Add numcodecs namespace and built-in shuffle/delta codecs

.changeset/numcodecs-namespace.md

@@ -0,0 +1,27 @@
+---
+"zarrita": minor
+---
+
+Add `numcodecs.` namespace for v2 codec registry and built-in shuffle/delta codecs
+
+V2 filters and compressors are now registered under a `numcodecs.` prefix
+(e.g., `numcodecs.blosc`, `numcodecs.zlib`) matching zarr-python's convention.
+This separates v2-specific codecs from the v3 codec namespace.
+
+Adds built-in `numcodecs.shuffle` and `numcodecs.delta` codecs for reading v2
+data that uses these common numcodecs filters. Both are pure JS with no WASM
+dependencies.
+
+Custom v2 codecs can be registered under the same namespace:
+
+```ts
+zarr.registry.set("numcodecs.my-filter", async () => ({
+  fromConfig(config) {
+    return {
+      kind: "bytes_to_bytes",
+      encode(data) { /* ... */ },
+      decode(data) { /* ... */ },
+    };
+  },
+}));
+```

fixtures/v2/data.zarr/.zmetadata

@@ -166,6 +166,56 @@
             ],
             "zarr_format": 2
         },
+        "1d.contiguous.delta.i2/.zarray": {
+            "chunks": [
+                4
+            ],
+            "compressor": {
+                "id": "zlib",
+                "level": 1
+            },
+            "dtype": "<i2",
+            "fill_value": 0,
+            "filters": [
+                {
+                    "astype": "<i2",
+                    "dtype": "<i2",
+                    "id": "delta"
+                }
+            ],
+            "order": "C",
+            "shape": [
+                4
+            ],
+            "zarr_format": 2
+        },
+        "1d.contiguous.delta.shuffle.i2/.zarray": {
+            "chunks": [
+                4
+            ],
+            "compressor": {
+                "id": "zlib",
+                "level": 1
+            },
+            "dtype": "<i2",
+            "fill_value": 0,
+            "filters": [
+                {
+                    "astype": "<i2",
+                    "dtype": "<i2",
+                    "id": "delta"
+                },
+                {
+                    "elementsize": 2,
+                    "id": "shuffle"
+                }
+            ],
+            "order": "C",
+            "shape": [
+                4
+            ],
+            "zarr_format": 2
+        },
         "1d.contiguous.f4.be/.zarray": {
             "chunks": [
                 4
@@ -277,6 +327,28 @@
             ],
             "zarr_format": 2
         },
+        "1d.contiguous.shuffle.i2/.zarray": {
+            "chunks": [
+                4
+            ],
+            "compressor": {
+                "id": "zlib",
+                "level": 1
+            },
+            "dtype": "<i2",
+            "fill_value": 0,
+            "filters": [
+                {
+                    "elementsize": 2,
+                    "id": "shuffle"
+                }
+            ],
+            "order": "C",
+            "shape": [
+                4
+            ],
+            "zarr_format": 2
+        },
         "1d.contiguous.u1/.zarray": {
             "chunks": [
                 4

fixtures/v2/data.zarr/1d.contiguous.delta.i2/.zarray

@@ -0,0 +1,23 @@
+{
+    "chunks": [
+        4
+    ],
+    "compressor": {
+        "id": "zlib",
+        "level": 1
+    },
+    "dtype": "<i2",
+    "fill_value": 0,
+    "filters": [
+        {
+            "astype": "<i2",
+            "dtype": "<i2",
+            "id": "delta"
+        }
+    ],
+    "order": "C",
+    "shape": [
+        4
+    ],
+    "zarr_format": 2
+}

fixtures/v2/data.zarr/1d.contiguous.delta.shuffle.i2/.zarray

@@ -0,0 +1,27 @@
+{
+    "chunks": [
+        4
+    ],
+    "compressor": {
+        "id": "zlib",
+        "level": 1
+    },
+    "dtype": "<i2",
+    "fill_value": 0,
+    "filters": [
+        {
+            "astype": "<i2",
+            "dtype": "<i2",
+            "id": "delta"
+        },
+        {
+            "elementsize": 2,
+            "id": "shuffle"
+        }
+    ],
+    "order": "C",
+    "shape": [
+        4
+    ],
+    "zarr_format": 2
+}

fixtures/v2/data.zarr/1d.contiguous.shuffle.i2/.zarray

@@ -0,0 +1,22 @@
+{
+    "chunks": [
+        4
+    ],
+    "compressor": {
+        "id": "zlib",
+        "level": 1
+    },
+    "dtype": "<i2",
+    "fill_value": 0,
+    "filters": [
+        {
+            "elementsize": 2,
+            "id": "shuffle"
+        }
+    ],
+    "order": "C",
+    "shape": [
+        4
+    ],
+    "zarr_format": 2
+}