Support read-only bitround codec

Fixes #233. Introduces `BitroundCodec` with a no-op for decoding because
bit-rounding is lossy, and precision cannot be restored. Encoding is not
yet implemented, reflecting zarrita's current focus on read-heavy
contexts. Open an issue if encoding support is needed.

packages/core/src/codecs.ts

@@ -6,6 +6,7 @@ import { Crc32cCodec } from "./codecs/crc32c.js";
 import { JsonCodec } from "./codecs/json2.js";
 import { TransposeCodec } from "./codecs/transpose.js";
 import { VLenUTF8 } from "./codecs/vlen-utf8.js";
+import { BitroundCodec } from "./codecs/bitround.js";
 
 type ChunkMetadata<D extends DataType> = {
 	data_type: D;
@@ -31,7 +32,8 @@ function create_default_registry(): Map<string, () => Promise<CodecEntry>> {
 		.set("bytes", () => BytesCodec)
 		.set("crc32c", () => Crc32cCodec)
 		.set("vlen-utf8", () => VLenUTF8)
-		.set("json2", () => JsonCodec);
+		.set("json2", () => JsonCodec)
+		.set("bitround", () => BitroundCodec);
 }
 
 export const registry: Map<string, () => Promise<CodecEntry>> =

packages/core/src/codecs/bitround.ts

@@ -0,0 +1,53 @@
+import type { Chunk, Float32, Float64 } from "../metadata.js";
+
+/**
+ * A codec for bit-rounding.
+ *
+ * Reduces floating-point precision by truncating mantissa bits during encoding.
+ * Decoding is a no-op as the process is lossy and precision cannot be restored.
+ *
+ * Note: {@link BitroundCodec.encode} is not yet implemented since Zarrita is
+ * primarily used in read-only contexts (web browser). If you need encoding support,
+ * please open an issue at {@link https://github.com/manzt/zarrita.js/issues}.
+ *
+ * @see {@link https://github.com/zarr-developers/numcodecs/blob/main/numcodecs/bitround.py}
+ * for the original Python implementation.
+ *
+ * @remarks
+ * Data types are not validated, and `float16` arrays are not supported (reflecting browser support).
+ */
+export class BitroundCodec<D extends Float64 | Float32> {
+	kind = "array_to_array";
+
+	constructor(configuration: { keepbits: number }, _meta: { data_type: D }) {
+		if (configuration.keepbits < 0) {
+			throw new Error("keepbits must be zero or positive");
+		}
+	}
+
+	static fromConfig<D extends Float32 | Float64>(
+		configuration: { keepbits: number },
+		meta: { data_type: D },
+	): BitroundCodec<D> {
+		return new BitroundCodec(configuration, meta);
+	}
+
+	/**
+	 * Encode a chunk of data with bit-rounding.
+	 * @param _arr - The chunk to encode
+	 */
+	encode(_arr: Chunk<D>): Chunk<D> {
+		throw new Error(
+			"`BitroundCodec.encode` is not implemented. Please open an issue at https://github.com/manzt/zarrita.js/issues.",
+		);
+	}
+
+	/**
+	 * Decode a chunk of data (no-op).
+	 * @param arr - The chunk to decode
+	 * @returns The decoded chunk
+	 */
+	decode(arr: Chunk<D>): Chunk<D> {
+		return arr; // No-op as bit-rounding is lossy
+	}
+}