) packed.byteSize()];
-		MemorySegment.copy(packed, ValueLayout.JAVA_BYTE, 0, packedBytes, 0, packedBytes.length);
 
 		long[] longs = new long[(int) rowCount];
 		if (rowCount > 0) {
 			longs[0] = baseValue;
-			long[] deltas = unpack(packedBytes, (int) rowCount - 1, bitWidth, deltaFor);
+			long[] deltas = unpack(packed, (int) rowCount - 1, bitWidth, deltaFor);
 			for (int i = 0; i < deltas.length; i++) {
 				longs[i + 1] = longs[i] + deltas[i];
 			}

core/src/main/java/io/github/dfa1/vortex/encoding/DictCodec.java

@@ -237,14 +237,12 @@ public Array decode(DecodeContext ctx) {
 		}
 
 		ByteBuffer meta = ctx.metadata();
-		byte[] metaBytes = new byte[meta.remaining()];
-		meta.duplicate().get(metaBytes);
 
 		// 1-byte = legacy Java format; multi-byte = Rust proto format
-		if (metaBytes.length == 1) {
-			return decodeLegacyJava(ctx, metaBytes[0]);
+		if (meta.remaining() == 1) {
+			return decodeLegacyJava(ctx, meta.get(0));
 		}
-		return decodeRustProto(ctx, metaBytes);
+		return decodeRustProto(ctx, meta.duplicate());
 	}
 
 	private Array decodeLegacyJava(DecodeContext ctx, byte codeTypeByte) {
@@ -265,10 +263,10 @@ private Array decodeLegacyJava(DecodeContext ctx, byte codeTypeByte) {
 		return new Array(ctx.dtype(), rowCount, new MemorySegment[]{out.asReadOnly()}, Array.NO_CHILDREN, ArrayStats.empty());
 	}
 
-	private Array decodeRustProto(DecodeContext ctx, byte[] metaBytes) {
+	private Array decodeRustProto(DecodeContext ctx, ByteBuffer metaBuf) {
 		EncodingProtos.DictMetadata meta;
 		try {
-			meta = EncodingProtos.DictMetadata.parseFrom(metaBytes);
+			meta = EncodingProtos.DictMetadata.parseFrom(metaBuf);
 		} catch (InvalidProtocolBufferException e) {
 			throw new VortexException(CodecId.VORTEX_DICT, "invalid proto metadata", e);
 		}

core/src/main/java/io/github/dfa1/vortex/encoding/SequenceCodec.java

@@ -90,12 +90,9 @@ public Array decode(DecodeContext ctx) {
 		if (metaBuf == null || !metaBuf.hasRemaining()) {
 			throw new VortexException(CodecId.VORTEX_SEQUENCE, "missing metadata");
 		}
-		byte[] metaBytes = new byte[metaBuf.remaining()];
-		metaBuf.duplicate().get(metaBytes);
-
 		EncodingProtos.SequenceMetadata meta;
 		try {
-			meta = EncodingProtos.SequenceMetadata.parseFrom(metaBytes);
+			meta = EncodingProtos.SequenceMetadata.parseFrom(metaBuf.duplicate());
 		} catch (InvalidProtocolBufferException e) {
 			throw new VortexException(CodecId.VORTEX_SEQUENCE, "invalid metadata", e);
 		}

core/src/main/java/io/github/dfa1/vortex/encoding/SparseCodec.java

@@ -135,12 +135,9 @@ public Array decode(DecodeContext ctx) {
 		if (rawMeta == null || !rawMeta.hasRemaining()) {
 			throw new VortexException(CodecId.VORTEX_SPARSE, "missing metadata");
 		}
-		byte[] metaBytes = new byte[rawMeta.remaining()];
-		rawMeta.duplicate().get(metaBytes);
-
 		EncodingProtos.SparseMetadata sparseMeta;
 		try {
-			sparseMeta = EncodingProtos.SparseMetadata.parseFrom(metaBytes);
+			sparseMeta = EncodingProtos.SparseMetadata.parseFrom(rawMeta.duplicate());
 		} catch (InvalidProtocolBufferException e) {
 			throw new VortexException(CodecId.VORTEX_SPARSE, "invalid metadata", e);
 		}
@@ -157,10 +154,9 @@ public Array decode(DecodeContext ctx) {
 
 		// Fill value from buffer[0]
 		MemorySegment fillBuf = ctx.buffer(0);
-		byte[] fillBytes = fillBuf.toArray(ValueLayout.JAVA_BYTE);
 		ScalarProtos.ScalarValue fillScalar;
 		try {
-			fillScalar = ScalarProtos.ScalarValue.parseFrom(fillBytes);
+			fillScalar = ScalarProtos.ScalarValue.parseFrom(fillBuf.asByteBuffer());
 		} catch (InvalidProtocolBufferException e) {
 			throw new VortexException(CodecId.VORTEX_SPARSE, "invalid fill value", e);
 		}