Support data alignment in the custom extensions

Author: EddiG

src/Decoder.ts

@@ -687,8 +687,16 @@ export class Decoder<ContextType = undefined> {
       throw new DecodeError(`Max length exceeded: ext length (${size}) > maxExtLength (${this.maxExtLength})`);
     }
 
-    const extType = this.view.getInt8(this.pos + headOffset);
-    const data = this.decodeBinary(size, headOffset + 1 /* extType */);
+    let padding = 0;
+    let extType = this.view.getInt8(this.pos + headOffset);
+
+    // 0xc1 => -63 (Int8) (noop byte)
+    while (extType === -63) {
+      padding++;
+      extType = this.view.getInt8(this.pos + headOffset + padding);
+    }
+
+    const data = this.decodeBinary(size, headOffset + padding + 1 /* extType */);
     return this.extensionCodec.decode(data, extType, this.context);
   }
 

src/Encoder.ts

@@ -427,6 +427,14 @@ export class Encoder<ContextType = undefined> {
     } else {
       throw new Error(`Too large extension object: ${size}`);
     }
+    if (ext.align && Number.isInteger(ext.align)) {
+      const align = ext.align;
+      const dataPos = this.pos + 1; // + extType size
+      const padding = (align - (dataPos % align)) % align;
+      for (let i = 0; i < padding; i++) {
+        this.writeU8(0xc1); // noop byte
+      }
+    }
     this.writeI8(ext.type);
     this.writeU8a(ext.data);
   }

src/ExtData.ts

@@ -5,5 +5,6 @@ export class ExtData {
   constructor(
     readonly type: number,
     readonly data: Uint8Array,
+    readonly align: number | undefined | null = null,
   ) {}
 }

src/ExtensionCodec.ts

@@ -34,24 +34,28 @@ export class ExtensionCodec<ContextType = undefined> implements ExtensionCodecTy
   // custom extensions
   private readonly encoders: Array<ExtensionEncoderType<ContextType> | undefined | null> = [];
   private readonly decoders: Array<ExtensionDecoderType<ContextType> | undefined | null> = [];
+  private readonly aligns: Array<number | undefined | null> = [];
 
   public constructor() {
     this.register(timestampExtension);
   }
 
   public register({
     type,
+    align,
     encode,
     decode,
   }: {
     type: number;
+    align?: number;
     encode: ExtensionEncoderType<ContextType>;
     decode: ExtensionDecoderType<ContextType>;
   }): void {
     if (type >= 0) {
       // custom extensions
       this.encoders[type] = encode;
       this.decoders[type] = decode;
+      this.aligns[type] = align;
     } else {
       // built-in extensions
       const index = 1 + type;
@@ -80,7 +84,8 @@ export class ExtensionCodec<ContextType = undefined> implements ExtensionCodecTy
         const data = encodeExt(object, context);
         if (data != null) {
           const type = i;
-          return new ExtData(type, data);
+          const align = this.aligns[type];
+          return new ExtData(type, data, align);
         }
       }
     }

test/ExtensionCodec.test.ts

@@ -196,4 +196,32 @@ describe("ExtensionCodec", () => {
       ]);
     });
   });
+
+  context("custom extensions with alignment", () => {
+    const extensionCodec = new ExtensionCodec();
+
+    extensionCodec.register({
+      type: 0x01,
+      align: 4,
+      encode: (object: unknown): Uint8Array | null => {
+        if (object instanceof Float32Array) {
+          return new Uint8Array(object.buffer);
+        }
+        return null;
+      },
+      decode: (data: Uint8Array) => {
+        return new Float32Array(data.buffer, data.byteOffset, data.byteLength / Float32Array.BYTES_PER_ELEMENT);
+      },
+    });
+
+    it("encodes and decodes Float32Array type with zero-copy", () => {
+      const data = {
+        position: new Float32Array([1.1, 2.2, 3.3, 4.4, 5.5]),
+      };
+      const encoded = encode(data, { extensionCodec });
+      const decoded = decode(encoded, { extensionCodec });
+      assert.deepStrictEqual(decoded, data);
+      assert.strictEqual(decoded.position.buffer, encoded.buffer);
+    });
+  });
 });