From 062879ea7a6662b9b2c41761cf998f4db54bd012 Mon Sep 17 00:00:00 2001
From: hahmann <hahmann@mpi-cbg.de>
Date: Mon, 21 Sep 2026 14:57:26 +0200
Subject: [PATCH] Support gzip and zlib 'level' of -1 in v2 metadata

Other implementations write zlib's Z_DEFAULT_COMPRESSION (-1) as the
'level' of gzip and zlib, meaning "let the library choose". Both codecs
rejected it while parsing .zarray, so such an array could not be opened.
Deflater takes -1 as-is, so only the lower bound of the check moves.

Reported for gzip in https://github.com/mobie/mobie-io/issues/181.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
---
 .../zarrjava/v2/codec/core/GzipCodec.java     |  5 +--
 .../zarrjava/v2/codec/core/ZlibCodec.java     |  4 +--
 .../java/dev/zarr/zarrjava/ZarrV2Test.java    | 35 +++++++++++++++++++
 3 files changed, 40 insertions(+), 4 deletions(-)

diff --git a/src/main/java/dev/zarr/zarrjava/v2/codec/core/GzipCodec.java b/src/main/java/dev/zarr/zarrjava/v2/codec/core/GzipCodec.java
index 42e8f505..3579c22f 100644
--- a/src/main/java/dev/zarr/zarrjava/v2/codec/core/GzipCodec.java
+++ b/src/main/java/dev/zarr/zarrjava/v2/codec/core/GzipCodec.java
@@ -8,6 +8,7 @@
 import dev.zarr.zarrjava.v2.codec.Codec;
 
 import java.nio.ByteBuffer;
+import java.util.zip.Deflater;
 
 public class GzipCodec extends dev.zarr.zarrjava.core.codec.core.GzipCodec implements Codec {
 
@@ -20,8 +21,8 @@ public class GzipCodec extends dev.zarr.zarrjava.core.codec.core.GzipCodec imple
     public GzipCodec(
             @JsonProperty(value = "level", defaultValue = "" + DEFAULT_LEVEL) int level)
             throws ZarrException {
-        if (level < 0 || level > 9) {
-            throw new ZarrException("'level' needs to be between 0 and 9.");
+        if (level < Deflater.DEFAULT_COMPRESSION || level > 9) {
+            throw new ZarrException("'level' needs to be between -1 and 9.");
         }
         this.level = level;
     }
diff --git a/src/main/java/dev/zarr/zarrjava/v2/codec/core/ZlibCodec.java b/src/main/java/dev/zarr/zarrjava/v2/codec/core/ZlibCodec.java
index 4e62817e..4117952c 100644
--- a/src/main/java/dev/zarr/zarrjava/v2/codec/core/ZlibCodec.java
+++ b/src/main/java/dev/zarr/zarrjava/v2/codec/core/ZlibCodec.java
@@ -27,8 +27,8 @@ public class ZlibCodec extends BytesBytesCodec implements Codec {
     @JsonCreator(mode = JsonCreator.Mode.PROPERTIES)
     public ZlibCodec(
             @JsonProperty(value = "level", defaultValue = "1") int level) throws ZarrException {
-        if (level < 0 || level > 9) {
-            throw new ZarrException("'level' needs to be between 0 and 9.");
+        if (level < Deflater.DEFAULT_COMPRESSION || level > 9) {
+            throw new ZarrException("'level' needs to be between -1 and 9.");
         }
         this.level = level;
     }
diff --git a/src/test/java/dev/zarr/zarrjava/ZarrV2Test.java b/src/test/java/dev/zarr/zarrjava/ZarrV2Test.java
index 2d7c4bcc..310fae62 100644
--- a/src/test/java/dev/zarr/zarrjava/ZarrV2Test.java
+++ b/src/test/java/dev/zarr/zarrjava/ZarrV2Test.java
@@ -108,6 +108,41 @@ public void testReadBloscAutoShuffle(String dataType, String expectedShuffle)
         Assertions.assertEquals(127, readData.getInt(127));
     }
 
+    @ParameterizedTest
+    @ValueSource(strings = {"gzip", "zlib"})
+    public void testReadDefaultCompressionLevel(String compressorId)
+            throws IOException, ZarrException {
+        String arrayName = "v2_default_level_" + compressorId;
+        StoreHandle storeHandle = new FilesystemStore(TESTOUTPUT).resolve(arrayName);
+        ucar.ma2.Array testData =
+                ucar.ma2.Array.factory(ucar.ma2.DataType.UBYTE, new int[]{16, 16});
+        for (int i = 0; i < testData.getSize(); i++) {
+            testData.setInt(i, i % 128);
+        }
+        ArrayMetadataBuilder metadataBuilder = Array.metadataBuilder()
+                .withShape(16, 16)
+                .withDataType(DataType.UINT8)
+                .withChunks(8, 8);
+        Array array = Array.create(
+                storeHandle,
+                (compressorId.equals("gzip")
+                        ? metadataBuilder.withGzipCompressor(1)
+                        : metadataBuilder.withZlibCompressor(1)).build()
+        );
+        array.write(testData);
+
+        Path zarrayPath = TESTOUTPUT.resolve(arrayName).resolve(ZARRAY);
+        String zarray = new String(Files.readAllBytes(zarrayPath));
+        String defaultLevelZarray = zarray.replaceFirst("\"level\"\\s*:\\s*1", "\"level\": -1");
+        Assertions.assertTrue(defaultLevelZarray.contains("\"level\": -1"), zarray);
+        Files.write(zarrayPath, defaultLevelZarray.getBytes());
+
+        Array reopenedArray = Array.open(storeHandle);
+        ucar.ma2.Array readData = reopenedArray.read();
+        Assertions.assertEquals(16 * 16, readData.getSize());
+        Assertions.assertEquals(127, readData.getInt(127));
+    }
+
     @ParameterizedTest
     @CsvSource({
             "BOOL", "FLOAT64"